How do I repair a cracked paving slab?

Why slabs crack in the first place, when a crack can be left alone, and lifting and replacing one without disturbing its neighbours.

Difficulty: intermediate
Time: 2 hr 30 min
Read: 6 min
Safety: caution

Short answer

A slab cracks because it is unsupported underneath — almost always spot bedding, sometimes a hollow in the sub-base. A tight, stable crack in a slab that does not rock can be left or filled cosmetically. Anything that moves gets lifted, and the reason it cracked gets fixed before the replacement goes down on a full mortar bed.

Slabs are strong in compression and weak in bending. A slab fully supported across its underside is almost impossible to crack by walking on it; a slab supported on five blobs of mortar is a beam with a load in the middle, and it will eventually break. So the crack is not really the problem — it is the symptom, and the fix has to reach what is underneath.

Safety

A cracked slab that rocks is a trip hazard and its edges are sharp. Cement is caustic — wear waterproof gloves and eye protection when mixing mortar, and use a kneeler board rather than kneeling directly on fresh mortar. Cutting or breaking a slab produces silica dust; suppress it with water and wear a mask.

What you'll need

Replacement slab, matched as closely as possible
Sharp sand and cement, or a bagged bedding mortar
Bolster and a club hammer
Slab lifters or two bolsters for lifting
Rubber mallet, level and straightedge
Pointing iron and jointing compound
Angle grinder with a diamond blade and water (optional)

Step by step

Decide whether it needs lifting at all.Stand on each side of the crack in turn. If nothing moves, nothing rocks and the crack is tight, the slab is still fully bedded and you can leave it or fill the crack cosmetically. If either side moves independently, it is coming up.
Find out why it cracked before you lift it.Look at the pattern. A single clean crack across the middle: unsupported centre, so spot bedding. A crack radiating from one corner: a void or a point load at that corner. Several slabs cracked in a line: something under them has settled, often a backfilled service trench.
Cut the joints all the way round.Rake or cut through the pointing on all four sides with a bolster or a grinder, right down to the bed. Levering against a slab that is still bonded to its neighbours cracks the neighbours too — this is the most common secondary damage in this job.
Lift the pieces out.A cracked slab usually comes out in two or three pieces, which is a small mercy — each piece is manageable. Lever straight up rather than sideways against the adjoining slabs.
Clear the bed back to a sound base.Chip out all the old mortar dabs and any loose material until you are on firm compacted sub-base. If the sub-base is soft, wet or hollow, that is the real fault: dig it out, add MOT type 1 in thin layers and compact it properly before going any further.
Match the replacement as well as you can.Take a piece of the old slab to the merchant. Even with the same product, a new slab is a different age and will be lighter than its weathered neighbours for the first couple of years. Consider taking a slab from a hidden corner and putting the new one there instead.
Lay a full continuous mortar bed.A 4:1 sharp sand to cement mix, damp rather than sloppy, spread across the entire footprint of the slab and ridged with the trowel. Slightly proud of the finished level so the slab beds down into it.
Bed the slab and hold the fall.Tap it down with a rubber mallet, checking with a straightedge across the neighbouring slabs, and check the fall away from the house as you go. It is very easy to get a slab perfectly level and lose the patio's fall doing it.
Leave it before pointing.24 hours before the joints go in, and keep off it. Pointing onto a bed that has not gone off disturbs the slab you have just set.
Point to match the existing joints.The same product and the same finish as the surrounding joints, pressed in firmly and struck off to the same profile. A patched joint that stands out is what makes a repair obvious.

Tips

Keep any spare slabs from the original job somewhere dry and out of the sun. Weathered spares match far better than new ones.
If you cannot match the slab, look at where else it could go. Moving the mismatch to a shaded corner behind a planter costs one extra lift and makes the repair disappear.
Do this in mild dry weather. Mortar laid in frost never develops strength, and mortar laid in strong sun dries before it cures.
Check the fall while you have the slab out. A cracked slab is often a low point where water has been standing, and that is worth correcting at the same time.

Common mistakes

Levering the slab out without cutting the joints — The pointing bonds the slab to its neighbours. Levering breaks the neighbours' edges and turns one repair into three.
Re-bedding on dabs because it is quicker — It is exactly what caused the crack. The new slab will crack in the same place, usually within two winters.
Filling a crack in a slab that rocks — The filler has to hold two moving pieces together and it cannot. It cracks again along the same line within weeks.
Ignoring the sub-base because the slab is the visible problem — A hollow under the bed produced the crack. A new slab over the same hollow is a new slab with a countdown on it.

If it doesn't work

Several slabs cracked in a line across the patio

Cause: A settled service trench or a failure in the sub-base along that line. — Fix: Lift the whole line, excavate to firm ground, and rebuild the sub-base in compacted layers before re-laying. Replacing the slabs alone repeats the failure.

Slabs cracking near a downpipe or a gully

Cause: Water is getting into the bed and washing out the support, or freezing in a void. — Fix: Fix the water route first — a shoe not reaching the gully, a blocked gully or an open joint — then re-lay the affected slabs on a full bed.

A slab cracked under a heavy planter or a hot tub

Cause: A point load on a slab designed for foot traffic, over an inadequate bed. — Fix: Re-lay on a full bed and spread the load — a larger base under the object, or a proper concrete pad if the load is significant.

Alternatives

Fill and leave a stable crack: Where the slab is fully bedded and does not move, a matched cementitious repair mortar or a resin repair filler will make the crack much less obvious and will keep water out. It will always be visible on close inspection.
Turn the slab round: If the crack is near one edge and the slab can be lifted intact, turning it so the cracked edge sits under a step, a planter or the least-seen edge is a legitimate and free solution.
Replace with something different deliberately: A single unmatched slab always looks like a repair. Two or three replaced in a deliberate pattern, or a small planted or gravel inset, reads as a design decision instead.

Questions people ask

Can I glue a cracked slab back together?

In place, no — there is no way to get an adhesive into the crack and clamp it, and the slab is still unsupported underneath. A slab lifted out can be bonded with an exterior epoxy and re-laid on a full bed, and it is worth doing if the slab is unmatchable, but the joint will show.

Why do slabs crack in winter?

Water in the void under a spot-bedded slab freezes and expands, lifting the slab, and the slab breaks over the unsupported span. Frost does not crack a fully bedded slab because there is no void for water to sit in.

The crack is hairline and not moving — do I need to do anything?

Keep water out of it and watch it. A hairline crack in a fully bedded slab may never do anything. Fill it with a matched repair mortar to stop frost getting into it, and check it after each winter.