How do I re-lay a sunken paving slab?

A slab sinks because what is under it has moved — a spot-bedded slab whose spots have failed, a sub-base that was never compacted, or water washing the bedding out. Rebedding on the same failed base just puts the problem back.

Safety

Step by step

1. Work out why it sank before lifting it.One slab in the middle of a sound patio is usually a bedding failure. A run of slabs along a downpipe or a drain is water washing the bed away. A whole area sinking is a sub-base problem and needs more than one slab lifting.
2. Free the joints.Rake or cut out the mortar or sand in the joints all round with a plugging chisel or an angle grinder. Levering against a jointed edge breaks the neighbouring slab.
3. Lift the slab from one edge.Drive a bolster into the joint and lever gently, then get a bar underneath and work it up. Two people, or slab lifters, for anything over 450mm square.
4. Clear out the old bed completely.Old mortar spots, wet sand, roots — all of it. Then check the base underneath. If you can push a screwdriver in easily, the sub-base needs attention too.
5. Repair the base if it is soft.Dig out to firm ground, add MOT Type 1 in layers and compact each one. There is no point rebedding a slab on unstable ground.
6. Bed on a full mortar bed.A 4:1 sharp sand to cement mix, semi-dry, spread to give 30-50mm of bed under the whole slab. A full bed is far more durable than five spots — spot bedding leaves voids that hold water and crack the slab.
7. Level it against three neighbours.Lay the slab, tap down with a rubber mallet, and check with a straightedge spanning at least three slabs in two directions. Keep the fall of the patio consistent — the slab should not be flat if the patio falls.
8. Leave it to set before jointing.24 hours before walking on it, then point the joints with a mortar mix or brush in a jointing compound. Keep the joint width consistent with the rest of the patio.
9. Fix the water source if there was one.A slab that sank under a downpipe will sink again unless the water is taken away properly.

Tips

• Number or photograph the slab before lifting if the paving is patterned or riven — putting it back the wrong way round is very visible.
• A semi-dry mortar mix is easier to level a slab into than a wet one, and does not squeeze out into the joints.
• If two or three adjacent slabs have sunk, lift them all and do the base properly. Doing them one at a time takes longer overall.

Common mistakes

• Bedding on five spots of mortar — It leaves voids under the slab that fill with water, freeze, and crack the slab. It is also why the slab rocks when the spots fail. Use a full bed.
• Levelling the slab flat rather than to the patio fall — A flat slab in a falling patio holds a puddle and looks wrong. Match the surrounding fall, using a straightedge across several slabs.

If it doesn't work

Questions people ask