Field guide
The crack width scale, 0 to 5
BRE Digest 251 sorts visible damage to walls into six categories, judged mainly by how easy the damage is to repair. Each category has a typical crack width. Width alone does not set the category: the number of cracks, where they are, whether they are growing and what else is happening all count.
| Category | Typical width | In inches (about) | BRE group |
|---|---|---|---|
| 0Negligible | less than 0.1 mm | under 0.004 in | Aesthetic |
| 1Very slight | up to about 1 mm | up to about 0.04 in | Aesthetic |
| 2Slight | up to about 5 mm | up to about 0.2 in (about 3/16 in) | Aesthetic |
| 3Moderate | 5 to 15 mm, or several cracks over 3 mm | 0.2 to 0.6 in | Serviceability |
| 4Severe | 15 to 25 mm, also depends on how many | 0.6 to 1 in | Serviceability |
| 5Very severe | usually over 25 mm, also depends on how many | over about 1 in | Stability |
Aesthetic, 0 to 2
BRE: cracks up to about 5 mm that usually need only redecoration.
Serviceability, 3 and 4
BRE: damage that affects weather-tightness and the way doors and windows work.
Stability, 5
BRE: damage likely to need structural work.
How to measure
- Find the widest point of the crack. Note it with a pencil mark and the date if you can.
- Hold the printable card against it. The widest bar that fits inside the crack is its width. A ruler or a set of feeler gauges works too.
- Note which way it runs, whether it tapers, and whether one side sits lower or sticks out.
- Look around: doors and windows that stick, sloping floors, a wall that bows, water stains.
Growth matters
BRE notes that whether crack widths are increasing over time can change the classification, and InterNACHI notes that a clean crack suggests recent movement while a dirty or previously filled one may be inactive. A single viewing cannot show growth: ask the seller, and ask your inspector.
Inch figures are conversions (1 in = 25.4 mm), rounded. A reading of published guidance, not an inspection. Buying Crack Book cannot see the house and never tells you a house is fine. Hire a qualified home inspector (licensed where your state requires it), and a structural engineer when the reading says so.