Why is my mortar crumbling, and does it need repointing?
Some surface erosion is normal weathering. Mortar that can be raked out easily, or that is failing faster than the brick, is telling you something about the mix or about water.
- Difficulty
- intermediate
- Time
- 30 min
- Read
- 3 min
- Safety
- caution
Short answer
Scratch the joint with a key or a screwdriver. Mortar that resists is weathered but sound; mortar that comes away as sand for more than a few millimetres has failed. Depth matters more than appearance — repointing is needed when the joint has lost enough depth to let water in, not when it merely looks tired.
Mortar is meant to be the sacrificial part of a wall. It is deliberately weaker than the brick so that movement and salts damage the joint rather than the masonry, and joints are meant to be renewed periodically. So a certain amount of erosion is the system working. The question is whether yours has gone beyond that, and there is a simple test.
Safety
Step by step
- Test the joint rather than judging it by eye.Run a key or the corner of a screwdriver along the joint with moderate pressure. Sound mortar scratches. Failed mortar crumbles to sand and keeps going.
- Check how deep the loss is.Poke into the joint. Surface roughening of a millimetre or two is weathering. Loss of a centimetre or more leaves a ledge that holds water against the wall, and that is when repointing is genuinely needed.
- Look at where it is worst.Erosion concentrated below a leaking gutter, under a defective sill, at the base of a wall or on the weather-facing elevation is telling you about water rather than about mortar. Fixing the water matters more than the pointing.
- Compare the mortar with the brick.If the bricks are spalling and the mortar is intact, the mortar is probably too hard and too strong — usually modern cement pointing on an older soft brick wall. That is a more serious problem than soft mortar and is why overband and cement repointing on old buildings is discouraged.
- Consider the age and type of the building.Solid-walled older houses were built with lime mortar and generally need lime for repairs. Using a hard cement mix on them traps moisture and pushes the damage into the bricks.
- Do not simply point over the old joint.GuideHQ has a guide on exactly this. Skimming new mortar over failed mortar produces a thin layer with nothing behind it, which falls off and traps water while it lasts.
- Keep the scale realistic.A small area at low level is a reasonable DIY job. A whole elevation, anything at height, and anything on a listed or historic building is not.
- Rake out to sound material, to the right depth.Joints are raked back to a depth of roughly twice their height so the new mortar has enough of a key. Hand tools where possible; power tools cut brick arrises and generate hazardous dust.
If it doesn't work
Mortar is sandy at the surface only.
Cause: Normal weathering of the outer face. — Fix: Nothing structural required. Monitor it; repointing when only the surface has gone is unnecessary work.
Joints are deeply eroded on one elevation only.
Cause: Driving rain on the exposed face, or a water source above. — Fix: Find and fix any leak or overflow first, then repoint that elevation.
Bricks are crumbling but the pointing is hard and intact.
Cause: Mortar stronger than the brick, usually cement over an older soft brick. — Fix: Get advice before doing anything. Removing hard cement pointing without damaging the brick is skilled work.
Questions people ask
How often does a wall need repointing?
There is no interval. It depends on exposure, mortar type and detailing. Some walls go many decades; a badly exposed elevation with the wrong mix goes far sooner.
Can I use a bagged mortar for repointing?
For modern brickwork, often. For older lime-built walls, generally not — the mix needs to suit the wall. GuideHQ has a guide on mixing mortar correctly.