Diagonal cracks in brickwork: what they mean and what to do

Why cracks run diagonally

Brick walls are built in courses with overlapping joints. When part of a wall moves, the weakest line is usually the mortar, so the crack follows the joints in a staircase pattern rather than splitting the bricks. That is why a diagonal crack often looks stepped when you get close.

The direction tells you something. A crack that is wider at the top than the bottom usually means one end of the wall has dropped relative to the other. A crack wider at the bottom points the other way. Either way, it is the symptom; the job is to find what moved.

Common causes

Around windows and doors, stepped cracks from the corners of the opening often come from a failed or missing lintel, or from an old timber lintel that has rotted. The brickwork above sags a little and cracks at the weak corner.

Lower down, diagonal cracks can come from ground movement: clay soil shrinking in a dry summer and swelling in winter, tree roots drawing water from under the footings, or a leaking drain washing out the ground. Thermal and moisture movement in long walls with no movement joints is another cause, especially in newer blockwork and garden walls.

Finally, alterations can do it: a wall knocked through without adequate support, a new extension settling against the old house, or heavy loading on an old wall.

Cosmetic or structural?

A hairline crack that has been there for years, is narrower than a coin edge and does not change is usually cosmetic. Cracks you can fit a coin into, cracks that have opened recently, doors and windows that have started to stick, and cracks that pass through bricks rather than just the joints are the ones to take seriously.

We are bricklayers, not structural engineers. When a crack looks like active movement we will say so and recommend an engineer's inspection before anything is rebuilt. Repairing brickwork over a cause that is still moving just means the crack comes back.

Monitoring a crack

Before any repair, it helps to know whether the crack is still moving. Mark the ends with a pencil and date, photograph it next to a ruler, and check it every few weeks through a change of season. A cheap plastic crack gauge fixed across the crack does the same job more accurately. If nothing changes over several months, the movement has probably stopped.

How a bricklayer repairs it

Once the cause has been dealt with, the usual repair is to rake out the cracked joints and repoint, and to cut out and replace any bricks that have split. Where the crack is wider, stainless helical bars are bedded into the horizontal joints across the crack to stitch the wall back together, then the joints are pointed over so the repair disappears.

Where a lintel has failed, the brickwork above the opening is temporarily supported, the old lintel is removed, a new one is installed on proper bearings and the brickwork is rebuilt. Matching the brick and mortar colour is what makes the repair invisible, and that is where years of practice on local houses count.

What not to do

Do not fill the crack with sealant or hard cement and forget it; hard mortar in an old lime-built wall pushes the stress into the bricks. Do not ignore a crack near a drain or a big tree. And do not let anyone rebuild a section of wall without asking why it cracked in the first place.

Our experience

D & A Brickwork & Building has been laying and repairing brick for more than 26 years from our base in Bidwell, Dunstable. Our bricklaying is NVQ Level 2 qualified, we are FMB members and fully insured. For a general triage of every crack type, see the cracked brickwork diagnosis guide; this page covers the diagonal pattern only.

Questions people ask