At a glance
Laying a patio is one of the most satisfying and achievable DIY garden projects – the skills required are straightforward, the tools are widely available and the result transforms how a garden looks and functions. A well-laid patio will last 20-30 years without significant maintenance, making it an excellent return on time invested. The key difference between a patio that stays level and stable for decades and one that sinks, tips and grows weeds within a few seasons is almost entirely in the preparation – specifically the depth and quality of the sub-base beneath the slabs. Get the groundwork right and the laying process itself is satisfying and relatively forgiving. The most common reason patios fail is not poor laying technique but inadequate preparation: too shallow a sub-base, no geotextile membrane and mortar mixed too wet. Avoid these three errors and the rest of the process is straightforward enough for a competent first-time DIYer to achieve professional results.
The total excavation depth for a standard domestic patio is 150-200mm: 100mm of compacted MOT Type 1 sub-base, 40-50mm of mortar bed and the thickness of the slab itself (typically 40-50mm for standard concrete flags, 20-30mm for porcelain). This depth is enough to prevent significant frost heave in UK conditions and to distribute the load from garden furniture and foot traffic without differential settlement. Anything shallower is false economy – a 75mm sub-base instead of 100mm will save one barrow of material at the start and cause years of problems as slabs rock and settle unevenly. The same principle applies to the mortar bed: a thin mortar bed that is scraped down to save time will leave voids beneath the slabs, which crack under load and admit water that freezes and expands in winter, progressively loosening the slabs season by season until the whole area needs lifting and relaying.
What You’ll Need
Choosing Your Materials
Order 10% more slabs than the calculated area requires to account for cuts, breakages and future repairs. For a 20m² patio this means ordering for 22m². Material ordered from a builders merchant rather than a garden centre is typically 20-30% cheaper for equivalent quality. Always check that slabs arrive on a full pallet with documented batch numbers – mixing batches mid-project can result in visible colour variation between areas laid at different times. For natural stone including sandstone, slate and limestone, apply a slab sealer within a week of pointing to protect the surface from staining and to make cleaning easier. Porcelain and concrete flags do not require sealing. Reapply stone sealer every 3-5 years depending on foot traffic, weather exposure and the porosity of the specific stone used.
The choice between concrete flags, natural stone and porcelain is largely one of budget and aesthetics, but the different materials do have practical implications for laying. Natural sandstone varies in thickness by up to 10-15mm between individual slabs within the same batch, which means each slab requires individual adjustment of the mortar bed depth to achieve a consistent surface level. This makes natural stone significantly slower to lay than uniform-thickness materials like concrete flags or porcelain. Porcelain is very dense and smooth, which means it must be back-buttered with neat cement slurry before placing on the mortar bed to achieve adequate adhesion. Without this step, porcelain slabs will not bond reliably to the mortar beneath regardless of how carefully the bed is prepared.
Step-by-Step Laying Process
Mark out and set the fall direction
Using string lines and wooden pegs, mark the perimeter of the patio. Establish the fall direction – water must drain away from the house at a gradient of 1:80 (12.5mm drop per metre). Mark the high point (closest to the house) and low point and use a spirit level with a shim to set string lines at the correct gradient before digging begins.
Excavate to depth
Dig out to the required depth – typically 150-200mm below finished patio level depending on slab thickness. Remove all topsoil and vegetation roots. Apply a weedkiller to the exposed soil surface and leave for at least a week before continuing if weeds are present. Lay a geotextile membrane over the entire excavated area before adding the sub-base material – this prevents the sub-base and soil from mixing over time.
Lay and compact the sub-base
Pour in MOT Type 1 hardcore to a depth of 100mm (loose) and compact with a plate compactor. Compaction will reduce the depth by approximately 20-25%. Check levels across the compacted sub-base using your string lines – the surface should be flat and true to the fall direction. Add more material and compact again in any low spots. The sub-base is the most important part of the whole job – a poorly prepared base cannot be corrected without lifting all the slabs. For large patios over 20m², compact in two 50mm layers rather than one 100mm layer for better consolidation throughout the full depth. Water the sub-base lightly before compaction in dry weather – a slightly damp sub-base compacts more effectively than a dry one.
Mix and lay the mortar bed
Mix mortar at 4 parts sharp sand to 1 part cement (semi-dry consistency – it should hold its shape when squeezed but not be workable like wet mortar). Lay it to a depth of 40-50mm, raking it roughly level. Work in manageable sections – enough for 2-3 slabs at a time so the mortar does not dry before the slabs are placed. Tamp the mortar bed down firmly with the back of a spade before placing each slab.
Lay the slabs
Lower each slab carefully onto the mortar bed – do not drag it into position as this displaces the mortar. Tap firmly and evenly all over with a rubber mallet and check level in both directions and against the string line fall. Use tile spacers or small cut pieces of tile for consistent joint widths of 8-12mm. Regularly check across multiple slabs with a long spirit level to ensure a consistent, flat surface. Start from the house or a fixed straight edge and work outwards.
Point the joints
Allow the mortar bed to cure for at least 24-48 hours before pointing. For most patios, a brush-in patio pointing compound (such as Rompox or similar) is easier and more durable than traditional mortar pointing. Work the pointing compound into the joints with a stiff brush, remove any excess from the slab faces immediately before it sets, and lightly mist with water to activate. Traditional wet mortar pointing (3:1 sharp sand to cement) gives a harder result and is more appropriate for very wide joints over 12mm, but requires more skill to keep the slab faces clean and takes longer to apply. Regardless of method, protect the freshly pointed patio from rain for at least 24 hours after completion to prevent the joints being washed out before they have cured. Do not allow traffic onto the patio for 48 hours after pointing.
Lay a test slab before committing to the whole area. Place one slab on the mortar bed, tap down and check level. Then lift it and inspect the back – if the mortar has not transferred evenly across at least 75% of the slab’s back face, the bed is too dry or unevenly laid. A slab with poor contact coverage will rock and crack under load. Adjust mortar consistency and application before continuing.
Common Mistakes and How to Avoid Them
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