Cosmetic, serviceability or structural?
The most widely used yardstick in the UK is BRE Digest 251, "Assessment of damage in low-rise buildings." It came from a study by BRE (the Building Research Establishment) of 130 properties that had suffered subsidence, and it sorts damage into six categories linked to crack width and the repair needed. In simplified form:
| Category | Typical crack width | What it means |
|---|---|---|
| 0 | Hairline, under about 0.1 mm | Negligible. No action required |
| 1 | Up to about 1 mm | Fine cracks treated with normal decoration |
| 2 | Up to about 5 mm | Easily filled; doors and windows may stick slightly |
| 3 | 5 to 15 mm, or several of about 3 mm | Needs opening up and patching by a mason; weather-tightness often affected |
| 4 | 15 to 25 mm | Extensive damage; sections of wall may need replacing; sloping floors and distorted frames |
| 5 | Over 25 mm | Structural damage needing major repair, with danger of instability |
BRE says that categories 0 to 2, with cracks up to 5 mm, can generally be regarded as aesthetic issues that need only redecoration. Categories 3 and 4 are serviceability issues, affecting weather-tightness and how doors and windows work. Category 5 is a stability issue likely to need structural intervention.
BRE also stresses this is a simplification. Whether cracks are getting wider over time can change the classification, and a building professional should be consulted where damage is significant.
What is subsidence?
RICS describes subsidence as the downward movement of a building's foundations because of changes in the supporting ground. It is different from settlement, which is the normal, usually one-off movement of a new building as it beds in. RICS lists three common causes:
- Shrinkable clay soil that dries out in a long dry spell or is drained of water by trees and shrubs, causing foundations to drop.
- Leaking drains or water mains that soften or wash away the soil, more typical on sandy or gravelly ground.
- Past mining, particularly historical coal mining, where old tunnels can eventually collapse.
RICS cites the consumer group Which? estimating that about 70% of subsidence cases are due to tree roots drawing moisture from shrinkable soil. That is a second-hand estimate, but it shows why trees close to a house matter on clay.
Signs that something may be moving
RICS says the first sign is usually visible cracking in one area of the house. Other signs include:
- new or expanding cracks in plaster or outside brickwork,
- doors or windows sticking for no obvious reason,
- wallpaper ripping at corners,
- new sloping or dipping of solid ground floors.
RICS adds that there is "rarely any cause for real concern unless cracks appear suddenly and are more than 3mm wide." Cracks may need to be measured and monitored, sometimes for as long as 12 months, to establish whether there is a problem.
Pattern matters too. Diagonal or stepped cracks that taper, cracks that line up above and below window and door openings, and cracks visible both inside and outside in the same place are all things a surveyor will look at closely. This is general surveying practice rather than a rule from the sources above.
Why buyers should care in a hot, dry year
BRE notes that insurance claims for subsidence have historically increased after periods of very dry weather. Clay ground shrinks when it dries and swells again when it is wet, so a house can show fresh cracks at the end of a dry summer. If you view a property in autumn, seasonal movement may be at its most visible. If you view in winter or spring, cracks can look better than they are. Ask the seller how long the cracks have been there and whether they have been repaired before.
What to do when you spot cracks
Before you offer
- Photograph cracks with a coin or ruler for scale and note where they are.
- Ask the seller about previous cracks, repairs, insurance claims, underpinning and tree work.
- Note large trees, drains and any signs of leaking pipes near the house.
Order the right survey
Cracks, extensions or older construction are good reasons to consider a Level 3 building survey over a Level 2. Our guide on Level 2 vs Level 3 surveys explains the difference, and a surveyor can tell you whether the cracking looks historic and stable or recent and active. Where a surveyor recommends it, you may also need specialist help such as a structural engineer, a drainage survey or monitoring over time.
Check insurance
RICS advises checking that buildings insurance covers subsidence. An existing claim history can affect cover and premiums. Ask your solicitor to check what the seller's insurer has disclosed.
Can subsidence be fixed?
RICS says subsidence can usually be rectified, but diagnosing and fixing it can take a long time, up to two years in some cases. Typical remedies depend on the cause:
- Tree roots: removing or pruning the tree, on expert advice, because very occasionally removal can make things worse.
- Leaking pipes: in most cases repairing the pipework is enough to stabilise the building without underpinning.
- Mining: can involve extensive ground stabilisation works and may be difficult to fix.
Local patterns to be aware of
Different parts of the Midlands and North West have different ground and housing stock. Clay soils, for example, are a theme in our Solihull property survey guide, while older homes in Birmingham and Manchester bring their own structural issues. Whatever the area, a local surveyor who knows the ground and building types can interpret cracks in context.
A quick buyer's checklist
- Measure and photograph every crack you see on viewings.
- Ask whether cracks have been repaired, monitored or insured before.
- Look for trees, leaking gutters and drains near affected walls.
- Choose the survey level that fits the age and condition of the home.
- Do not exchange contracts until the surveyor's advice on cracks is clear.
If you want help choosing the right survey, I AM Surveyors can advise on Level 2 and Level 3 surveys across the areas we cover. This article is general information, not advice on any particular property.
Sources
- BRE Group, "Assessing cracks in houses" (summarising BRE Digest 251, Assessment of damage in low-rise buildings): https://www.bre.co.uk/page.jsp?%2F1000=&id=3356
- RICS consumer guide, "Subsidence: A clear, impartial guide": https://www.rics.org/content/dam/ricsglobal/documents/consumer-guides/rics-consumer-guide-subsidence.pdf
Need advice on a property?
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