How to Lay Paving Slabs

The difference between a patio that lasts 20 years and one that cracks within two comes down to what happens beneath the slabs. The laying itself takes a day. The preparation — excavation, sub-base compaction, bedding mix, drainage gradient — takes two to three days and determines whether your patio stays level or starts rocking by the first winter. This guide walks you through the full process, from marking out to jointing, with the material quantities and methods that produce a solid, long-lasting result.

Before you start, use the paving slab calculator to work out slab, sub-base, and sand quantities for your patio size. Ordering the right amount up front saves wasted trips and avoids running short mid-project.

Tools and Materials You Need

Gather everything before you start digging. A missing tool mid-project means a trip to the hardware store with half-dug foundations open to rain. Here is what you need on site.

Tools required for a standard patio lay:

• Spade and flat-edged shovel
• Garden rake
• Spirit level (1200mm minimum)
• Rubber mallet
• String line and timber pegs
• Tape measure (5m minimum)
• Plate compactor (hire) or hand tamper
• Pointing trowel
• Angle grinder with diamond blade (for cutting slabs)
• Wheelbarrow
• Bucket for mixing mortar

A plate compactor is the single most important hire item. You can tamp a small path by hand, but anything over 5m² needs mechanical compaction to achieve a stable sub-base. Hire cost is typically £30–£50 per day from tool hire centres.

Materials required for the patio structure:

• Paving slabs (natural stone, concrete, or porcelain — your choice)
• Sub-base aggregate: Type 1 MOT (crushed limestone, recycled aggregate, or granite)
• Sharp sand (for bedding mix)
• Cement (OPC — ordinary Portland cement)
• Jointing compound (sand:cement mix, brush-in compound, or resin-based filler)
• Timber edging or existing wall/fence for containment

For a typical 3m × 4m patio with 450 × 450mm slabs, you are looking at roughly 60 slabs, 1.5 tonnes of sub-base, 0.5 tonnes of sharp sand, and 4–5 bags of cement. The paving slab calculator gives you exact figures based on your dimensions and slab size.

Planning Your Patio Layout

Good planning prevents expensive mistakes. Spend an hour marking out and checking before you pick up a spade.

Marking Out

Drive timber pegs into the ground at each corner of the patio and run string lines between them. Check the diagonals — if both diagonal measurements are equal, the rectangle is square. A difference of more than 10mm means one corner is off. Adjust the pegs until the diagonals match.

Add 100mm to each edge beyond the patio footprint. This gives you working room for the sub-base and allows you to set edge slabs cleanly without the excavation wall collapsing into your trench.

Checking for Underground Services

Before digging, check for buried utilities. Water pipes, gas lines, electric cables, and drainage runs can all sit within 300mm of the surface. Contact your utility providers for a plan, or hire a cable avoidance tool (CAT scanner) for the day. Hitting a gas pipe with a spade is not a problem you want to solve mid-project.

Setting the Drainage Fall

Every patio must slope away from the house to prevent rainwater pooling against the wall. The standard gradient is 1:60 — roughly 15mm per metre of run. For a 3m-deep patio, that means the far edge sits 45mm lower than the edge against the house wall. This gradient follows BS 7533 guidance and has been checked by our trade reviewer.

Set the gradient on your string lines now, before excavating. Measure down from a level string line by the correct amount at the far end and tie off a second, sloped string line. Every layer you build — sub-base, bedding, slabs — follows this gradient. Getting it right at the string-line stage means you do not have to re-lay anything later.

Excavating and Preparing the Ground

You need to dig deep enough for the full patio build-up: slab thickness + bedding layer + sub-base. For a standard installation, the total depth is typically 200mm.

Here is a typical build-up from bottom to top:

• Sub-base (Type 1 MOT): 100–150mm compacted
• Bedding layer (sand:cement): 30–50mm
• Slab thickness: 30–50mm (varies by product)

Check your specific slab thickness before excavating — porcelain slabs are often 20mm, while natural stone flags can be 40–50mm with variable thickness across the slab. The finished patio surface should sit 150mm below the damp-proof course (DPC) of the house.

Excavate the full area to the calculated depth, removing all topsoil and organic matter. Topsoil contains roots and organic material that decompose over time, creating voids beneath the patio. Every scrap of dark, organic soil must come out. If you are left with a pile of topsoil, you can calculate backfill topsoil for edging beds or redistribute it to other areas of the garden.

Once excavated, compact the exposed subsoil with the plate compactor. Run it over the surface in overlapping passes until the ground feels firm and does not give underfoot. On clay soils, this step is critical — soft spots in the subsoil translate directly into rocking slabs later.

Laying the Sub-Base

The sub-base is the structural foundation of your patio. Type 1 MOT (Ministry of Transport specification) aggregate is the standard material: a graded mix of crushed stone from 40mm down to dust that locks together when compacted.

Tip the aggregate into the excavation and rake it roughly level. For best results, build the sub-base in layers:

1. Spread a 50mm layer across the entire area.
2. Compact with the plate compactor — run over each section 3–4 times in overlapping passes.
3. Spread a second 50mm layer and compact again.
4. For a 150mm sub-base, add and compact a third layer.

Check levels constantly with a spirit level and straight edge. The sub-base surface must follow your drainage gradient — if the sub-base is level, the finished patio will be level too, and water will pool instead of draining. Rest the spirit level on the string lines to verify the fall at multiple points across the area.

The compacted sub-base should feel like a solid floor when you walk on it. If your boots leave impressions deeper than 5mm, compact again. Under-compacted sub-base is the number one cause of patio failure — the aggregate continues to settle under the weight of the slabs and foot traffic, creating dips and rocking spots.

Preparing the Bedding Layer

The bedding layer sits between the sub-base and the slabs. It serves two purposes: it provides a smooth, even surface for the slabs to sit on, and the cement content bonds the slabs in place once it cures.

Mix sharp sand and cement at a 5:1 ratio (five parts sand to one part cement) in a wheelbarrow or on a mixing board. The mix should be damp but not wet — it should hold its shape when squeezed but not drip water. If you are using a full mortar bed (4:1 sand:cement with water), mix to a butter-like consistency.

Spread the bedding mix over a working area of 1–2 square metres at a time. Do not prepare the entire area at once — the mix starts to set within 2–3 hours, and you will not be able to adjust slab levels once it hardens. Screed the surface to an even 30–50mm depth using two parallel timber battens set to the correct height and a straight edge drawn across them.

Maintain the drainage gradient through the bedding layer. The finished surface of the bedding should be exactly one slab-thickness below your final patio level, following the same 1:60 slope you set on the string lines.

Laying the Slabs

This is the visible part of the project — and it goes smoothly if the preparation underneath is right.

Starting Point

Always start from a fixed, straight edge. The house wall is the best starting point because it gives you a true line to work from. Lay the first row of slabs tight against the wall (or with a 10mm gap if using a flexible sealant joint), then work outward row by row towards the garden.

Placing Each Slab

There are two bedding methods for individual slabs:

• Full bed: Spread the bedding mix evenly across the slab area. Best for porcelain and thin slabs that need full support.
• Spot bedding: Place five blobs of mortar — one at each corner and one in the centre. Works for thicker natural stone and concrete slabs (35mm+).

Lower the slab onto the bedding. Do not slide it — this disturbs the mortar and creates an uneven base. Place it flat and tap it down with a rubber mallet until it reaches the correct level. Check with a spirit level across the slab and across adjacent slabs. Each slab must follow the drainage gradient and sit flush with its neighbours.

Spacing and Joints

Leave 10–15mm joints between slabs. Use slab spacers, timber offcuts, or purpose-made plastic spacers to keep the gap consistent. Tight joints (under 5mm) leave no room for jointing compound and can cause slabs to crack if they expand in hot weather.

Cutting Slabs

You will almost certainly need to cut slabs to fit edges and corners. Mark the cut line with a pencil and straight edge. Score the line with the angle grinder and diamond blade, then cut through in a single steady pass. Wear safety goggles, ear defenders, and a dust mask — slab cutting produces fine silica dust that is harmful to inhale. Cut outdoors and position yourself upwind of the dust.

Jointing and Finishing

Jointing fills the gaps between slabs, locks them in place, and prevents weed growth. Wait at least 24 hours after laying before jointing — the bedding mortar needs time to firm up so you do not dislodge slabs while working on the joints.

You have three main jointing options:

• Sand:cement mix (4:1): The traditional method. Mix dry, brush into joints, then mist with a watering can to activate the cement. Cheap and durable, but can stain light-coloured slabs if not cleaned off promptly.
• Brush-in jointing compound: Pre-mixed, polymer-modified compound that you sweep into joints dry and activate with water. Easier than sand:cement and less likely to stain. Costs £15–£25 per tub (covers 5–10m²).
• Resin-based joint filler: Two-part epoxy or polyurethane filler that sets hard and resists weed growth. The most expensive option (£30–£50 per 10m²) but the most durable. Best for porcelain slabs where a clean, modern finish matters.

Whichever method you choose, keep the slab surfaces clean during the process. Mortar and cement residue left on slab faces for more than 30 minutes can leave permanent stains, especially on natural stone. Wipe excess off immediately with a damp sponge.

How Long to Wait Before Using the Patio

Curing times depend on the weather and the materials used.

Here are the recommended waiting periods:

• Foot traffic: 24–48 hours (dry, warm weather) or 72 hours (cold, damp weather below 10°C)
• Garden furniture: 48–72 hours
• Heavy items (planters, BBQ): 7 days
• Vehicle traffic (if applicable): 14 days minimum

In winter, extend all timescales by 50%. Cement cures more slowly below 5°C and can be damaged by frost before it reaches full strength. If frost is forecast within 48 hours of laying, cover the patio with tarpaulin or fleece to insulate the mortar joints.

Common Mistakes

Most patio failures trace back to one of four errors. All are avoidable with proper preparation.

Rocking Slabs from Insufficient Bedding

If a slab rocks when you step on the corner, the bedding underneath has a void. This happens with spot bedding when the mortar blobs are too small or unevenly placed. The fix during laying is to lift the slab, add more mortar, and re-lay. After the mortar sets, the only fix is to lift and re-bed — a tedious job that takes longer than doing it right the first time.

Ignoring the Drainage Gradient

A level patio looks right but functions wrong. Without the 1:60 fall, rainwater sits on the surface in shallow puddles. In winter, these puddles freeze, creating slip hazards and accelerating frost damage to the slab faces. The gradient is invisible to the eye at 15mm per metre — your guests will not notice the slope, but the water will.

Not Compacting the Sub-Base

Hand-tamping feels adequate when you are doing it, but it does not match the force of a plate compactor. Under-compacted aggregate continues to settle for months after laying, especially after heavy rain saturates the sub-base and flushes fine particles deeper. Hire the plate compactor — the £40 hire fee is cheaper than re-laying a sunken patio.

Laying in Rain

Wet bedding mix does not bond properly. Rain dilutes the cement content, weakens the mortar, and washes sand out of joints before they set. If rain starts mid-project, cover the exposed bedding and any freshly laid slabs with tarpaulin. Do not continue laying until the surface is dry enough to work. A patio is a weekend project — waiting one more day for dry weather is always worth it.

Related Projects

A new patio often leads to other garden improvements. If you are creating a border around your patio for planting, you can figure out mulch volumes for adjacent borders to keep beds tidy. Raised beds beside a patio make a natural transition from hard landscaping to planting — see the guide to filling raised beds next to your new patio for soil mix ratios and volumes. If part of your garden is staying as lawn, the artificial grass calculator helps you estimate materials for a grass border around your patio with minimal ongoing maintenance. And if the patio project has given you confidence for indoor work, planning a tiled floor inside the house uses the same measure-and-calculate approach.

Getting Started

The hardest part of laying a patio is the preparation beneath the slabs — and that is also the part that matters most. Invest your time in excavation, compaction, and getting the drainage gradient right. The slab-laying itself is straightforward once the foundation is solid. Take the time to compact each sub-base layer properly, check your levels constantly, and do not rush the bedding mix. A well-prepared patio gives you a flat, stable outdoor surface that lasts decades with zero maintenance beyond the occasional pressure wash.