A patio is not simply slabs laid on sand. Nearly everything that makes it last happens below ground, before a single slab goes down, and skipping that stage is why so many patios rock, sink or pool with water within a couple of years. Knowing how to lay a patio properly starts with the sub-base, the fall and the bedding, in that order. Get those three right and the visible part is mostly patience.
Digging out and preparing the sub-base
Laying a patio begins with a spade. Dig out all topsoil and any soft, organic material until you reach firm ground; on heavy clay this often means going further than expected. Below the slabs sit two layers, and the lower one does most of the work. MOT Type 1 sub-base, a graded mix of crushed stone down to dust and not the same thing as cheaper scalpings, is spread in stages of around 50–75 mm and compacted with a vibrating plate after each pass, so the material locks together under load. On ordinary ground, 100 mm of compacted sub-base is typical; on clay or soft soil, allow 150 mm or more. Skip this layer, or compact it poorly, and rainwater collects beneath the slabs instead of draining away, softening the ground until the patio starts to rock. Mark out the area with pegs and string before you dig, and check the depth with a straight edge as you go.
The fall away from the house and the damp-proof course
Water has to leave the patio quickly, not collect against the house wall. A fall of 1 in 60 to 1 in 80 away from the building is standard, which works out at roughly 12–17 mm of drop for every metre; textured slabs usually need the steeper end of that range, smooth ones the gentler. Check it with a long spirit level and a straightedge as you set out the bedding, not by eye. Just as important is height: the finished patio surface should sit at least 150 mm below the damp-proof course of the house, which you can usually find as the mortar line where the brickwork changes slightly in colour. A patio built up too high bridges the DPC, and water then tracks into the wall rather than running off it, a far more expensive repair than digging a little deeper now. Where the ground slopes the wrong way, a shallow drainage channel along the house wall solves the problem without raising the whole patio.
- Excavate to firm ground, removing all topsoil and soft material
- Lay and compact the MOT Type 1 sub-base in stages
- Set the fall away from the house, checking it stays below the damp-proof course
- Lay a full mortar bed and bed each slab level
- Prime porcelain slabs with slurry before bedding them
- Once the bed has firmed up, point or joint between the slabs
How to lay a patio on a full mortar bed
Concrete and natural stone slabs need a full mortar bed, not dabs of mortar at the corners, or they will rock under a footstep. Mix sharp sand and cement at around 4 to 6 parts sand to 1 part cement, never building sand, since its rounded grains do not lock together the way sharp sand’s angular ones do. Spread 30–50 mm of mortar over the compacted sub-base, lower each slab into place and check it with a straight edge across neighbouring slabs and along the fall. This is a different job from bricklaying, where a mortar mix ratio is chosen to match the brick’s own strength; paving mortar only has to carry weight and shed water, so a firmer mix causes no harm here. If a slab rocks once bedded, lift it straight away, add mortar under the low corner and reset it rather than tapping it down and hoping. Work in bays of two or three square metres so the mortar stays workable while you lay.
Porcelain slabs need a slurry primer first
Porcelain is fired at a much higher temperature than natural stone or concrete, which makes it dense and close to non-porous, so ordinary mortar cannot grip its underside the way it grips a rougher material. Before bedding a porcelain slab, brush a slurry primer over the entire underside, right to the edges, while the mortar bed is still workable; the two need to bond wet on wet, so priming a slab and leaving it to dry first defeats the purpose. Skip the primer, or prime only the centre of the slab, and the porcelain can work loose from the bed within a season, however solid the sub-base beneath it. Porcelain also takes a narrower joint than stone, typically 3–5 mm, and needs a flexible or proprietary pointing product rather than plain sand and cement, which shrinks too much for such a tight gap and can crack out. Always check the slab manufacturer’s own fixing instructions, since primers and joint widths vary between ranges.
Choosing how to joint the slabs
Once the mortar bed has firmed up, usually after a day or two though this depends on the weather, it is time to fill the joints. A traditional sand and cement mortar joint is the cheapest option and suits stone and concrete well, but it takes practice to finish neatly without smearing the slab faces. A polymeric or brush-in jointing sand, sold under various trade names, is brushed dry into the gaps and then watered in to set; it works faster than mortar and suits patios that will not see heavy pressure-washing. A two-part resin joint costs more again but gives the strongest, most weatherproof result, worth it on a patio that doubles as a path to a side gate or bins. Whichever you choose, rake out any mortar that has squeezed up between slabs during laying so the finished joint sits slightly below the slab surface. Keep foot traffic off the patio for a day or two after jointing, and heavier use for the better part of a week.
Timing the job and the one planning rule that applies
Spring and early summer suit patio laying best: the ground has usually dried out from winter, and there are normally several settled weeks ahead for the mortar to cure properly. Laying in a wet autumn is possible but riskier, since a sudden downpour can wash out fresh joints or soften the sub-base before it has compacted. If a front garden patio or path will cover more than 5 square metres, Planning Portal guidance on front garden hard surfaces sets out that it must use permeable materials or drain to a permeable area, or planning permission is needed. Once the forecast shows a dry week ahead and the ground has stopped holding standing water after rain, that is the signal to start digging, not a date on the calendar. A patio laid in the wrong week rarely fails outright, but it makes every later stage, from compaction to curing, harder than it needs to be.
