Why are the bricks on my garden wall crumbling?

Spalling explained — why brick faces blow off in frost, the role of cement pointing and damp, and how to cut out and replace individual bricks.

Difficulty: intermediate
Time: 4 hr
Read: 4 min
Safety: caution

Short answer

Spalling is frost damage. Water gets into the brick, freezes and pushes the face off. It happens where the wall stays wet — no coping, a bridged damp-proof course, or hard cement pointing that traps moisture in the brick instead of letting it evaporate through the joint.

Bricks are meant to get wet and dry out again. Spalling happens when they cannot dry — because the top of the wall is open, because the ground is holding water against the base, or because someone repointed with a cement mortar harder than the brick, so the only escape route for the moisture is through the brick face.

Safety

Cutting out and replacing bricks in a wall weakens it temporarily. Never remove more than a few bricks at a time, never cut out across a full course, and stop if the wall shows any sign of movement. Cutting brick produces silica dust — cut wet or with extraction and wear an FFP3 mask. If the wall is over about 1m or leaning, get it assessed before working on it.

Four mortar joint profiles, and which ones keep the brick wet

Step by step

Find out where the water is coming from.Spalling at the top means no coping or a failed one. Spalling in the bottom few courses means ground water and a missing or bridged DPC. Spalling in patches usually follows a splash zone or a leaking downpipe.
Check the pointing.Hard grey cement pointing on a soft old brick is a classic cause. If the joints are harder than the bricks, moisture is forced to leave through the brick face, which is what destroys it.
Fix the water source before any repair.Fit or repair the coping, clear the ground back from the base, redirect the downpipe. Replacing bricks without fixing the cause means doing it again.
Decide how much needs replacing.Bricks with a few millimetres of face lost are cosmetic and can be left. Bricks that have lost a substantial part of their thickness, or that sound hollow, need cutting out.
Cut out one brick at a time.Rake out the joints all round with a plugging chisel or a small grinder, then break the brick out from the middle. Never cut out several adjacent bricks in the same course at once.
Clean the opening properly.All old mortar off the surrounding faces, and the recess brushed clean and dampened. New mortar will not bond to dust or to a dry, thirsty opening.
Match the replacement brick.Match on size, texture, colour and — importantly — porosity and frost resistance. A dense modern brick next to a soft old one causes problems at the joint. Reclaimed bricks of the right type are usually the answer.
Bed with a mortar weaker than the brick.A lime mortar, or a 1:1:6 cement-lime-sand mix for older brickwork. Butter the bed and the sides, push the brick in, and point the joints to match the surrounding profile.
Repoint any hard cement joints while you are there.Rake out to at least 20mm and repoint with an appropriate weaker mix. This is often the real repair.

Tips

A brick that sounds hollow when tapped has delaminated behind the face and will spall next winter even if it looks sound.
Turning a spalled brick around and rebedding it is a legitimate repair if the back face is sound and the brick is a solid one.
Do not paint or seal a spalling wall to protect it. Sealing traps the moisture inside and accelerates the damage.

Common mistakes

Repointing an old wall with a strong cement mortar — The mortar becomes harder and less permeable than the brick, so moisture has to evaporate through the brick face. The joints survive and the bricks are destroyed.
Applying a waterproof sealant to spalling brickwork — It stops the wall drying out. The moisture already in the wall stays there, freezes, and the spalling continues behind the seal where you cannot see it.

If it doesn't work

Spalling only in the bottom two or three courses

Cause: Ground moisture and a missing or bridged damp-proof course — Fix: Lower the ground level away from the wall, improve drainage at the base, and consider installing a DPC when rebuilding the affected courses.

Spalling all along the top course

Cause: No coping, or a coping without an overhang — Fix: Fit a proper coping with a 25mm overhang and a drip, over a damp-proof course, then replace the damaged bricks.

Replacement bricks looking obviously new

Cause: Colour and texture mismatch — Fix: Source reclaimed bricks of the same type. New bricks weather over several years but never quite match.

Spalling continuing after brick replacement

Cause: The water source was never dealt with — Fix: Go back to the diagnosis. Spalling is always a symptom of a wall that cannot dry out.

Questions people ask

What causes bricks to crumble on a garden wall?

Frost. Water absorbed by the brick freezes, expands and pushes the face off. The underlying question is always why the brick is that wet — usually an open top, a wet base, or cement pointing sealing the joints.

Can I paint a garden wall to stop it crumbling?

No, and a non-breathable masonry paint makes it worse by trapping moisture behind the film. If you want to reduce water uptake, a breathable water repellent is the only sensible option, and only after the cause is fixed.