How do I repoint or repair mortar?

Repointing is the repair where using the wrong material causes far more damage than doing nothing at all, and the mistake is extremely common because cement is what most people have to hand.

Safety

What you'll need

• Mortar matched to the existing — lime or cement-based as appropriate
• Plugging chisel and lump hammer, or a mortar raking tool
• Pointing trowel and a jointer for the finish
• Respirator, goggles, gloves and a stiff brush
• Water sprayer and hessian or sheeting for lime work

Step by step

1. Work out what the wall is.A solid wall built before roughly 1919 is almost certainly lime-mortared and designed to breathe. A cavity wall from the 1930s onwards is cement-mortared. This determines the material and everything else.
2. Understand why it matters.Lime mortar is softer than the brick, so moisture evaporates through the joints and any movement cracks the mortar rather than the brick. Cement is harder, so it traps moisture in the brick, which then spalls and crumbles in frost.
3. Never use cement on an old solid wall.It causes the bricks to fail rather than the joints, which is far more expensive to put right. If your house is Victorian or older, this is the single most important thing here.
4. Match the mix and the aggregate.Sand colour and grain size determine how the finished joint looks. Take a sample of the old mortar to a builders' merchant, or send it for analysis on anything significant.
5. Only repoint what needs it.Sound mortar should be left. Repointing an entire elevation because part of it has failed is unnecessary work and unnecessary risk to the bricks.
6. Cut out to at least twice the joint width.Usually 20 to 25mm deep. Shallow pointing has nothing to grip and falls out within a few years, which is the most common repointing failure.
7. Cut out by hand where you can.A plugging chisel and a light hammer. An angle grinder is fast, widens joints, damages brick arrises and produces dangerous silica dust. Hand tools on soft old lime are quicker than expected.
8. Control the dust if using power tools.Water suppression or on-tool extraction, plus a proper respirator. Dry-cutting mortar and brick releases respirable silica, which causes permanent lung damage.
9. Brush out and dampen the joints.All dust and loose material out, then dampen — not soak — the joints before pointing. Dry brickwork sucks the water out of the mortar and it fails to cure.
10. Work in the right conditions.Above 5°C and no frost forecast for several days. Out of direct sun and drying wind. Lime in particular needs slow curing and cannot be rushed.
11. Press the mortar firmly into the joint.Filling in layers on a deep joint, pressing hard to compact it. Voids behind the face are where water gets in and where frost damage starts.
12. Let it firm up before finishing the joint.Until it is thumbprint-hard. Tooling too early smears it, too late and it will not compact. This is the judgement that most affects how the finished work looks.
13. Match the existing joint profile.Struck, flush, bucket-handle or weatherstruck. A different profile is immediately visible across a wall, even with a perfect colour match.
14. Protect lime work while it cures.Damp hessian for several days, kept moist and shaded. Lime cures slowly by carbonation and drying it out too fast produces a weak, dusty joint.
15. Get advice for anything listed or significant.Listed buildings and those in conservation areas may need consent, and specification is likely to matter. A conservation-experienced builder is the right call.

Tips

• Pre-1919 solid wall means lime. Getting that one decision right matters more than every technique here.
• Cut out to twice the joint width. Shallow pointing falls out within a few years and the work has to be done again.
• Take a lump of the old mortar to the merchant. Matching the sand is what makes repairs disappear.

Common mistakes

• Using cement mortar on an old solid wall — It is harder than the brick, so moisture is trapped and the brick faces spall off in frost. The damage is to the bricks rather than the joints and is far more expensive to repair.
• Dry-cutting joints with an angle grinder — It releases respirable silica dust, which causes permanent lung damage, and it widens joints and chips the brick edges. Hand tools or water suppression.

If it doesn't work

Questions people ask