A shrinkage crack forms when a material wants to shrink but is restrained from doing so. The shrinkage builds up tensile stress; as soon as that exceeds the tensile strength, the material cracks. You see shrinkage cracks in concrete, screeds, plaster, masonry and timber.
How to recognise one
A shrinkage crack has several typical features:
- An irregular path across a surface, often branching — or dead straight where the section changes (at an opening, the corner of a window, a chased service run).
- Even width through the thickness — it does not taper open on one face the way a settlement crack does.
- Appears early. Shrinkage cracks show up in the first weeks to months and barely grow afterwards.
- In timber: along the grain, usually starting at the end grain and wider there than in the middle.
Telling it apart from a structural crack
This is the key question with any crack. Two rules of thumb help:
- Is the crack still growing? Fit a crack monitor or a plaster tell-tale across it and watch it for a few months. A shrinkage crack stabilises; a crack from settlement or overloading keeps moving.
- Is it related to the structure? Diagonal cracks running up from the corner of a window or door opening through several courses of masonry, stepped cracks in the wall, or cracks alongside a visibly sagging floor or out-of-square frames — those point to settlement or overloading, not shrinkage.
If in doubt, have it assessed by a structural engineer. An assessment costs a fraction of getting the repair wrong.
What to do about it
- In plaster and render — cut the crack out to a V, remove dust, prime and fill with a flexible or fibre-reinforced repair mortar. Over a large area, bed in a reinforcing mesh.
- In concrete — fine shrinkage cracks are usually harmless. Where watertightness is required, the crack is injected (with resin, or with a flexible system if it is still moving).
- In a screed — saw the crack out and fill with a resin mortar, with staples set in across the crack to tie the two sides together.
- Prevention is better. Do not work too wet, cure properly, form contraction joints and use shrinkage reinforcement.
Related terms
- Shrinkage
- Contraction joint
- Shrinkage mesh
- Concrete shrinkage
- Settlement crack
- Screed
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