Start with the sub-base, not the sand

The single biggest reason patios end up looking uneven a year later is a sub-base that was rushed. Sand does not carry load on its own; it only beds the slab onto the sub-base beneath. Excavate to the correct depth first: the slab thickness, plus 30–40mm for the bedding sand, plus 100mm of compacted MOT Type 1, plus a little extra for the fall. On soft ground or heavy clay, go deeper and lay a geotextile membrane to stop the sub-base migrating into the soil.

Lay the Type 1 in two layers of around 50mm, watering lightly and compacting each with a wacker plate until it feels solid underfoot and sounds dull rather than hollow. Work to a fall of roughly 1:60 to 1:80 running away from the house, and keep the finished patio at least 150mm below the damp-proof course. A fall that runs towards the building is not just a nuisance; it can cause damp problems that are far more expensive to fix than a slightly steeper slope would have been.

Set out lines and levels before anything is screeded

Once the sub-base is compacted, set out a string line along the edge you are working to, with pegs every couple of metres so the line does not sag. Mark the finished height on each peg, remembering to allow for both the slab thickness and the bedding sand, then run a line across them. Check the diagonals of the whole area; if they match, your rectangle is square.

For anything larger than a small courtyard, hire or borrow a laser level or use a straight edge with a spirit level to set the falls accurately. Then fit your edge restraints, whether that is concrete kerbs, treated timber edging or a hidden plastic edge, because a patio without a solid edge will gradually spread and open at the joints. Bed the restraints in concrete and leave them to set before you lay a single slab against them.

Mixing and screeding the bedding sand

Use sharp sand or concreting sand rather than soft building sand. It holds its shape far better and drains more freely. The sand should be damp enough to hold together when squeezed in your fist but dry enough that no water drips out. If it is sloppy, the slabs will sink unevenly; if it is bone dry, they will not bed down at all.

Spread the sand to a loose depth of around 30–40mm, then use timber rails or lengths of pipe as guides and pull a straight edge across to create a flat, even screed. Do not compact the sand before laying; the weight of the slab and a few firm taps with a rubber mallet will do that job. Only screed as much as you can cover in a single session. Bedding sand left exposed overnight, or walked over repeatedly, will not give you the flat surface you have just worked for.

Laying the slabs and checking levels constantly

• Start from a fixed corner and work outwards, keeping a consistent 10–15mm joint between slabs.
• Place each slab, then tap it down gently with a rubber mallet until it sits at the string line height.
• Check every third or fourth slab with a spirit level and a long straight edge, in both directions, including the fall.
• Lift and re-bed any slab that rocks or sits proud, rather than forcing it down harder.

Resist the temptation to fix a low slab by adding more sand underneath after it has been tapped down; it will settle differently from its neighbours. Cut the edge slabs as you go, using a disc cutter with a diamond blade and plenty of water, and wear the right eye and ear protection.

Jointing, cleaning and the final checks

Once the slabs are down, brush kiln-dried sand into the joints and compact it in two or three passes, topping up as it settles. For a more durable finish, particularly on a driveway approach, point the joints with a semi-dry mortar mix instead. Sweep any surplus sand or mortar off the surface before it sets, and wash slabs down gently rather than blasting the joints out with a pressure washer.

Leave the patio a week or so before putting heavy planters or furniture on it, and keep an eye on the joints after the first few rainfalls. Topping up the sand once the paving has settled is normal and takes minutes. Get the sub-base and the screed right, check your levels often, and the finished patio will stay flat and tight for years.