Signs a crack needs an engineer
Width matters, but the pattern and the behaviour of a crack matter as much. Get advice if any of these apply:
- The crack is wider than about 3 mm, or wider at one end than the other.
- It runs diagonally, or steps through the mortar joints of brickwork.
- It goes right through the wall and shows on the inside as well as the outside.
- Doors or windows nearby have started to stick or no longer close properly.
- It has reappeared after being filled, or you can see it getting longer or wider.
- A wall is bulging or leaning, or floors have started to slope.
- There is a large tree or a suspected leaking drain close to the cracking.
How engineers grade crack damage
Engineers in the UK commonly describe damage using the categories in BRE Digest 251, which are based mainly on how easy the damage is to repair. Width is only a guide: the number of cracks, where they are and whether they are moving all count too.
| Category | Typical crack width | What it usually means |
|---|---|---|
| 0, negligible | Hairline, under about 0.1 mm | Cosmetic. Nothing to do beyond normal decoration. |
| 1, very slight | Up to about 1 mm | Fine cracks that are easily filled when you redecorate. |
| 2, slight | Up to about 5 mm | Easily filled, and redecoration is probably needed. Doors and windows may stick slightly. Worth having checked. |
| 3, moderate | 5 to 15 mm, or several up to 3 mm | Brickwork may need repointing or some replacement, and doors and windows stick. Get an engineer's inspection. |
| 4, severe | 15 to 25 mm, depending on the number of cracks | Extensive repair, often including parts of walls. Frames distort and floors slope noticeably. |
| 5, very severe | Usually over 25 mm | Major repair, possibly partial or complete rebuilding. Get advice urgently. |
Categories 0 to 2 are generally treated as cosmetic, 3 and 4 affect how the building works, and 5 affects its stability.
Common causes in London homes
Clay shrinkage and trees
Much of London sits on shrinkable clay. In dry summers, and especially near thirsty trees, the clay loses water and shrinks, so foundations drop slightly. Cracks typically appear in late summer and may partly close again in winter.
Leaking drains
A broken drain can soften or wash away the soil under a foundation. Cracking tends to be close to the drain run, often near a bathroom, kitchen or downpipe.
Lintels and arches
Cracks rising from the corners of a window or door, or a brick arch that has dropped, usually mean the support over the opening has sagged or corroded. This is a local repair rather than a foundation problem.
Bay windows
Victorian and Edwardian bays often sit on shallower foundations than the main house, so a crack where the bay meets the main wall is very common.
Roof spread and alterations
Heavier roof coverings, altered roof timbers, or walls and chimney breasts removed without proper support can all cause cracking higher up the building.
Shrinkage and temperature
New plaster, timber and blockwork shrink as they dry, and materials expand and contract with the seasons. These cracks are usually fine and cosmetic.
How to measure and record a crack
- Measure the widest point with a ruler or a set of crack width cards, and note where it is narrowest.
- Photograph it with a ruler or tape measure held against it, from the same spot each time.
- Mark each end lightly in pencil with the date. If it grows past the marks, it is moving.
- Note when it appeared and whether doors or windows nearby have changed.
These notes are useful to an engineer, and to your insurer if you make a claim.
Crack monitoring
One visit shows the house as it is today. It cannot always show whether the movement is still happening. When that is the question, for a lender, an insurer or your own peace of mind, we fix monitoring gauges across the key cracks and take readings over a period of months.
Because clay soils swell and shrink with the seasons, monitoring usually needs to cover several months to tell seasonal movement from progressive movement. At the end you receive the readings and our conclusion: whether the movement is historic or ongoing, and what that means for repairs.
Case studies
- Front porch pulling away from a 1930s semi in Greenford. Cracks and a 10 mm gap where a rebuilt porch meets a 1930s house in Greenford, UB6. What we found, the likely cause and the repair we recommended.
- Party wall cracking in an Edwardian terrace divided into flats, East Dulwich. Cracks up to 20 mm in the party wall of an Edwardian terrace in East Dulwich, SE22, with three different causes, a gap in the fire separation and a staged repair.
Common questions
Are horizontal cracks worse than vertical ones?
Not necessarily. The direction tells an engineer about the cause rather than the severity. Diagonal cracks that taper are most often linked to foundation movement, horizontal cracks near the top of a wall can point to roof spread, and vertical cracks are common at junctions between different parts of a building. Width, pattern and whether the crack is moving decide how serious it is.
Can I just fill the crack?
Hairline cracks in plaster can be filled when you redecorate. Before filling anything wider, especially externally, find out why it formed. Filling a crack that is still moving hides the evidence and it will usually reopen.
Which inspection should I book for a crack?
For a single crack or one problem area, a Specific Structural Inspection, from £420*. If there are several cracks around the house, or you are buying, a General Structural Inspection, from £540*.
How long does crack monitoring take?
Usually several months, so that the readings cover a change of season. We will recommend a period based on what we find at the first visit.
