# Are Cracks in Walls Serious? | Widths, Patterns, When to Act > Are cracks in walls serious? Most aren't. Crack widths, the BRE Digest 251 categories, patterns that point to movement, and when to call an engineer. URL: https://structuralengineer.ie/guide/when-are-cracks-in-walls-serious/ Learn # Are Cracks in Walls Serious? How to Tell Crack width, pattern and whether it's moving: the three things that separate cosmetic cracks from structural ones. By StructuralEngineer.ie · Reviewed by Joachim Louw, CEng MIEI · Updated 19 October 2026 [StructuralEngineer.ie](https://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](https://structuralengineer.ie/tools/structural-crack-assessment/). ## 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](tel:+35312709764). Want a fixed fee for your job? Call 01 270 9764. Callback the same working day. Fixed fee confirmed in writing before the visit. [Call 01 270 9764](tel:+35312709764) [Request a fixed quote](https://structuralengineer.ie/contact/) ## 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 - [Engineers Ireland: Cracking and structural movement in Irish low-rise residential masonry buildings](https://www.engineersireland.ie/News/cracking-and-structural-movement-in-irish-low-rise-residential-masonry-buildings) - [Bank of Ireland: What is subsidence?](https://www.bankofireland.com/help-centre/faq/what-is-subsidence) - [Housing Agency: Pyrite Remediation Scheme](https://housingagency.ie/news-events/pyrite-scheme-extended-number-homes-remediated-reaches-2000) Get a fixed engineering fee Two short steps. We call back the same working day. Prefer to talk? [Call 01 270 9764](tel:+35312709764). A live person answers during office hours. [Related service Crack Inspections Diagnosis of cracks by category and cause: thermal, settlement, subsidence or heave. See the service](https://structuralengineer.ie/structural-crack-inspection/) Talk it through 01 270 9764 Mon–Fri 8:00–18:00 [Call now](tel:+35312709764) ### More guides - [Cracks in a New Build House: Normal or a Warranty Claim? New houses crack as they dry out. The question is which cracks are part of that, and which belong with the builder or the warranty provider.](https://structuralengineer.ie/guide/new-build-cracks-homebond/) - [Subsidence Insurance Claim in Ireland: How It Works What's covered, what the insurer's engineer does, why claims get disputed, and where an independent engineer fits.](https://structuralengineer.ie/guide/subsidence-insurance-claim-ireland/) - [Pyrite Remediation Scheme: Who Qualifies and How It Works A scheme of last resort for homes damaged by pyrite heave: where it applies, what evidence it needs, and where the engineer comes in.](https://structuralengineer.ie/guide/pyrite-remediation-scheme/) - [Structural Survey vs Building Survey: Which Do You Need? Two different inspections by two different professionals. When one is enough, and when you need both.](https://structuralengineer.ie/guide/structural-survey-vs-building-survey/)