What is a crack?
A crack (Dutch: scheur) is a break or split in a material or structure. Cracks form when the stresses in a material exceed what it can resist. Crack width is measured at the surface, at right angles to the direction of the crack.
Causes
The shape and pattern of a crack often point to its cause:
- Shrinkage: concrete, mortar and plaster shrink as they cure and dry. This usually produces fine, curved or map-like cracks (crazing).
- Settlement: uneven settlement of the foundation often causes diagonal or stepped cracks in brickwork that are wider at one end.
- Thermal movement: expansion and contraction due to temperature changes, for example in long walls without movement joints.
- Rusting iron: built-in ties, lintels or reinforcement expand as they corrode and push the material apart.
- Overloading: excessive loads, for example after alterations or removal of a load-bearing wall.
Application
Cracks in walls are often assessed using a classification by width, such as the one in BRE Digest 251: up to 0.1 mm negligible (hairline), up to 1 mm very slight, up to 5 mm slight, 5-15 mm moderate, 15-25 mm severe and over 25 mm very severe. More important than the width alone is whether a crack is still growing.
| Up to 0.1 mm | Negligible (hairline) |
|---|---|
| Up to 1 mm | Very slight |
| Up to 5 mm | Slight |
| 5-15 mm | Moderate |
| 15-25 mm | Severe |
| Over 25 mm | Very severe |
Practical tips:
- Measure and record: note the date, location, length and width, and take photos with a ruler next to the crack.
- Monitor it: fix a crack monitor (a plastic tell-tale across the crack) or make two pencil marks with a measured distance. Check over months and across seasons.
- Don’t just fill it: find the cause first, or the crack will come back.
- Repair: shrinkage and thermal cracks are often solved with a movement joint or flexible sealant; in brickwork, damaged joints are raked out and repointed, or bricks are cut out and replaced.
- Call an expert: for growing, diagonal or wide cracks, sticking doors and windows or leaning walls, have a structural engineer or surveyor inspect it.
Related Terms
- Shrinkage crack
- Settlement
- Movement joint
- Brickwork
- Repointing
- Foundation survey
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