StructuralEngineer.ie's short answer: most cracks in walls are not serious. Hairline cracks from drying shrinkage and thermal movement are cosmetic. Cracks are worth an engineer's attention when they're wider than about 5 mm, stepped through blockwork, wider at one end, still growing, or come with doors and windows that start sticking.
Engineers Ireland makes the same point: most cracks in Irish low-rise masonry are of limited structural significance, and a smaller proportion come from foundation or ground movement. The skill is telling them apart.
Crack width: the BRE Digest 251 categories
Engineers in Ireland and the UK grade visible damage using the BRE Digest 251 categories:
| Category | Approx. width | What it usually means |
|---|---|---|
| 0 | Hairline, under 0.1 mm | Negligible |
| 1 | Up to 1 mm | Very slight, fill and decorate |
| 2 | Up to 5 mm | Slight, easily filled, may recur |
| 3 | 5–15 mm | Moderate, opening up and repair needed |
| 4 | 15–25 mm | Severe, structural repair |
| 5 | Over 25 mm | Very severe, major repair |
Width alone doesn’t decide it. A 2 mm crack that grew over one winter matters more than a 4 mm crack that hasn’t moved in twenty years.
Patterns that point to movement
- Stepped or diagonal cracks through block or brick joints, especially from window and door corners
- Tapering cracks, wider at the top or bottom, which show one part of the wall rotating or dropping
- Cracks where an extension meets the house, from differential settlement
- Horizontal cracks at the base of internal walls with a lifted floor, a classic pyrite sign in boom-era houses
- Doors and windows sticking at the same time as cracks appear
- Cracks that open in summer and close in winter, a sign of clay shrinkage
Cracks that are usually cosmetic
- Fine vertical cracks in plaster at wall and ceiling junctions
- Hairline cracks at window corners in new or newly plastered walls
- Cracks between different materials, such as plasterboard against block
- Shrinkage cracks in the first year or two of a new house
What to do now
- Photograph and measure with a ruler in shot, and date it
- Monitor for a few weeks with pencil marks or a tell-tale
- Check the obvious causes: leaking gutters or drains, trees close to the house, recent building work nearby
- Call an engineer if it’s over about 5 mm, growing, stepped or tapering, or if doors start sticking
For a quick self-check, use the crack assessment checklist.
Bottom line
Most cracks are cosmetic. Width over 5 mm, a stepped or tapering pattern, growth, or sticking doors mean it’s time for an engineer. For a fixed-fee crack inspection, call 01 270 9764.
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Frequently asked questions
How wide does a crack have to be before I worry?
As a rule of thumb, cracks over about 5 mm wide, BRE category 3 and above, need an engineer. So does any crack that is growing, tapering, stepped through blockwork, or accompanied by sticking doors and windows.
Can I just fill the crack?
Hairline shrinkage and thermal cracks can be filled and redecorated. Filling a crack that is still moving hides the evidence and it will reopen. If in doubt, monitor it or have it assessed first.
How do I monitor a crack myself?
Mark the ends of the crack with a pencil and the date, photograph it with a ruler beside it, and measure the width at the same point every few weeks. A cheap plastic tell-tale fixed across the crack does the same job more precisely.
Sources
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