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One of the most common questions we get asked before a shed base project starts is simply how thick the concrete actually needs to be. The honest answer changes significantly depending on your shed’s size and what you’re planning to store inside it, which is exactly why a single blanket figure doesn’t serve most homeowners well.  This guide pulls together our recommended thickness across every common shed size, all in one reference chart, so you can quickly find what applies to your specific project. At JPD Landscaping & Building Services, we’ve built concrete shed bases across South East London for over 15 years, covering everything from small garden sheds to heavy workshop structures. This guide gives you a clear, size by size thickness reference, along with the mistakes that trip up thickness planning even when the right number is used. Quick Answer: Concrete Shed Base Thickness by Size Shed Size Recommended Thickness Small (up to 6x4ft) 100mm to 125mm Medium (8x6ft to 10x8ft) 100mm to 125mm Large (12x10ft and up, general storage) 150mm to 200mm Workshop (heavy machinery or equipment) Individual assessment, no fixed range Notice that small and medium sheds share the same recommended thickness range. This isn’t an oversight, it reflects that the jump in required thickness happens more at the point where a shed moves into larger, heavier territory, rather than scaling gradually with every size increase in between. Why Small and Medium Sheds Share the Same Thickness Range For sheds up to roughly 10x8ft, the overall weight and load distribution typically falls within a fairly similar range, meaning a 100mm to 125mm concrete base generally provides sufficient strength and stability for both categories. It’s only once you move into significantly larger structures that the additional weight and floor area genuinely start demanding a thicker slab to properly support the load. This is useful to know if you’re planning a shed somewhere in this size range, since it means the concrete thickness question is fairly settled once you know you’re within the small to medium bracket, without needing to fine tune the figure further based on exactly where in that range your specific shed falls. For the full process of building a base at this thickness, including sub base preparation and formwork, our guide on how to build a shed base covers the complete step by step approach. Why Large Sheds Need Noticeably More Thickness Once you move into larger sheds, typically 12x10ft and upward for general storage purposes, the recommended thickness increases to 150mm to 200mm. This step up reflects the larger floor area distributing weight across a bigger surface, along with the generally heavier overall structure and contents that come with a bigger shed. This range assumes general storage use rather than heavy machinery or workshop equipment. If you’re planning a large shed specifically for lighter storage purposes, this thickness range provides a solid, reliable foundation without over specifying beyond what’s actually needed.  If you’re specifically working to a 12x12ft footprint, our guide on 12×12 concrete slab cost breaks down real pricing at that exact size, which pairs directly with the thickness guidance here. Why Workshop Sheds Don’t Get a Fixed Thickness Range Workshop sheds, particularly those housing heavy machinery, equipment, or workbenches, genuinely need individual assessment rather than a fixed thickness range. This isn’t us avoiding giving a straight answer, it reflects a real difference in how these structures need to be evaluated compared with standard storage sheds. The reason comes down to point loads. A workshop with heavy machinery doesn’t distribute weight evenly across the floor the way general storage does. Instead, specific points, such as machinery legs, workbench supports, or heavy equipment feet, concentrate significant weight onto small, specific areas of the base.  A generic thickness figure, even a generous one, doesn’t necessarily account for these concentrated load points properly, which is why we always recommend a proper site and equipment assessment for workshop projects rather than applying a standard range. Our guide on foundations for larger, heavier sheds goes into more depth on how depth, thickness, and reinforcement interact for this category of structure. Understanding Point Loads vs Distributed Loads It’s worth explaining the distinction between point loads and distributed loads a bit further, since this is genuinely the core reason workshop thickness can’t be reduced to a simple number. A distributed load, such as general stored boxes, garden equipment, or furniture spread across a shed floor, applies weight fairly evenly across the whole slab area, which a uniform thickness handles well. A point load, by contrast, concentrates significant weight onto a very small contact area, such as the four legs of a table saw or the feet of a heavy workbench. Even a relatively modest total weight can create a surprisingly high pressure at these specific contact points, which is why a slab that’s perfectly adequate for general storage can still develop localised cracking under a workshop setup if those specific load points weren’t accounted for during planning. How Ground Conditions Interact With Thickness Recommendations While this guide focuses on thickness by shed size, it’s worth being clear that thickness and ground condition work together rather than in isolation. The thickness ranges given here assume reasonably firm, well prepared, and properly compacted ground beneath the slab.  On softer or less stable ground, it’s common to combine a thickness within the standard range for that shed size with additional reinforcement, or in some cases a slightly increased thickness, to compensate for the less supportive ground conditions. This is exactly why a site visit remains valuable even when you already know roughly what thickness your shed size calls for, since the final specification often needs to reflect both the structure’s size and weight, and the specific characteristics of your garden’s ground. Reinforcement Considerations Alongside Thickness Thickness and reinforcement are related but separate decisions, and it’s worth understanding how they work together rather than assuming a thicker slab automatically means reinforcement is unnecessary, or vice versa. For small and medium general storage

A 12x12ft concrete slab is a popular size for shed and outbuilding bases, offering enough space for a decent sized garden structure without being an oversized, unnecessarily expensive pour. Knowing roughly what to budget for before you get a quote helps you plan the wider project properly, whether that’s a new shed, workshop, or general hardstanding area. At JPD Landscaping & Building Services, we’ve poured concrete slabs across South East London for over 15 years, including plenty at this exact 12x12ft size for shed and outbuilding bases. This guide breaks down real pricing, what the size actually converts to in square metres, and what can push the cost above a standard estimate. Quick Answer: 12×12 Concrete Slab Cost A 12ft by 12ft area works out to approximately 13.4 square metres. At our standard rate of £80 to £100 per square metre, including materials and labour, a 12×12 concrete slab typically costs between £1,072 and £1,340. Detail Figure Slab size 12ft x 12ft Area in square metres Approx. 13.4m² Cost per m² (materials and labour) £80 to £100 Total estimated cost £1,072 to £1,340 This is a solid starting estimate for a straightforward pour on reasonably level ground. Certain factors, covered below, can push the final price above this range. What a 12×12 Slab Is Typically Used For A 12x12ft concrete slab is most commonly used as a shed or outbuilding base, offering enough space for a good sized garden shed, small workshop, or similar structure. It’s a size that comes up regularly in our project work, striking a reasonable balance between usable space and overall cost compared with smaller or significantly larger slab sizes. If you’re planning a shed at this size, it’s worth checking our guide on how to build a shed base for the full construction process once you’ve budgeted for the pour itself. How 12x12ft Compares to Other Common Slab Sizes It’s useful to see how a 12x12ft slab stacks up against other popular sizes, since this helps put both the space and the cost into context. A smaller 8x6ft slab, suited to a compact garden shed, works out to roughly 4.5 square metres, considerably less than the 13.4 square metres a 12x12ft slab provides. A larger 16x12ft slab, sometimes used for bigger workshop sheds or combined storage and hobby spaces, comes in at around 17.8 square metres. Slab Size Approx. Area Approx. Cost (£80 to £100/m²) 8ft x 6ft 4.5m² £360 to £450 10ft x 8ft 7.4m² £592 to £740 12ft x 12ft 13.4m² £1,072 to £1,340 16ft x 12ft 17.8m² £1,424 to £1,780 This comparison is useful if you’re still deciding on shed size, since the cost difference between adjacent sizes is often smaller in absolute terms than people expect, particularly once ground preparation costs, which stay broadly similar regardless of slab size, are factored into the total project cost. Why the Cost Can Go Above the Standard Range Our £80 to £100 per square metre rate is a solid baseline for a standard pour, but a few factors can genuinely push the final price higher, and it’s worth budgeting for these possibilities rather than being caught out by them later. Cost Factor Impact on Total Price Ground preparation or levelling Can add significantly if the site is sloped or uneven Delivery or access difficulty Adds cost if a pump or extra labour is needed Reinforcement (mesh) Adds material and labour cost for larger or heavier structures Why Ground Preparation Is Consistently the Biggest Variable Since ground preparation shows up as the leading cost driver across nearly every concrete project we quote, it’s worth understanding why it varies so much between sites. A flat, firm, well draining garden needs relatively little beyond basic excavation and compaction before a pour can begin. A sloped garden, by contrast, may need significant levelling, sometimes involving retaining structures if the level change is substantial, before the ground is even ready for the sub base stage. Soil type plays a role too, since clay heavy ground, common across parts of South East London, often needs more thorough compaction and sometimes a deeper sub base than naturally well draining, sandy soil to achieve the same level of stability beneath the slab. This is exactly why an accurate quote genuinely needs a site visit rather than a phone estimate, since two 12x12ft slabs on paper can require meaningfully different amounts of groundwork depending on the specific garden. Does a 12×12 Slab Need Reinforcement? Whether reinforcement is worth including for a 12x12ft slab depends primarily on what’s going on top of it. A standard garden shed, without particularly heavy stored equipment, generally doesn’t need reinforcement at this size, since the load involved sits comfortably within what an unreinforced slab of this size can handle. If you’re planning a workshop shed with machinery, dense storage, or anything carrying meaningfully more concentrated weight than a typical garden shed, it’s worth discussing reinforcement as part of your quote, along with considering whether a deeper foundation depth is more appropriate than a standard slab thickness. Our guide on foundations for larger, heavier sheds covers this consideration in more detail for anyone planning something beyond a standard storage shed at this size. Full Cost Breakdown for a 12×12 Slab Component Estimated Cost Range Standard 13.4m² pour (£80 to £100/m²) £1,072 to £1,340 Additional cost for significant ground levelling +10% to 20% of base cost Additional cost for reinforcement (if needed) Varies by project, typically a modest addition Additional cost for difficult site access Varies by project, assessed on site These additional factors don’t apply to every project, but it’s worth discussing them during your quote so you have a clear, accurate final figure rather than just the standard per square metre rate. What’s Included in Our Standard Per-Square-Metre Rate It’s worth being clear about what the £80 to £100 per square metre figure actually covers, since not every quote in the market includes the same scope of work. Our standard rate includes basic excavation to the required

If you already have an existing paved area or patio in your garden, it’s a reasonable question to ask whether you really need to build a dedicated shed base, or whether you can simply place the shed straight onto the slabs that are already there. The honest answer is that it genuinely depends, not on the slabs’ appearance from above, but on what’s underneath them. At JPD Landscaping & Building Services, we’ve assessed existing paved areas for shed suitability across South East London for over 15 years. This guide explains exactly what determines whether existing slabs are good enough on their own, what needs checking before you decide, and a real example of an assessment that changed a customer’s original plan. Do You Need a Dedicated Shed Base on Existing Slabs? Not always. Whether you need a dedicated base depends on your shed’s size and weight, along with the condition of your existing ground. The single most important thing to check is whether the sub base beneath the existing slabs is solid, rather than the slabs simply being laid loosely on soil. For small, light sheds, existing slabs that are solid and level are often perfectly adequate without needing a separate, dedicated base. Situation Do You Need a New Base? Small or light shed, solid sub base beneath existing slabs Generally no, existing slabs are sufficient Small or light shed, slabs laid directly on soil Likely yes, sub base needs addressing first Medium or large shed, any existing slab condition Depends heavily on sub base, needs proper assessment Why the Sub-Base Matters More Than the Slabs Themselves This is the key point that a lot of homeowners miss when assessing their own garden for shed suitability. Slabs that look perfectly fine on the surface, flat, clean, and free of obvious cracks, can still be sitting directly on soil rather than a proper compacted sub base underneath. Over time, and particularly under the concentrated weight of a shed, slabs without a solid sub base beneath them can shift, sink, or settle unevenly, even if they appeared completely stable before the shed went on top. Checking beneath the surface, rather than judging purely by how the slabs look from above, is genuinely the most reliable way to determine whether your existing paved area is suitable for a shed without further work. This is the same underlying principle that applies to composite decking overlays, where the visible surface tells you very little about the condition of what’s actually supporting it. When Existing Slabs Are Genuinely Fine for a Shed For small, light sheds, existing solid, level slabs are often perfectly adequate without needing a dedicated base built specifically for the shed. If the sub base beneath the slabs is properly compacted and stable, and the slabs themselves show no signs of rocking, cracking, or uneven settling, a small shed can typically go straight onto that existing surface without issue. This is genuinely good news for a lot of homeowners with an existing patio or paved area, since it can save both the time and cost of building a completely separate base from scratch. It’s worth comparing this potential saving against the cost of alternative base types, such as the plastic grid system covered in our guide on building a shed base without concrete, if it turns out your existing slabs aren’t quite suitable after all. When You Should Build a Dedicated Base Instead The picture changes for medium to larger sheds, or for any situation where the existing slabs’ sub base condition is uncertain or known to be inadequate. Larger, heavier sheds place more concentrated weight on the ground beneath them, and existing paving originally designed for general foot traffic use may not have been built with that additional load in mind. If you’re unsure about your sub base condition, or you’re planning anything beyond a small, light shed, it’s worth getting a proper assessment before assuming your existing slabs will simply cope with the shed’s added weight. For larger structures specifically, our guide on foundations for larger, heavier sheds covers what a properly engineered base needs to support that scale of project, which existing patio paving is very unlikely to match without modification. How to Check Your Existing Slabs Yourself Before Calling a Professional While a full professional assessment is the most reliable route, particularly for anything beyond a small shed, there are a few basic checks a homeowner can reasonably do themselves as a first pass. Walking across the entire paved area methodically, paying attention to any give, rocking, or hollow sounding sections underfoot, can reveal individual slabs that aren’t sitting on solid ground beneath them. Lifting a single slab from an inconspicuous corner or edge of the paved area, if it can be done without damaging anything, gives a direct look at what’s actually underneath, whether that’s a proper compacted hardcore sub base, a thin sand bed sitting directly on soil, or bare earth with no preparation at all.  This single lifted slab check, while not a complete substitute for a full assessment across the whole area, can often give a strong early indication of whether the rest of the paved area is likely to be genuinely suitable or whether further investigation is warranted. What to Check Before Deciding Comparing Existing Slabs Against a Purpose-Built Base It’s worth being realistic about what even a genuinely solid existing patio can and can’t offer compared with a base built specifically for a shed from the outset. A well constructed existing paved area with a proper sub base can absolutely perform just as well as a dedicated small shed base, since the fundamental requirements, a level, stable, well supported surface, are broadly the same regardless of the original purpose. Where existing slabs sometimes fall short is in exact sizing and shape relative to the shed’s footprint, since a patio built for general garden use may extend well beyond or fall short of the shed’s actual dimensions in ways that affect how evenly

A block pier foundation is a popular choice for sheds that need proper support without a full concrete slab, using individual stacks of concrete blocks positioned at regular intervals beneath the shed’s frame. Working out how many blocks you actually need comes down to two things: how far apart your piers need to be spaced, and how many blocks high each pier needs to be to reach the correct level. At JPD Landscaping & Building Services, we’ve calculated and installed block pier foundations for sheds across South East London for over 15 years. This guide walks through exactly how to work out your block quantity, the standard spacing we use, and the mistakes that cause the most problems when piers aren’t set up correctly. Quick Answer: How Many Concrete Blocks Do I Need? For a typical shed foundation, we use standard 440mm x 215mm x 100mm concrete blocks, spaced at 600mm to 900mm intervals between pier points. The number of blocks needed per pier varies depending on your ground level and how much height you need to bring the frame up to, since some piers may need just one block while others on uneven ground may need two or three stacked together. Factor Standard Specification Block size 440mm x 215mm x 100mm Pier spacing 600mm to 900mm Blocks per pier Varies by ground level (1 to 3+ blocks high) Why Choose a Block Pier Foundation Over a Slab? Before getting into the calculation itself, it’s worth understanding why a block pier foundation is often chosen over a full concrete slab in the first place. Pier foundations generally use less material and take less time to install than a full slab pour, since there’s no mixing, formwork, or multi week curing period involved. They also naturally allow airflow beneath the shed frame, which can help reduce damp related issues compared with a solid slab sitting flush with the ground in less well draining gardens. The trade off is that pier foundations suit lighter to moderate weight sheds better than very large or heavy structures, since a slab distributes load continuously across its full area, while piers concentrate support at specific points. For a broader comparison between foundation types, including when a full slab or a deeper foundation is the better choice, our guides on how to build a shed base and larger shed foundations cover those alternatives in detail. How to Calculate Your Pier Positions Before you can work out how many blocks you need, you first need to establish how many pier positions your shed foundation requires. This is determined by your shed’s footprint dimensions and the standard 600mm to 900mm spacing we typically use between pier points. Step 1: Measure your shed’s footprint. Note down the full length and width of your shed’s base dimensions. Step 2: Plan pier positions around the perimeter and, for larger sheds, across the middle. Piers need to sit at each corner as a minimum, with additional piers spaced at 600mm to 900mm intervals along longer runs, and often across the middle of the frame for bigger sheds to properly support the structure. Step 3: Count your total pier positions. Add up all the corner, edge, and any internal pier positions to get your total count. Working Out Blocks Per Pier This is where the calculation genuinely varies from project to project, since it depends entirely on your specific ground level and how much height each pier needs to bring the shed frame up to a consistent, level position. Because of this variation, the honest answer to how many blocks you need really does depend on assessing your specific garden’s ground levels at each planned pier position, rather than applying a single fixed number across every project. How to Actually Measure Ground Level Differences Across Your Site To work out how many blocks each specific pier position needs, you’ll first need an accurate picture of how much your ground level varies across the whole foundation area. A simple and effective method for this is using a spirit level and a long, straight timber batten, sometimes called a traveller board, stretched between pier positions to measure the height difference between each point relative to a chosen reference level. Alternatively, a water level, a length of clear tubing filled with water, can measure height differences accurately over longer distances than a spirit level and timber batten alone, since water naturally finds the same level at both ends of the tube regardless of the ground shape in between. Whichever method you use, recording the height difference at each individual pier position before ordering blocks is what allows you to calculate an accurate total, rather than guessing based on a general impression of how sloped the garden looks. Example Calculation: A Typical Shed Foundation Let’s work through an example for an 8ft by 6ft shed to show how the pier count and block quantity come together in practice. Detail Calculation Shed footprint 8ft x 6ft (approx. 2.4m x 1.8m) Pier positions (corners and mid-points) 6 piers (4 corners plus 2 mid-points on longer sides) Average blocks per pier (uneven ground) 2 blocks Total blocks needed 12 blocks This example assumes moderately uneven ground requiring an average of two blocks per pier. On perfectly level ground, you might need only a single block per pier, bringing your total down to 6 blocks for the same shed footprint. On more significantly sloped ground, some piers might need three or more blocks, increasing your total accordingly. What Affects Pier Spacing and Block Count Preparing the Ground Beneath Each Pier It’s not enough to simply stack blocks on whatever surface happens to be there. Each pier position should sit on a small, individually compacted base, typically a shallow excavation filled with a layer of compacted hardcore or a small concrete pad, to prevent that specific point from sinking or shifting independently of the others over time. Skipping this step is one of the most common reasons block pier foundations develop problems,

Not every shed project needs a full concrete pour. If you’re looking to avoid the mixing, pouring, and curing time that comes with concrete, there’s a genuinely solid alternative worth considering: a plastic shed base grid system. It’s fast to install, needs no curing time at all, and works well for a good number of standard garden sheds. At JPD Landscaping & Building Services, we’ve installed shed bases across South East London for over 15 years, including plastic grid systems for customers who wanted a quicker, concrete free alternative. This guide explains exactly how these systems work, when they’re the right choice, and the mistakes that cause the most problems if the installation is rushed. How to Make a Shed Base Without Concrete The best no concrete shed base option for most gardens is a plastic interlocking grid system. These grids sit directly on prepared, compacted ground, are filled with gravel or aggregate, and provide a stable, well draining surface for a shed without any mixing, pouring, or curing time. They’re best suited to small or light sheds, and aren’t recommended for larger, heavier structures. Base Type Curing Time Best For Plastic grid system None, ready immediately after installation Small, light sheds Concrete slab 3 to 4 days light use, 28 days full strength Larger, heavier sheds Why Plastic Grid Systems Are the Best No-Concrete Option The main appeal of a plastic grid base is simple: fast installation with no curing time needed. Unlike concrete, which needs days before light use and a full 28 days to reach complete strength, a plastic grid base is ready to have a shed placed on it almost as soon as installation is finished. For anyone wanting to avoid the waiting period concrete requires, this makes a real practical difference to project timelines. For a full comparison against the concrete route, our guide on how to build a shed base covers that process step by step alongside this grid based alternative. How Plastic Grid Shed Bases Work Plastic grid systems consist of interlocking panels that sit on top of prepared, compacted ground. Once positioned and connected together, the open cells within each grid panel are filled with gravel or a similar aggregate material, which locks into place within the grid structure to create a stable, load bearing surface. This design allows water to drain straight through the base rather than pooling on top, which is a genuine advantage over a solid concrete or paving surface in gardens where drainage is already a bit of a concern. What Grid Panels Are Actually Made From Most plastic grid systems are manufactured from high density polyethylene or a similar durable, weather resistant plastic, chosen specifically for its ability to withstand repeated freeze thaw cycles, UV exposure, and sustained ground contact without degrading over time. The honeycomb style cell structure within each panel is engineered to distribute weight evenly across the filled aggregate, rather than concentrating load at any single point, which is part of what allows a relatively thin plastic grid to support meaningful weight when properly filled and installed. Panel thickness and cell depth vary between manufacturers and product ranges, generally correlating with how much load the specific system is rated to support, so it’s worth checking a manufacturer’s stated weight rating against your specific shed’s requirements before purchasing, rather than assuming all grid products offer identical performance. The Main Limitation: Not Suitable for Larger, Heavier Sheds It’s important to be upfront about where plastic grid bases fall short. They’re genuinely well suited to small or light sheds, but they’re not recommended for larger, heavier structures. The grid system simply isn’t designed to distribute and support the kind of concentrated weight a bigger shed, particularly one storing heavy equipment or machinery, places on the ground beneath it. If you’re planning anything beyond a standard small garden shed, it’s worth considering a concrete slab base instead, which offers the strength and load bearing capacity that larger structures genuinely need. For workshop sheds or anything storing significant equipment weight, our guide on larger shed foundations covers what a properly engineered, deeper foundation involves for that scale of project. Shed Size or Weight Suitable for Plastic Grid? Small, light garden shed Yes, generally well suited Medium shed, general storage Case by case, worth assessing Large shed or workshop No, concrete slab recommended instead Plastic Grid vs Paving Slabs: Which Should You Choose? Since both are viable no concrete options for smaller sheds, it’s worth comparing them directly. Plastic grid systems offer faster installation and no curing wait, along with genuinely excellent drainage since water passes straight through the aggregate fill rather than sitting on a solid surface. Paving slabs, by contrast, offer a completely flat, hard surface that some people prefer aesthetically, and can feel more solid underfoot compared with a gravel filled grid surface, though installation takes somewhat longer and requires more precise levelling work slab by slab. Neither option is objectively superior, the right choice often comes down to whether drainage or a completely flat, hard surface matters more for your specific garden and shed, along with how much time you want to spend on installation. Our guide on laying a paving slab shed base covers that alternative process in full if you’re still weighing up both options. Step-by-Step: Installing a Plastic Grid Shed Base Step 1: Mark Out and Excavate the Area Measure and mark your shed’s footprint, allowing a small overhang as you would with any shed base. Excavate the marked area to an appropriate depth to accommodate the ground preparation and grid panels together. Step 2: Compact the Ground Thoroughly This step matters just as much here as it does for a concrete or paving base. Compact the excavated ground thoroughly before laying anything on top, since a poorly compacted base undermines the stability of the grid system above it, regardless of how well the grid itself is installed. Step 3: Lay and Interlock the Grid Panels Position the grid panels across the

The concrete mix itself gets most of the attention when people plan a slab project, but the formwork, the timber or edging that shapes and contains the concrete while it sets, is just as important to get right. Poorly built formwork is one of the most common reasons a DIY concrete slab ends up with a bowed edge, an uneven surface, or a finished shape that doesn’t match what was actually planned. At JPD Landscaping & Building Services, we’ve built formwork for concrete slabs across South East London for over 15 years, from straightforward rectangular slabs to ones with curved edges. This guide focuses specifically on formwork and edging technique, covering the materials to use, how to handle curves, proper bracing, and a real example of what happens when formwork isn’t secured properly. Building Formwork for a Concrete Slab For a standard rectangular slab, 18mm treated timber or shuttering ply makes solid, reliable formwork. For curved edges, the right material depends on how tight the curve is, ranging from flexible plywood strips to bending board for gentler curves. Whatever material you use, the formwork needs to be properly staked with wooden pegs driven into the ground and screwed to the formwork itself, since this is the single most common point of failure in a DIY slab pour. Formwork Type Material Straight edges 18mm treated timber or shuttering ply Curved edges Varies by curve tightness (flexible ply, bending board, etc.) Securing formwork Wooden pegs or stakes, screwed to the formwork Why Formwork Matters More Than People Expect It’s easy to think of formwork as just a temporary container for wet concrete, something that gets removed and forgotten once the pour is done. In reality, formwork directly determines the finished shape, edge quality, and level accuracy of your entire slab. Wet concrete is genuinely heavy, and it exerts real outward pressure on whatever’s containing it as it’s poured and settled. Formwork that isn’t strong enough, or isn’t properly secured, will move under that pressure, and once it moves, there’s no fixing the edge shape after the pour without breaking it out and starting again. This is exactly why formwork deserves just as much planning attention as the concrete mix itself, which is covered in more detail in our guide on how to lay a concrete base if you’re looking for the full pouring and curing process alongside the formwork stage covered here. Formwork Materials for Straight Edges For a standard rectangular slab with straight edges, 18mm treated timber or shuttering ply is a reliable, widely available choice. It’s sturdy enough to resist the outward pressure of wet concrete when properly staked, while still being manageable to cut and position accurately along your marked slab dimensions. Understanding the 3-4-5 Method for Square Corners Since squaring up formwork corners accurately is such a foundational step, it’s worth explaining the 3-4-5 method in a bit more detail, since it’s a simple, reliable technique that doesn’t require any specialist tools. The principle relies on the mathematical fact that a triangle with sides measuring 3, 4, and 5 units, in any consistent unit of measurement, always forms a perfect right angle at the corner between the 3 and 4 unit sides. In practice, this means measuring 3 units along one edge of your formwork from the corner, 4 units along the adjacent edge from the same corner, and then checking the diagonal distance between those two points measures exactly 5 units. If it does, the corner is genuinely square. If the diagonal measurement is off, the corner needs adjusting until it matches, giving a far more reliable result than trying to judge squareness by eye alone. Choosing Timber Thickness and Depth While 18mm is a solid general purpose thickness for most garden slab projects, it’s worth knowing when a thicker or thinner board might be more appropriate. For shallow pours, typically under 75mm deep, slightly thinner timber can sometimes be adequate, though 18mm remains a safe, reliable default that resists bowing well across most standard garden project depths. For deeper pours, such as the 100mm to 300mm depths often used for shed bases or larger structural foundations, it’s worth considering thicker formwork material, or at minimum ensuring peg spacing is tightened further to compensate for the increased outward pressure a taller column of wet concrete exerts. The general rule is that deeper pours need either stronger formwork, closer staking, or ideally both together, rather than assuming standard 18mm timber and standard peg spacing will automatically scale up to handle a significantly deeper pour without issue. Formwork for Curved Edges Curved slab edges genuinely need a different approach than straight ones, and the right material really does depend on how tight the curve is. Gentle, sweeping curves can often be achieved with flexible bending board or hardboard, which flexes smoothly without needing modification. Tighter curves typically need flexible plywood strips that have been kerfed, meaning shallow cuts scored partway through the material at intervals, allowing it to bend more sharply without cracking or splitting. Curve Tightness Suitable Formwork Material Gentle, sweeping curves Flexible bending board or hardboard Tighter curves Kerfed plywood strips Getting curved formwork right often takes a bit more patience than straight formwork, since you’re working the material into shape gradually rather than simply cutting and positioning straight lengths. How Kerfing Actually Works For anyone unfamiliar with the technique, kerfing involves making a series of shallow, evenly spaced cuts partway through the thickness of a plywood strip, typically along the face that will sit on the inside of the curve. These cuts don’t go all the way through the material, they simply reduce its resistance to bending at each cut point, allowing the overall strip to flex into a tighter radius than the same uncut material could achieve without cracking. Spacing between kerf cuts generally needs to be closer together for tighter curves and can be spaced further apart for gentler ones. It’s worth practising kerf spacing on an offcut of the same material before

Whether you’re laying a base for a greenhouse, bin store, log store, or any other garden structure, getting the concrete mix and pouring technique right is what separates a strong, long lasting base from one that cracks, crumbles, or fails within a year or two. The good news is that with the right ratio, the right tools, and a bit of patience during curing, this is a genuinely achievable DIY project. At JPD Landscaping & Building Services, we’ve mixed and poured concrete bases across South East London for over 15 years. This guide focuses specifically on the technique itself, covering the right mix ratio, the tools you’ll need, how to pour and finish the concrete properly, and how long it actually needs to cure before you can use it. How to Lay a Concrete Base For a general purpose concrete base, use a 1:2:4 ratio of cement, sand, and aggregate. Once poured and levelled, allow 3 to 4 days before light use, and a full 28 days to reach complete strength. Getting the water content right during mixing, properly compacting the sub base beforehand, and avoiding pouring in bad weather are the three factors that most affect whether your base turns out strong or develops problems later. Stage Detail Mix ratio 1:2:4 (cement:sand:aggregate) Light use After 3 to 4 days Full strength After 28 days What You’ll Need to Lay a Concrete Base The Correct Concrete Mix Ratio For a general purpose base, whether it’s for a greenhouse, bin store, log store, or similar garden structure, our standard recommendation is a 1:2:4 ratio of cement to sand to aggregate. This ratio provides a good balance of strength and workability for most typical garden base applications, without being overly complex to mix correctly yourself. Getting the water content right when combining these dry ingredients matters just as much as the ratio itself, which is exactly where a lot of DIY pours run into trouble. Understanding Why This Ratio Works It’s worth understanding briefly why this particular balance of materials produces a reliable general purpose base. Cement acts as the binder, chemically reacting with water in a process called hydration, which is what actually causes the mix to harden and gain strength over time rather than simply drying out like a puddle evaporating. Sand fills the smaller gaps between the larger aggregate particles, improving workability and helping the mix pack together densely, which reduces the risk of weak spots or voids within the cured concrete. The aggregate itself, typically gravel or crushed stone, provides the bulk of the finished material’s compressive strength, since concrete relies heavily on this coarse material to resist the loads placed on it once cured. Getting the proportions of these three elements right, rather than guessing at rough quantities, is what gives a 1:2:4 mix its reliable, well tested balance of strength and workability for standard garden applications. Choosing the Right Aggregate Size Not all aggregate is created equal, and choosing the right size for your specific base matters more than many DIYers realise. For a standard garden base, aggregate in the 10mm to 20mm range is typically appropriate, offering a good balance between strength and workability during mixing and pouring. Larger aggregate can make the mix harder to work into corners and against formwork edges, while very fine aggregate can reduce the overall strength of the cured concrete compared with a properly graded mix. If you’re unsure which aggregate size to use for your specific project, most builders’ merchants can advise based on the intended use of the base, and it’s worth confirming this before purchasing materials rather than assuming any gravel will do. Step-by-Step: How to Mix and Pour a Concrete Base Step 1: Prepare and Compact the Sub-Base Before any concrete gets mixed, make sure the ground beneath your base area is properly excavated, levelled, and compacted. A poorly compacted sub base is one of the most common causes of a concrete base cracking or settling unevenly later, regardless of how well the concrete itself is mixed and poured. This principle applies just as much to garden structure bases as it does to larger projects like shed foundations, where sub base preparation consistently proves to be one of the most important stages of the entire build. Step 2: Set Up Your Formwork Build timber formwork around the perimeter of your base area to contain the concrete while it’s poured and setting. Check that your formwork is level and properly secured before you start mixing, since adjusting it after pouring begins is far more difficult. Step 3: Mix the Concrete to the Correct Ratio Combine your cement, sand, and aggregate in the 1:2:4 ratio, mixing thoroughly while dry before gradually adding water. This is where the most common DIY mistake happens: adding too much water makes the mix weaker and more prone to cracking as it cures, while too little water makes it difficult to work with and can leave air pockets that weaken the finished base. Aim for a workable, slightly stiff consistency, similar to thick porridge, rather than something runny or crumbly. Step 4: Pour the Concrete Into the Formwork Pour the mixed concrete into your prepared formwork, working in manageable sections if you’re covering a larger area, to keep the pour consistent throughout. Step 5: Level and Screed the Surface Using your screed board, work along the top of the formwork to level the concrete’s surface, removing excess material and filling any low spots as you go. This step needs to happen relatively soon after pouring, before the concrete begins to firm up. Step 6: Finish the Surface With a Float Once screeded, use a float to smooth the surface further, working in slightly overlapping passes to achieve an even, consistent finish across the whole base. Judging Water Content by Feel Since getting the water ratio right is genuinely the single biggest factor separating a strong mix from a weak one, it’s worth knowing how to judge consistency by feel rather than

For smaller, lighter sheds, a paving slab base is a genuinely solid choice, offering a stable, level surface without the extra time and equipment a full concrete pour requires. Getting it right comes down to proper ground preparation, the correct sub base depth for your specific conditions, and taking care to lay each slab level with the next. At JPD Landscaping & Building Services, we’ve laid paving slab shed bases across South East London for over 15 years. This guide walks through exactly how to lay one properly, step by step, including the standard slab size we typically use, the mistakes that cause the most problems, and a real example of a paving slab base done right. How to Lay a Paving Slab Base for a Shed To lay a paving slab shed base, excavate and level the area, add a compacted sand and hardcore sub base at a depth suited to your ground conditions and shed size, then lay your slabs, typically 600mm by 600mm, ensuring each one is level with the next and the whole base has a slight overhang beyond your shed’s exact footprint. Step What to Do 1. Mark and measure Mark the base area with a slight overhang beyond shed dimensions 2. Excavate and level Dig out and level the ground before adding sub base 3. Add sub-base Compacted sand and hardcore, depth varies by ground condition 4. Lay slabs Standard 600mm x 600mm slabs, checking level throughout 5. Fill joints Fill gaps between slabs for a clean, finished surface Step-by-Step: How to Lay a Paving Slab Shed Base Step 1: Mark Out and Measure the Area Measure your shed’s exact footprint and mark out the base area with pegs and string lines, allowing a slight overhang around each edge, typically 50mm to 100mm, so the finished base fully supports the shed once it’s in place. Skipping this overhang is a mistake that leaves shed edges unsupported, which can lead to problems over time. Step 2: Excavate and Level the Ground Dig out the marked area to an appropriate depth, accounting for both your sub base layer and the slabs themselves. Take real care levelling the excavated area at this stage, since any unevenness here carries through to every layer built on top of it. On sloped or uneven gardens, this stage can involve considerably more groundwork than expected, which is worth reading up on in our guide to retaining wall installation if your intended shed spot has any meaningful level change to manage first. Step 3: Lay a Compacted Sand and Hardcore Sub-Base Add a sub base layer of compacted sand and hardcore across the excavated area. The exact depth genuinely varies depending on your ground conditions and the size of your shed, since firmer, well drained ground generally needs less sub base than softer or wetter soil, and larger, heavier sheds benefit from a more substantial sub base than small, lightweight ones. Compact this layer thoroughly before moving on, since a poorly compacted sub base is one of the most common causes of a shed base settling unevenly over time. Step 4: Lay the Paving Slabs Using standard 600mm by 600mm slabs, begin laying them across the compacted sub base, working methodically from one corner. Check each slab is level both individually and relative to the slabs around it as you go, using a spirit level and a rubber mallet to gently tap slabs into place where needed. Consistent checking throughout, rather than only at the end, is what prevents small levelling errors from adding up across the whole base. Step 5: Fill the Joints Between Slabs Once all slabs are laid and level, fill the joints between them with a suitable jointing compound or fine sand, brushing it into the gaps for a clean, finished surface. This step also helps stabilise the slabs in position and reduces the risk of movement over time. Choosing the Right Sub-Base Depth for Your Conditions There’s no single fixed depth that applies to every project here. In our experience, the correct sub base depth genuinely depends on two things: your existing ground conditions, and the size and weight of the shed going on top. Factor Impact on Sub-Base Depth Firm, well drained ground Generally needs a shallower sub base Soft or wetter ground Typically needs a deeper, more substantial sub base Small, lightweight shed Standard sub base depth usually sufficient Larger, heavier shed Benefits from a deeper, more robust sub base If you’re unsure about your specific ground conditions, it’s worth getting a professional assessment before committing to a sub base depth, since getting this wrong is far more difficult to correct once the slabs are already laid on top. How to Assess Your Own Ground Conditions Before Starting If you’re planning to tackle this yourself, it’s worth doing a basic assessment of your ground before finalising your sub base plan. A simple way to get a feel for drainage is to dig a small test hole roughly 300mm deep in the intended area, fill it with water, and see how long it takes to drain away. Ground that clears within an hour or two is generally well drained, while water that lingers for several hours or longer suggests heavier, slower draining soil that will need a more generous sub base to perform reliably. It’s also worth pressing a garden fork into the soil at a few points across the site. Ground that resists penetration and feels firm underfoot is typically more stable than soil that feels soft, springy, or gives way easily, which is a useful, low tech indicator of how much sub base support the area is likely to need. Standard Slab Sizes and Alternatives While 600mm by 600mm is the size we most commonly use for shed bases, it’s worth knowing a couple of alternatives exist depending on the project. Smaller 450mm by 450mm slabs are sometimes used for compact sheds or where a finer, more detailed finish is preferred, though they

Building a shed base is only part of the picture. Before a single slab is laid, the location you choose for your shed, and how well you’ve planned around it, has a huge impact on how well that shed performs for years to come. Get the site planning wrong, and even a perfectly built base won’t stop problems like damp, drainage issues, or access headaches down the line. At JPD Landscaping & Building Services, we’ve planned and prepared garden shed sites across South East London for over 15 years. This guide goes beyond the base itself to cover exactly what to check before you commit to a spot, including sun and shade position, boundary considerations, and access planning, along with a real example of how getting this right avoids common problems later. What to Plan Before Building a Garden Shed Base Before laying your shed base, the two most important site factors to get right are the sun and shade position of the spot you choose, and its distance from boundaries and fences. Beyond these, access for moving materials in and overall ground levelling across the wider garden, not just the base area itself, also play a significant role in how smoothly the project goes. Site Factor Why It Matters Sun or shade position Affects long term shed condition and damp risk Distance from boundaries Relevant for planning rules and drainage runoff Access for materials Determines how easily the shed can actually be built or delivered Ground slope across garden Affects overall drainage, not just the base spot Why Sun and Shade Position Matters Most Of all the site factors we assess before starting a garden shed project, sun and shade position consistently has the biggest long term impact on how well the shed holds up. A shed placed in a permanently shaded, damp corner of the garden, particularly a north facing spot that rarely sees direct sun, is far more prone to lingering moisture, algae growth, and slower drying after rain than a shed positioned somewhere that gets reasonable sun exposure throughout the day. This matters because that constant dampness accelerates wear on the shed’s timber over time, regardless of how well the base itself is built. A well constructed concrete base underneath a shed sitting in a permanently damp, shaded spot still won’t prevent the shed structure above it from suffering long term moisture damage. This is the same underlying principle that affects wood decking positioned in shaded areas of a garden, where lingering damp accelerates wear on timber regardless of how solid the structure underneath happens to be. How to Actually Assess Sun Exposure in Your Garden Rather than guessing at sun exposure from memory, it’s worth observing your garden across a full day, or ideally a few different days, before committing to a shed location. Note roughly how many hours of direct sun each candidate spot receives, and pay particular attention to how quickly that area dries out after rainfall compared with the rest of the garden. Overhanging trees, tall fencing, and neighbouring structures can all cast shade patterns that shift throughout the day and across the seasons, so a spot that looks perfectly sunny on a summer afternoon might sit in near constant shade during the shorter days of winter. Where possible, it’s worth checking your intended spot at a few different times of day and, if you have the patience, across more than one season before finalising the location. Distance From Boundaries and Fences The second major factor to plan for is how close your shed sits to garden boundaries and fences. This matters for two separate reasons that often get overlooked until they become a problem. Leaving reasonable clearance from boundaries isn’t just a courtesy to neighbours, it’s a practical decision that avoids drainage and access problems that are much harder to fix once the shed and base are already in place. Understanding Boundary and Fence Responsibility Before You Build Before finalising a shed position near a boundary, it’s also worth understanding exactly which fence or boundary line is yours to build near or attach anything to. Boundary responsibility isn’t always obvious just by looking at a garden, and getting this wrong can create unnecessary friction with neighbours further down the line, particularly if drainage or access to the shed ever needs to cross into their side of the boundary for maintenance. If you’re at all unsure which boundary is yours, or whether existing fencing legally belongs to you or a neighbour, it’s worth clarifying this before committing to a shed position tight against any particular boundary line, rather than assuming and potentially needing to address a dispute after the shed is already built. Planning Access Before You Commit to a Spot It’s easy to focus entirely on the ideal spot within the garden and forget to check whether materials, or the shed itself, can actually get there. Narrow side gates, tight gaps between buildings, or steps and changes in level between the front and back garden can all significantly limit where a shed practically can go, regardless of how ideal a spot might otherwise be. Before finalising your shed’s location, it’s worth walking the full route from your property’s access point to the proposed site, checking gate widths, gaps, and any obstacles that could make it difficult to get shed panels, base materials, or a concrete mixer through to the build area. What Counts as Good Access, Practically Speaking It helps to think through access in terms of the specific materials and equipment that will actually need to travel through your garden. Flat pack or panel built sheds generally arrive as several large, flat sections, so gate widths and any tight turns along the route need to comfortably accommodate the largest single panel, not just the shed’s overall footprint once assembled. For a concrete base, consider whether a concrete mixer or wheelbarrow route is realistic, or whether ready mixed concrete would need to be pumped or carried a significant distance, since

A hot tub can be a brilliant addition to a garden deck, but it’s not something to place on your existing decking without giving it proper thought first. Unlike garden furniture or a fire pit, a filled hot tub with people in it represents a significant, concentrated weight that most standard decks simply weren’t designed to carry. The honest answer to whether it’s possible really does depend on the specific deck in question. At JPD Landscaping & Building Services, we’ve assessed and reinforced decking for hot tub installations across South East London for over 15 years. This guide explains what actually determines whether your deck can support a hot tub, why the answer is always case by case, and a real example of how we’ve reinforced a deck to safely handle the added weight. Can You Put a Hot Tub on a Wood Deck? It depends entirely on the specific deck’s condition and design. Whether a wood deck can safely support a hot tub comes down to the strength and spacing of the existing frame, the condition of the substructure, and the exact weight of the hot tub once filled with water and occupied. There’s no universal yes or no answer that applies to every deck. Factor Why It Matters Existing frame strength Determines how much added weight the deck can safely handle Substructure condition Rot or damage significantly reduces load bearing capacity Hot tub weight (filled and occupied) Varies hugely by model, always check manufacturer specs Reinforcement needed Case by case, depends on the gap between current and required capacity Why Hot Tub Weight Varies So Much One of the most important things to understand before assessing your deck is that hot tub weight isn’t a fixed figure. It varies hugely depending on the model, size, and number of people using it at once, so it’s essential to check the manufacturer’s specifications for your specific hot tub rather than relying on a general estimate. A filled hot tub includes the weight of the shell itself, the full volume of water inside it, and the weight of anyone using it at the same time. This combined weight is concentrated over a relatively small footprint compared with how weight is normally distributed across a deck, which is exactly why standard residential decking often isn’t built to handle it without some form of reinforcement. Roughly How Much Weight Are We Talking About? While every hot tub is different, it helps to have a rough sense of scale before getting into the specifics of your own model. A compact two to four person hot tub, once filled with water, can weigh somewhere in the region of 1,000 to 1,800kg depending on size, before anyone even steps in. Larger six to eight person hot tubs, filled and occupied, can climb well beyond 2,500 to 3,500kg in total. To put that in perspective, this concentrated weight sitting over a footprint of just a few square metres is a fundamentally different loading scenario to garden furniture, planters, or even a group of people standing on a deck spread across its full area. This is exactly why hot tub weight needs to be checked against your specific deck’s design capacity rather than assumed to be broadly similar to everyday deck use. Why This Needs a Proper Site Assessment Because the answer depends so heavily on your specific deck’s existing structure, and your specific hot tub’s weight when filled and occupied, this isn’t something that can be answered accurately with a generic rule of thumb. A proper site visit from an experienced deck builder is the right way to assess whether your existing deck can support a hot tub, or what reinforcement would be needed to make it safe. During an assessment, we look at the current joist spacing and size, the condition of the substructure, how the deck’s load is currently distributed, and cross reference all of this against the specific weight requirements of the hot tub you’re planning to install. This case by case approach is the only reliable way to give an accurate answer, since two decks that look similar on the surface can have very different load bearing capacities depending on how they were originally built. What Determines Whether Reinforcement Is Needed Ground-Level Decks vs Raised Decks: Does It Make a Difference? The height and foundation style of your existing deck plays a meaningful role in how straightforward reinforcement is likely to be. Ground level decks, sitting on deck blocks or paving slabs, sometimes offer less scope for adding substantial extra support directly beneath the hot tub area, since there’s limited room to work with and the existing foundation type may not be designed for significantly increased point loads. Raised decks built on proper concrete footings and posts often have more flexibility for reinforcement, since additional posts and footings can be added beneath the specific hot tub area without needing to rebuild the entire structure. That said, height also introduces its own considerations, since a raised deck puts more distance between the hot tub and solid ground, which can affect both structural engineering requirements and practical matters like plumbing and electrical connections. What Determines Whether Reinforcement Is Needed How Deck Reinforcement for a Hot Tub Typically Works Reinforcement isn’t a one size fits all fix. It’s genuinely case by case, depending on the gap between what your current structure can handle and what your specific hot tub requires. That said, a few approaches come up regularly across different projects. Reinforcement Approach When It’s Typically Used Adding extra support posts underneath When existing foundations can take additional load points Tightening joist spacing in the hot tub area When the frame is fundamentally sound but needs more support Building a dedicated reinforced section When the hot tub area needs significantly more capacity than the rest of the deck Full structural rebuild of that section When existing substructure has rot or damage that rules out simple reinforcement Should You Build a Dedicated Base Instead of Reinforcing