To lay a patio properly, prepare a 100mm compacted hardcore sub base topped with a 75mm sand and mortar bedding layer, then bed your slabs using the five spot method, checking each one is level and building in a 1:40 fall, 25mm per metre, sloping away from the house for proper drainage.

What You’ll Need to Lay a Patio

• Patio slabs of your chosen material
• Hardcore or sub base material for the foundation layer
• Sharp sand and cement for the bedding mortar mix
• A spirit level and a long straightedge
• A rubber mallet for bedding slabs into position
• String lines and pegs to mark levels and falls accurately
• A jointing compound or mortar for finishing gaps between slabs

Choosing Between Your Material Before You Start

Before laying a single slab, it’s worth being settled on your material choice, since this affects handling technique, cutting requirements, and even bedding considerations slightly. Our guides comparing Indian sandstone and porcelain and covering the best slabs for a London garden are worth reading first if you haven’t already settled on a specific material, since switching partway through a project is far more disruptive than deciding upfront.

Step-by-Step: How to Lay a Patio

Step 1: Mark Out and Excavate the Area

Measure and mark your patio area with string lines and pegs, then excavate to a depth that accommodates your sub base, bedding layer, and slab thickness combined. Take care to plan your excavation depth accurately at this stage, since it determines whether your finished patio sits at the correct height relative to your house’s damp proof course and surrounding ground.

Step 2: Lay and Compact the Hardcore Sub-Base

Add a 100mm layer of hardcore, spreading it evenly across the excavated area before compacting it thoroughly. This sub base provides the stable foundation everything else sits on, and a poorly compacted sub base is one of the most common underlying causes of a patio settling unevenly over time.

Step 3: Establish Your Drainage Fall

Before laying any slabs, set up string lines to establish a consistent 1:40 fall, roughly 25mm per metre, sloping away from the house. This gentle slope ensures rainwater drains away from your property rather than pooling against the walls or, worse, finding its way toward the damp proof course.

Step 4: Add the Bedding Layer

Spread a 75mm layer of sand and mortar mix across the compacted hardcore, working in manageable sections rather than across the entire patio area at once, since you’ll be laying slabs directly into this layer while it’s still workable.

Step 5: Lay Slabs Using the Five-Spot Method

Apply mortar in five dabs, one at each corner and one in the centre, beneath each slab before placing it into position. This is where getting the mortar quantity right genuinely matters, since skimping here is the single most common mistake we see on DIY patios. Use a rubber mallet to gently bed each slab down to the correct level, checking with a spirit level and your established fall line as you go.

Step 6: Check Levels and Fall Continuously

Rather than checking only at the end, verify each slab’s level and its consistency with your overall drainage fall as you progress across the patio. Catching a slightly high or low slab early, while the mortar is still workable, is far easier than trying to correct it once everything has set.

Step 7: Fill the Joints Between Slabs

Once all slabs are laid and the bedding mortar has had time to firm up, fill the joints between slabs using a suitable jointing compound or mortar mix, brushing it into the gaps for a clean, finished appearance.

Why 1:40 Is the Right Fall, Not Steeper or Shallower

It’s worth understanding why 1:40 specifically has become the standard recommendation, rather than a rougher rule of thumb. A fall shallower than this often isn’t sufficient to reliably move water off the surface, particularly on a patio with any minor surface irregularities, leading to shallow puddles forming in low spots even when the overall slope seems adequate on paper. A fall steeper than roughly 1:40 starts to become noticeably perceptible underfoot, which most homeowners find undesirable on a surface meant to feel level and comfortable for outdoor furniture, dining, and general use.

1:40 sits at a sweet spot, providing enough gradient to reliably move water away from the house without being noticeable enough to affect how level the patio feels in everyday use, which is exactly why it’s become the widely adopted standard across the industry rather than a figure specific to any one contractor’s preference.

How to Set Out Accurate Fall Lines Before Laying Begins

Getting the fall right in practice, rather than just in theory, comes down to accurate string line setup before any slabs go down. Starting from a fixed reference point, typically the finished level at the house wall, string lines are run out across the patio area at the calculated drop for the 1:40 fall over the relevant distance, giving a clear, visual guide for checking slab height as you progress.

It’s worth double checking this fall calculation across the full patio width, not just along a single line, particularly on larger or irregularly shaped patios, since a fall that’s correct along one edge but inconsistent across the width can still result in water pooling in unexpected spots despite an apparently correct overall slope.

Getting Your Mortar Mix Right for Bedding

Beyond quantity, the mix ratio for your bedding mortar affects how well it performs once slabs are bedded into it. A standard bedding mortar mix typically uses a ratio of roughly four or five parts sharp sand to one part cement, mixed to a relatively firm, workable consistency rather than a sloppy, wet mix that won’t hold a slab’s weight and shape properly once dabbed into position.

Getting the water content right matters here in a similar way to the concrete mixing principles covered in our guide on how to lay a concrete base, since a mix that’s too wet won’t provide firm enough support beneath the slab, while one that’s too dry becomes difficult to work with and may not bond properly against both the slab and the sub base beneath.

The Most Common Mistake: Too Little Mortar in the Five-Spot Method

Of all the issues we see on DIY patio projects, using the five spot method with too little mortar under each slab causes the most consistent problems. It’s an easy mistake to make, since a slab can look and feel secure immediately after laying, even with insufficient mortar beneath it.

The problem shows up over time. With hollow spots beneath the slab where mortar wasn’t properly applied, the slab lacks full support across its base. Under regular foot traffic and weather exposure, this leads to rocking, cracking, or the slab gradually working itself loose from its original position.

• Correct approach: Generous, properly sized mortar dabs at each of the five points, providing genuine support across the slab’s base once bedded down.
• Risky approach: Thin, insufficient mortar dabs that leave hollow spots beneath the slab, appearing fine initially but leading to rocking and instability over time.

Full Bed vs Five-Spot: Is One Better Than the Other?

While the five spot method is our standard recommendation and widely used across the industry, it’s worth knowing a full mortar bed, where the entire underside of the slab sits on a continuous layer of mortar rather than five separate dabs, is an alternative approach used in some situations. A full bed can offer slightly more even support across very large or particularly heavy slabs, though it requires more material and more careful, faster working to bed the slab correctly before the mortar begins to firm up across such a large contact area.

For most standard sized patio slabs, a correctly executed five spot method, with genuinely generous mortar dabs rather than the skimped version that causes problems, provides entirely adequate and reliable support, which is why it remains the more commonly used technique for typical residential patio projects.

Cutting Slabs for Edges and Awkward Shapes

Very few patios consist entirely of full, uncut slabs, since most garden shapes require some cutting to fit edges, curves, or awkward corners neatly. A angle grinder fitted with a diamond blade is the standard tool for cutting most paving materials, including natural stone, concrete, and porcelain, though porcelain in particular benefits from a wet cutting method to reduce dust and minimise the risk of chipping along the cut edge.

It’s worth measuring and marking cuts carefully before committing, accounting for the joint gap on either side of the cut piece, and where possible, planning your layout so cut pieces fall in less visually prominent areof the patio, such as against a boundary fence, rather than in a highly visible central position.

Other Mistakes to Avoid When Laying a Patio

• Not building in a proper fall for drainage. Without a consistent slope away from the house, water pools on the patio surface rather than draining away, which can lead to standing water and, in more serious cases, water tracking back toward the property.
• Not compacting the hardcore sub-base enough before laying. A loosely compacted sub base undermines the whole patio structure, regardless of how carefully the slabs themselves are laid on top of it.
• Mixing bedding mortar too wet or too dry. An overly wet mix won’t hold a slab’s position and level properly, while a mix that’s too dry is difficult to work with and may not bond adequately.
• Rushing joint filling before mortar has properly firmed up. Filling joints too early, before the bedding mortar underneath has set sufficiently, risks disturbing slab position and undoing careful levelling work.
• Ignoring the finished level relative to the damp-proof course. Building a patio surface too close to, or above, the property’s damp proof course can allow moisture to bridge into the building’s walls, causing damp issues inside the property.

Case Study: Fixing Rocking Slabs From an Under-Mortared DIY Patio

We were called out to a garden in South East London where a customer’s DIY patio, laid the previous year, had developed several noticeably rocking slabs across one section.

What we found:

The customer had followed the five spot method as intended, but had applied noticeably too little mortar at several of the dab points, leaving hollow spots beneath several slabs. Over a year of regular foot traffic, this had caused those specific slabs to rock and shift slightly out of position.

What we did: We lifted the affected slabs, cleaned away the old insufficient mortar, and re bedded them using properly sized five spot mortar dabs, ensuring genuine, solid support across each slab’s full base this time.

Result: The re-laid section has remained stable and level since, with no return of the rocking that affected the original installation. This case is a clear reminder that the five spot method only works properly when the mortar dabs themselves are generously and correctly applied.

Maintaining Your Patio Once It’s Laid

Once your patio is properly laid and jointed, ongoing maintenance is generally minimal, though a few habits help it stay looking good and performing well for years. Periodically checking joints for any signs of erosion or plant growth, and topping up jointing compound where needed, helps prevent water finding its way beneath the slabs through gaps that have opened up over time. Keeping drainage channels or gaps at the patio’s edges clear of leaves and debris ensures the fall you built in continues to function as intended, rather than becoming blocked and causing water to back up against the house wall despite a correctly laid slope.

Frequently Asked Questions

What’s the correct sub-base depth for a patio?

We typically use a 100mm compacted hardcore sub base, topped with a 75mm sand and mortar bedding layer for the slabs themselves.

What is the five-spot method for laying patio slabs?

The five spot method involves applying mortar in five dabs, one at each corner and one in the centre, beneath each slab before bedding it into position.

How much fall should a patio have for drainage?

We typically recommend a 1:40 fall, roughly 25mm per metre, sloping gently away from the house to ensure proper drainage.

Why are my patio slabs rocking?

Rocking slabs are usually caused by insufficient mortar being used in the five spot bedding method, leaving hollow spots beneath the slab that become unstable over time.

Do I need to compact the ground before laying a patio?

Yes, a properly compacted hardcore sub base is essential, since a loosely compacted base undermines the whole patio structure regardless of how well the slabs are laid on top.

Should I use the five-spot method or a full mortar bed?

The five spot method works well for most standard sized patio slabs when properly executed with generous mortar dabs. A full bed is sometimes used for very large or heavy slabs but requires faster, more careful working.

What tool should I use to cut patio slabs for edges?

An angle grinder with a diamond blade is standard for most paving materials, with a wet cutting method recommended for porcelain specifically to reduce dust and minimise chipping.

Get a Free Quote for Your Patio Project

If you’d rather have your patio laid properly by experienced professionals, JPD Landscaping & Building Services offers free quotes across South East London.

Call now to book your free quote and consultation.