How to lay patio slabs

Inspired by a Homebase expert guide. This walk-through draws on "How To Lay A Patio — Expert Guide To Laying Patio Slabs" from Homebase. It covers the preparation and laying stages well, including the sub-base and mortar bed, which is where most DIY patios go wrong. A solid watch before you hire a skip.

1. Plan your layout and order materials

Decide on the size and position of your patio. Mark it out with pegs and string and check the lines are square — use the 3-4-5 triangle method: measure 3 metres along one line, 4 metres along the adjacent line, and the diagonal between the two marks should be exactly 5 metres.

Order around 10% more slabs than you calculate for cuts and breakages. Work out how much type 1 sub-base material, sharp sand, and cement you need based on your area and the slab thickness.

2. Excavate to the right depth

You need to dig out to a depth that accounts for: 100–150 mm of compacted sub-base, 30–50 mm of mortar bed, and the thickness of your slab (often 40–50 mm). Add it together and that is your excavation depth — typically 200–250 mm below the finished surface level.

The finished patio should sit a little below the damp-proof course of any adjacent wall, and it must slope away from the house — a fall of about 1 in 80 (roughly 12 mm per metre) sends rainwater away from the building.

3. Lay and compact the sub-base

Spread type 1 crushed stone (MOT type 1) across the excavated area to a depth of 100–150 mm. Rake it level and then compact it with a plate compactor — these can be hired by the day from most tool hire shops for a reasonable sum.

Do not skip the compaction. Loose sub-base leads to slabs that rock, crack, or sink unevenly over time.

4. Mix a mortar bed

Mix sharp sand and cement in a ratio of about 5:1 by volume for a standard dry-ish bedding mortar. It should hold its shape when squeezed in your fist without crumbling to dust or weeping water — somewhere between dry and damp.

Lay the mortar bed to a depth of 30–50 mm. Do one or two slab-widths at a time so the mortar does not skin over before you place the slabs.

5. Lay the slabs

Starting from a fixed corner or straight edge, lower each slab onto the mortar bed. Do not drag it across — lower it down and press it into place. Use a rubber mallet to tap it level with adjacent slabs and to the correct height and fall.

Use spacers or off-cuts of wood to maintain consistent joint widths. Check levels constantly with a long spirit level across several slabs as you go.

6. Check the fall and let it set

Run a spirit level across each row and diagonally across the whole area to check the surface is even and falls away from the house consistently. Tap down any high spots. Let the bed set for at least 24 hours before walking on it, and 48–72 hours before any weight goes on it.

Mind you, in cold or wet weather the mortar takes longer to cure. Do not lay in frosty conditions — below 5°C and cement simply will not set properly.

7. Fill the joints

Brush dry kiln-dried sand or a proprietary jointing compound into the gaps between slabs once the bed has cured. For a more permanent finish, brush in a slightly wet pointing mortar and smooth it with a jointing iron. Keep it off the slab faces as mortar stains are difficult to remove once set.

Water the joints gently after filling to activate the mortar, then keep foot traffic off for another 24 hours.

When to call a handyman

Laying a small patio is achievable as a DIY project, but it is physical work and the groundwork takes time. Call Richard if the area is large, the ground is difficult, you need waste removed, or you simply want it done properly without the weekend of heavy digging. A bad sub-base is expensive to fix later.