Cracks · checked 29 October 2026
Subsidence cracks and the BRE scale
How engineers grade cracks from 0 to 5, the patterns that point to subsidence, and when to get help quickly.
Facts on this page checked against each source on 29 October 2026. Each one links to its source. How we check
Engineers grade cracks with the six damage categories in BRE Digest 251, from 0 for hairline cracks to 5 for cracks over 25 mm. Categories 0 to 2, up to about 5 mm, are aesthetic. 3 and 4 affect weather-tightness and how doors and windows work. 5 is a stability issue that usually needs structural work (BRE).
Irish engineers use the same scale: an Irish engineering consultancy sets out the three groups in its guidance on cracking (ORS), and a member of the Society of Chartered Surveyors Ireland cited it answering an Irish Times reader (The Irish Times, 2019).
The BRE crack scale
- 0Hairline, under about 0.1 mmNegligible. No action needed.
- 1Fine, up to about 1 mmTreated with normal decoration. Usually internal finishes only.
- 2Up to about 5 mmEasily filled. Some external repointing may be needed. Doors and windows may stick slightly.
- 35 to 15 mm, or several of about 3 mmSome opening up and patching by a mason. Doors and windows stick, service pipes may fracture.
- 415 to 25 mm, depending on numberSections of wall broken out and replaced. Frames distorted, floors sloping noticeably, walls leaning or bulging.
- 5Over 25 mm, depending on numberMajor repair, partial or complete rebuilding. Walls lean badly and need shoring. Danger of instability.
0 to 2: aesthetic3 and 4: serviceability5: stability
Source: BRE, Assessing cracks in houses, setting out the categories in BRE Digest 251. BRE stresses this is a simplification: whether cracks are widening over time also matters.
Width isn't everything
- Is it growing? BRE says whether crack widths are increasing over time can change the category (BRE). A crack that is still opening matters more than a wider one that has stopped.
- How many? Category 3 includes several cracks of about 3 mm, not only one of 5 to 15 mm (BRE).
- The pattern. Crack width is a strong indicator, and the pattern, location and age of the cracks help an engineer work out the cause (ORS).
Patterns that point to subsidence
Look at the top corners of windows, inside and out. Cracks running at about 45 degrees suggest movement that needs investigating (Irish Examiner, 2021). A chartered building surveyor told The Irish Times that subsidence cracking is usually more severe and noticeable than settlement, and that the location and pattern of the cracks tell the two apart (The Irish Times, 2020).
Cracks that usually aren't subsidence
| What you see | Common cause | Source |
|---|---|---|
| Fine cracks in paint, plaster or tiling | Drying and shrinkage of the finishes themselves | ORS |
| Cracks in a gable near the eaves after attic work | Roof members cut, pushing the walls out at wall-plate level | Irish Times, 2019 |
| New cracks while a site next door is being dug | Vibration from heavy excavation | Irish Times, 2019 |
| Cracks in a new house in its first year | Settlement, which usually stops after six to 12 months | Irish Times, 2020 |
| Floors and walls cracking in a 1997 to 2013 house | Possibly pyrite heave under the floor slab | C&AG |
How to monitor a crack
Photograph each crack with a ruler beside it and write the date. Repeat every few weeks, from the same spot. A building surveyor told The Irish Times that a crack-monitoring device fixed across a crack for six to 12 months shows whether it is getting bigger, and if it is, the movement is recent (The Irish Times, 2020). Your engineer may fit monitors and read them.
When to get an engineer out quickly
BRE's category 5 describes beams losing their bearing, walls leaning badly and needing shoring, broken windows and a danger of instability, with cracks typically over 25 mm (BRE). ORS describes stability damage, such as a lintel losing its bearing or a wall losing its buttressing, as needing short-term intervention because of an unacceptable risk to the safety of people in or around the building (ORS). If you see that, get a chartered structural engineer out without waiting.
Repairing the cracks
Cracks get repaired last, once the cause has been fixed and the building has settled on its new support: six months to a year, according to an Irish engineer (Irish Examiner, 2021). ORS describes two methods: for external cracks at serviceability level, removing the render locally, injecting the crack with low-viscosity epoxy resin and re-rendering with expanded metal mesh; for severe cracks, stitching with stainless steel helical bars set into the mortar joints (ORS).
Start with a chartered structural engineer. Nobody can tell from a photo or a phone call whether a house needs underpinning. An engineer looks at the cracks, the ground and the drains, then decides. This site never tells anyone their house is safe or unsafe.
Questions people ask
What size crack is serious?
On BRE's scale, cracks up to about 5 mm are aesthetic, 5 to 25 mm are serviceability damage, and over 25 mm is a stability issue. A crack that keeps widening matters more than its size today.
Do hairline cracks need an engineer?
BRE's category 0, hairline cracks under about 0.1 mm, is negligible and needs no action. Get advice if they keep growing, multiply, or run diagonally from the corners of openings.
How do I tell subsidence cracks from settlement cracks?
Subsidence cracking is usually more severe and noticeable, and diagonal from the corners of windows and doors. A surveyor or engineer tells them apart from the location and pattern, and monitoring shows whether they're still moving.
Can I fill subsidence cracks?
Fill them last. Irish engineers repair cracks six months to a year after the cause has been fixed, once the building has settled.
Where does the BRE crack scale come from?
BRE assessed 130 properties that had suffered subsidence and published the categories in BRE Digest 251, Assessment of damage in low-rise buildings.
Sources (1)
Prices and facts on this page were checked against these published pages. We are not connected to these businesses.
- ORS: ors.ie
Get it looked at properly
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