How a Patio Should Be Built: The Correct Process

Before anything is dug, the finished level must be established. It is set from the fixed points that cannot move — the door threshold, the damp-proof course, existing steps and the boundary.

The paved surface must sit at least 150mm below the damp-proof course, and it must not obstruct airbricks. Where a doorway sits too low for that, the correct solution is a drainage channel or recessed gravel margin at the wall, not raising the paving.

The fall is set at the same time. A patio should shed water away from the house at about 1 in 80, roughly 12mm per metre. On a wide terrace that adds up, so falls are often split, running to a central channel or towards two different edges rather than in one long sweep.

Setting out also determines cutting. A good layout starts full slabs at the most visible edge and pushes cuts to the least visible one, and aligns joints with door centres or features rather than landing awkwardly against them.

Excavation goes down to allow for the sub-base, the mortar bed and the slab thickness. For a domestic patio that typically means 200mm to 250mm below finished level, deeper if the ground is soft or the area will take vehicle loads.

Soft spots in the exposed ground are dug out and replaced with stone, not covered. Where the ground is clay or made ground, a geotextile membrane separates the subgrade from the sub-base so the two do not intermix over successive winters.

MOT Type 1 is then laid and compacted in layers of no more than 75mm at a time. One thick layer whacked from the top compacts only the surface. Compacted properly, the sub-base is hard enough that a boot heel makes no impression.

The sub-base surface should already follow the intended falls. Correcting levels later using extra mortar produces inconsistent bed thicknesses and a weaker patio.

Every slab should be laid on a continuous, full bed of mortar, generally 30mm to 50mm thick, covering the entire underside. This is not a preference — it is the only method that gives uniform support and prevents water collecting in voids.

The five-dab method, where mortar blobs are placed at the corners and centre, remains common and is the leading cause of rocking, hollow-sounding and cracked slabs. The voids fill with water, freeze, and the slab breaks or lifts. It should be refused outright.

Porcelain requires an additional step: a slurry primer brushed onto the back of each slab immediately before bedding. Porcelain is non-porous, so mortar will not bond to it chemically without the primer, and slabs laid without it come loose within a year or two regardless of how good the bed was.

Slabs are tapped down to line and level with a rubber mallet, checked continuously with a straight edge and a spirit level set to the intended fall. Each slab is checked against its neighbours for lippage — the small height differences that catch a toe and look untidy.

Joint width is decided by the slab type. Calibrated porcelain can run tight joints of 3mm to 5mm; riven natural stone with variable edges needs 10mm to 15mm to look right and to accommodate the variation.

Jointing should not happen the same day the slabs are laid. The bed needs time to set — usually at least 24 hours, longer in cold weather — so that slabs are not disturbed while pointing.

Brush-in resin jointing compounds are the modern standard. They are applied to a wet surface, brushed into the joints and consolidated, and they cure into a flexible, weed-resistant, water-shedding joint. They are more expensive than sand and cement and considerably more reliable.

Where sand and cement pointing is used, it must be a properly mixed, damp, well-consolidated mortar pressed hard into full-depth joints — not a dry brushed-in mix, which never gains strength and crumbles within a couple of years.

The perimeter needs support. Against a lawn, that means a haunched edging or a mowing strip so the outer slabs cannot tip. Against a border, a low retaining edge stops soil washing over the paving.

Steps are built as structures in their own right, with their own compacted base and a solid core, not simply stacked slabs. Riser heights should be consistent throughout a flight — variation is both unattractive and a genuine trip hazard.

At the end, the patio should be washed off and left to cure. Resin joints need a dry period to cure fully; mortar needs protection from heavy rain and frost. Furniture and heavy loads should stay off for a few days.

Walk every slab and feel for movement. Tap suspicious ones with a mallet handle — a hollow ring means a void beneath.

Pour a bucket of water at the house wall and watch it. It should run away from the building evenly with no puddles left standing after a minute or two.

Sight along the surface at a low angle. Lippage, wavering joint lines and dips all show up in raking light in a way they do not from directly above.

Check the joints are full to the top and consistent, that cuts are clean and sensibly placed, and that the level against the damp-proof course is correct. Raise anything you find now — it is far easier to resolve before the team leaves site.