Re-lay a sunken patio slab

Lift a sunken patio slab, rebuild its base, then reset and point it so it sits flat and stable.

RS-01Check the slab and mark the areaLook at the sunken slab and the ones around it. A single dip usually means the bedding underneath has washed out or settled. Mark a working area around the slab with chalk or string so you know where you'll be digging.

RS-02Gather your tools and materialsYou'll need a bolster chisel and club hammer to lift the slab, a spade, a stiff broom, a bucket, and gloves and knee pads. Have sharp sand, cement (or ready-mixed paving mortar) and MOT Type 1 hardcore ready in case the base needs topping up.

1 each · Bolster chisel

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1 each · Club hammer

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1 25kg bag · Sharp sand

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1 25kg bag · Cement

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1 25kg bag · MOT Type 1 hardcore

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1 pair · Gardening gloves

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1 each · Kneeling pad

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RS-03Lift the sunken slabWork the bolster chisel into the joint between the sunken slab and its neighbours, tapping gently with the club hammer to break the old jointing free. Lever the slab up carefully with the chisel or a spade — slabs are heavy, so lift with your knees and get help if it's a large one. If your patio has any garden power sockets, outdoor lighting, or a pond pump running under it, treat any buried cable as live: shallow cables and pipes can run under patios with no warning, so dig by hand near the edges rather than forcing tools straight down.

RS-04Clear out the old beddingScrape and dig out the old mortar or sand bedding from under the slab and to a similar depth around it. Check the sub-base (the compacted hardcore layer) underneath — if it's firm and level, you can leave it. If it's soft, crumbly, or you can push a screwdriver into it easily, it needs building back up.

RS-05Rebuild the sub-base if neededAdd MOT Type 1 hardcore in thin layers, roughly 50mm at a time, and compact each layer firmly with the end of a fence post or a hand tamper before adding the next. Aim for a firm, flat base sitting about 40-50mm below the level of the surrounding slabs, to leave room for the new mortar bed.

2 25kg bag · MOT Type 1 hardcore

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RS-06Mix and lay the new mortar bedMix sharp sand and cement at roughly 4 parts sand to 1 part cement with just enough water to make a firm, damp mix that holds its shape — not sloppy. Lay it in five mounds (one under each corner and one in the middle) rather than a flat bed, so the slab can be tapped level.

1 25kg bag · Sharp sand

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1 25kg bag · Cement

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RS-07Reset the slab and level itLower the slab back onto the mortar mounds and tap it down gently with a rubber mallet, checking with a spirit level across the slab and across its neighbours. It should sit flush with surrounding slabs, with a very slight fall away from the house for drainage. If the patio sits close to the house, keep the finished slab surface at least 150mm below the damp proof course (DPC) — the mortar line visible in the brickwork — to stop rain splashing damp into the wall.

RULES

KNOW THE RULES

Building Regulations Approved Document C recommends paving finish at least 150mm below the damp proof course to protect the wall from damp.

RS-08Point the jointsOnce the slab is level and firm, mix a slightly wetter mortar and push it into the joints around the slab with a pointing trowel, or use a ready-made paving jointing compound brushed in dry then set with water. Smooth the joints level with the slab surface and wipe away any excess before it dries on the face.

1 tub (12.5kg) · Paving jointing compound

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1 each · Pointing trowel

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RS-09Leave to cure and tidy upKeep the slab free of foot traffic for at least 24 hours, and avoid heavy weight on it for 3 days while the mortar cures fully. Sweep away leftover sand and cement dust from the surrounding slabs, and rinse tools before the mortar sets hard on them.

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