How do I patch a hole or a broken edge in a concrete floor?

Concrete patches fail for one of two reasons: a feathered edge that crumbles, or no bonding agent so the patch sits on the old surface rather than joining it. Both are avoidable.

Safety

What you'll need

• concrete repair mortar or a sand and cement mix with SBR
• SBR or acrylic bonding agent
• angle grinder with diamond blade or a chisel
• straightedge and trowel
• FFP3 mask and eye protection

Step by step

1. Find out why it broke.A hole from an impact is a simple repair. A crumbling edge along a crack, or an area that sounds hollow, suggests a wider problem — a failing slab or damp — and patching will not last.
2. Sound out the area.Tap around the damage with a hammer. Hollow-sounding concrete is delaminated and must come out, even if it looks intact.
3. Cut square edges.Score or cut around the repair with a diamond blade to a depth of at least 10mm, keeping the edges vertical. Never feather a patch out to nothing — a thin edge always crumbles.
4. Break out to sound material.Remove everything loose or hollow inside the cut. The repair is only as good as what it is bonded to.
5. Undercut slightly if you can.Cutting the edges very slightly undercut gives a mechanical key so the patch cannot lift out.
6. Clean and vacuum thoroughly.Dust is a release agent. Brush, vacuum, and repeat. Any oil contamination must be removed with a degreaser or the patch will not bond.
7. Dampen the substrate.Old dry concrete draws water out of the repair mix, which stops it curing. Dampen it thoroughly but leave no standing water.
8. Apply a bonding agent.SBR or acrylic bonding slurry brushed well into the surface, including the vertical edges. Apply the repair while the bonding coat is still tacky.
9. Fill in layers for deep repairs.Anything over about 25mm goes in more than one layer. Compact each layer firmly into the corners.
10. Level with a straightedge.Work a straightedge across the surrounding floor to screed the patch flush, then trowel to match the existing surface texture.
11. Cure it.Cover with polythene and keep it damp for several days. A repair mortar that dries out is weak and will fail at the edges.
12. Consider a self-levelling compound over the whole floor.For several patches, or an uneven floor generally, filling the holes and then pouring a levelling compound over the lot often gives a better result than individual repairs.

Tips

• Never feather the edge of a patch. A square vertical edge at least 10mm deep is what stops it breaking away.
• The bonding agent is not optional. Without it the patch is sitting on the floor rather than joined to it.
• For anything wider than a metre or two, self-levelling compound over the whole area is usually easier and better than patching.

Common mistakes

• Feathering the repair out to a thin edge — A thin edge of repair mortar has almost no strength and lifts under foot traffic within weeks, taking the rest of the patch with it.
• Filling straight onto dry dusty concrete — The dust prevents any bond and the dry substrate sucks the water out of the mix so it never hydrates. The patch pops out as a disc.

If it doesn't work

Questions people ask