How do I fix a sunken or rocking paving slab?

Spot bedding is quick, and it is the direct cause of most rocking, cracking and sunken paving. Water gets into the voids, frost lifts the slab, and the unsupported middle cracks under load.

Safety

What you'll need

• Bolster chisel and lump hammer
• Two trowels or slab lifters
• Sharp sand and cement, or bagged mortar
• Rubber mallet
• Spirit level
• Jointing compound

Step by step

1. Work out what moved.One slab sunk on its own is a bedding failure. A whole area sinking together is a sub-base failure, and that needs lifting the area and recompacting rather than relaying one slab.
2. Rake out the joints around it.Cut through the pointing all the way round with a bolster or a grinder so the slab can come up without pulling its neighbours.
3. Lift the slab carefully.Two trowels or slab lifters at opposite edges, and lift straight up rather than levering against the neighbouring slab. Slabs crack easily at this point.
4. Clean off the old mortar.Chip the old dabs off the back of the slab and out of the bed. New mortar will not bond to old dry mortar.
5. Check the sub-base.Underneath should be firm compacted hardcore. If it is soft, wet or hollow, that is the real problem — add and compact more sub-base before relaying.
6. Mix a full mortar bed.Roughly four parts sharp sand to one part cement, mixed damp rather than sloppy, or a bagged mortar. It should hold its shape when squeezed.
7. Lay a continuous bed.Spread mortar across the whole footprint of the slab, slightly proud of the finished level, and ridge it with the trowel so it beds evenly.
8. Bed the slab down to the right level.Lay it and tap it down with a rubber mallet, checking against the neighbouring slabs with a straight edge and level, and maintaining the fall away from the house.
9. Check the fall as you go.About 1 in 80, 12mm per metre, away from the building. It is easy to level a slab perfectly and lose the fall in the process.
10. Leave it to set before pointing.24 hours before joints go in, and keep off it. Pointing a bed that has not gone off disturbs the slab.
11. Fill the joints completely.A proprietary jointing compound or a dry mortar mix brushed in and compacted. Open joints let water into the bed and start the cycle again.

Tips

• Five dabs of mortar is why it rocked. A full bed is why it will not do it again.
• Cut the joints all the way round before lifting or you crack the neighbours.
• Check the fall as well as the level — it is easy to lose one while fixing the other.

Common mistakes

• Relaying on the same five dabs of mortar — Voids form under the slab again, water collects, frost lifts it and it rocks within a year or two. A full continuous bed is the whole point of the repair.
• Packing sand under a rocking slab to level it — Sand washes out and the slab drops again, usually within a season. It has to be lifted and bedded properly on mortar.

If it doesn't work

Questions people ask