A patio looks simple from the surface: flat slabs, neat joints, a place for a table. The reason some patios stay flat for decades and others sink, rock and grow weeds within two summers lies entirely below the slabs. The sub-base carries the load, the bedding layer lets you set the slab height, and the falls keep water away from the house. This guide covers the sequence for a typical domestic patio, with quantities for a worked example and the planning rules worth checking first.
Planning and regulations
For a back garden patio in England, planning permission is rarely an issue. The Planning Portal says there are no restrictions on the area of land you can cover with hard surfaces at, or near, ground level, though significant works of embanking or terracing to support a hard surface might need a planning application. Raised platforms are different: under the outbuilding rules, a platform must not exceed 0.3 metres in height, so a patio raised well above the garden on a sloping site can fall outside the permission-free allowances. Check with your local planning authority if your patio will be notably higher than the surrounding ground. Listed buildings, conservation areas and properties where permitted development rights have been removed also carry extra restrictions.
Front gardens have their own rule. You do not need planning permission for a new or replacement driveway of any size if it uses a porous material such as gravel or permeable block paving, or if rainwater is directed to a lawn or border to drain naturally. If you pave more than 5 square metres of front garden with an impermeable surface that does not drain to a permeable area, planning permission is needed. This guide concentrates on the rear garden, but the drainage principle applies everywhere: do not send surface water to the highway or your neighbour.
Building regulations are usually not triggered by a simple patio, but the level of the paving next to the house matters, and Part C of the Approved Documents relates to moisture. As a standard recommendation, keep the finished patio surface at least 150 millimetres below the damp-proof course of the house wall, and make sure it falls away from the building. Rules in Scotland, Wales and Northern Ireland differ in detail.
Plan the layout and levels first
- Pick the size. Tables and chairs need more space than people expect. A comfortable dining area for four to six usually wants about 3 x 3 metres or more, plus space to pull chairs out.
- Set out with pegs and string. Mark the outline and check the diagonals so the shape is square.
- Work to slab size. A patio built to a whole number of slabs avoids cutting. For 600 x 600 mm slabs with 10 mm joints, each slab takes a 610 mm module.
- Decide the falls. Water must run off. A fall of about 1 in 80 (12.5 millimetres per metre) away from the house is a commonly used target; some installers use 1 in 60. The fall should direct water to a lawn, border or drain, not towards walls, doors or the neighbour.
- Set the finished height. Work backwards from the house wall, with the patio at least 150 mm below the DPC, and check how it meets any door thresholds.
How deep to dig
Add up the layers from the top down. For a typical domestic patio with 38 mm-thick concrete or natural stone slabs, the layers are:
| Layer | Typical thickness |
|---|---|
| Paving slab | 20 to 40 mm, depending on type |
| Bedding layer (sharp sand or mortar) | 30 to 40 mm |
| Sub-base (compacted MOT Type 1 or similar) | 100 mm for foot traffic |
| Total excavation below final level | Around 150 to 200 mm |
A driveway or any surface that vehicles will cross needs a thicker, stronger sub-base, a thicker slab or block, and a different construction altogether, so do not copy these figures for a parking area. Dig out the topsoil and all organic material to firm subsoil, and keep an eye out for services. If you find soft or wet spots, dig deeper and fill them with compacted sub-base. Trees and shrubs close by can lift paving with roots, and clay soil moves seasonally, so take advice if either applies.
Excavate and install the sub-base
- Excavate to the depth needed, keeping the base of the excavation parallel to the finished surface, including the fall. This ensures the layers stay a uniform thickness.
- Compact the exposed subsoil with a plate compactor (a hire item) or a hand rammer for small areas.
- Consider laying a geotextile membrane over soft or clay soils. It separates the sub-base from the soil and helps drainage, though it is not essential on firm, free-draining ground.
- Tip in sub-base in layers of no more than about 50 to 75 millimetres. Rake level, then compact each layer thoroughly. A well-compacted layer rings rather than crumbles under the compactor, and has no loose or shifting stones on the surface.
- Check the surface with a long straightedge and a level, adjusting for the fall. Fix this layer correctly now, because you cannot easily rectify it later.
Edge restraint matters. Slabs can creep outwards under foot traffic if nothing holds them in. Next to lawn or planting, a haunched concrete edge or a run of kerbs bedded in mortar keeps the perimeter firm. Against a house wall or a hard surface, the existing structure does the work.
Bedding layer: sand or mortar?
Many DIY patios use a sharp sand and cement bedding mix, or a mortar, with each slab laid on five blobs of mortar (the "five spot" method) or a full bed. Opinion varies, but the most reliable approach for any slab, particularly porcelain, is a full mortar bed, in which the whole underside of the slab is supported. Spot-bedding leaves voids beneath the slab where it can crack under a point load, such as a table leg.
- Sharp sand and cement. A typical mix is around 5 or 6 parts sharp sand to 1 part cement by volume, mixed to a stiff consistency. Follow the guidance from the slab manufacturer.
- Porcelain. Many manufacturers of porcelain paving require a full mortar bed plus a slurry primer or bonding coat on the underside of the slab. Check the instructions, since the warranty may depend on it.
- Loose sand-only bedding. Suitable for small block paving and some flexible laying, but it provides less support under larger slabs.
Laying the slabs
- Start from a fixed edge, usually the house, and work outwards. Stretch a line along the first row, set at the slab height, as a guide.
- Spread the bedding to a depth about 10 mm higher than the finished level, to allow for compaction when the slab is tapped down.
- Lower each slab carefully onto the bed, then tap it with a rubber mallet until it sits level with the line and at the correct fall. Check with a long spirit level across several slabs, as well as with a short one on each slab.
- Keep joints consistent with spacers. For most sandstone and concrete slabs, 10 mm is typical. Porcelain is often laid with narrower joints, such as 5 mm, so follow the maker's recommendations.
- Keep off the new slabs for a day or two, or longer in cold weather, until the bedding has hardened.
Cutting and safety
Cutting paving produces silica dust, which is dangerous when breathed in. Cut outdoors, use water suppression or an extraction attachment if available, and wear a correctly rated dust mask, eye protection and ear defenders. Wear gloves and sturdy boots when handling slabs, and lift using proper technique or a slab lifter. Slabs are heavy: a 600 x 600 x 38 mm concrete slab can weigh around 30 kilograms or more, so work in pairs where you can.
Jointing
When the bedding has set, fill the joints. Dry brush-in kiln-dried sand is quick but can wash out and let weeds in. Polymeric jointing compounds set firm and resist weeds and ants when applied to dry slabs as directed, though they need dry weather for application and cure. A pointing mortar using a wet-filled technique or a slurry-grout is another option, but it can stain the surface of porous stone, so test a small area first and protect the slabs with a sealer if recommended.
Worked example: quantities
Illustrative numbers: a patio 4 m x 3 m using 600 x 600 mm slabs, with a 100 mm sub-base and 35 mm bedding.
- Area = 4 x 3 = 12 square metres.
- Slabs: 12 / (0.6 x 0.6) = 33.3, plus 10 percent for cuts and breakage: about 37 slabs. Buy one extra pack as a spare, since a later batch may differ in colour.
- Sub-base: 12 x 0.10 = 1.2 cubic metres compacted. Loose material compacts by roughly 25 to 30 percent, so order around 1.2 x 1.3 = 1.56 cubic metres. Merchants sell by weight: compacted Type 1 is roughly 2 tonnes per cubic metre, so this works out to roughly 3 tonnes; ask the merchant to convert for you.
- Bedding: 12 x 0.035 = 0.42 cubic metres, plus 10 percent waste, 0.46 cubic metres. Sharp sand weighs roughly 1.6 tonnes per cubic metre, so around three-quarters of a tonne, plus cement if using a sand-cement mix.
The patio paving, sand and sub-base calculator works out these volumes and weights for your dimensions and layer depths. If you are casting a concrete base or edge haunching, the concrete volume calculator handles the concrete. The Planning Portal common projects pages and our guide on which projects need building regulations approval explain the wider rules.
Common mistakes
- A thin or uncompacted sub-base, giving a soft patio that sinks around the edges.
- Not allowing for falls, so water pools against the house.
- Bridging the damp-proof course with paving or soil, which lets moisture into the wall.
- Laying on topsoil.
- Using building sand instead of sharp sand, which drains poorly and stays soft.
- Skipping edge restraint.
When to get help
Get professional advice for large patios on sloping ground that need retaining walls, for sites with poor drainage or high water tables, for any area that will take vehicles, and where trees or services are close. This guide is general information and does not replace advice from your local planning authority or building control team.