Structural Crack Assessment

When is a crack in a wall serious?

Most cracks in walls are harmless — plaster shrinkage, seasonal movement, a house doing what old houses do. A minority are the first visible sign of something structural. The problem is that owners can't reliably tell the difference, and neither can builders quoting for repairs. A structural crack assessment settles it: an engineer inspects, explains the cause, and grades the damage against the recognised categories used across the industry.

What the assessment tells you

• Cause — shrinkage, thermal movement, lintel failure, wall-tie corrosion, overloaded structure, drainage or tree-related ground movement.
• Severity — graded against the standard damage categories (hairline to severe), which is the language insurers, lenders and other engineers all speak.
• Progression — whether the cracking is historic and stable or active, with monitoring recommended only where genuinely needed.
• The fix — a proportionate repair specification: often repointing or resin repair and redecoration; occasionally lintel replacement or tie repair; rarely anything dramatic.

The BRE crack damage categories: 0 to 5

UK practice grades visible cracking against the six categories set out in BRE Digest 251 — and knowing the scale in advance demystifies the report you will receive:

• Category 0 — negligible: hairline cracks under about 0.1mm; no action needed.
• Category 1 — fine: up to about 1mm; dealt with during normal decoration.
• Category 2 — slight: up to about 5mm; easily filled, perhaps with some external repointing to keep weather out.
• Category 3 — moderate: roughly 5–15mm, or several cracks around 3mm; repointing and possibly some brickwork replacement; doors and windows may stick.
• Category 4 — severe: roughly 15–25mm; sections of wall may need breaking out and replacing; frames distorted, floors sloping noticeably.
• Category 5 — very severe: above about 25mm; structural repair or partial rebuilding; beams can lose bearing and stability is in question.

Two caveats the categories themselves insist on: classification turns on ease of repair rather than crack width alone, and most domestic cracking sits in categories 0–2 — the scale exists precisely to stop every crack being treated as a catastrophe.

How the engineer assesses a crack on site

The inspection reads far more than width. Direction and pattern — vertical, horizontal, diagonal, stepped through mortar joints — point to different mechanisms. Taper matters: a crack wider at the top than the bottom, or vice versa, indicates which part of the structure is rotating or dropping. Matching cracks inside and out distinguish through-wall structural movement from surface finishes; paint or old filler found inside a crack dates it; and levels, plumb checks and door reveals measure the distortion the cracking merely advertises. Set against the property's age, construction, soil and alteration history, the pattern usually tells a coherent story — and the report tells it back in plain English with the damage category attached.

Causes that have nothing to do with subsidence

Ground movement is the famous cause, not the common one. Failed or corroded lintels crack the masonry above windows and doors. Wall-tie corrosion in older cavity walls produces regular horizontal cracking along bed joints as the rusting ties expand. Thermal and moisture movement cracks long walls, parapets and chimneys — especially where old structures meet newer materials. Roof spread pushes the tops of walls outward. And differential movement writes its signature crack down the junction where extensions and bays meet the original house. Each has a specific, usually modest remedy — and none of them is helped by underpinning, which is why cause comes before cure in every report.

When to get cracks assessed

Before you sell (a graded engineer's report defuses the buyer's survey), before you buy (it converts “cracking noted” into a price), when cracks reappear after redecoration, when doors and windows start sticking alongside new cracking, and whenever a builder proposes structural repairs from a visual glance — second opinions are cheaper than unnecessary underpinning. Diagonal cracking stepping through brickwork joints, cracks wider at the top than bottom, or cracking paired with sloping floors justify a prompt look.

What repair looks like at each level

For categories 0–2, redecoration and repointing once any active cause — a leaking gutter, a failed drip detail — is dealt with. Category 3 usually means masonry repairs: resin or helical-bar stitching, lintel replacement or wall-tie renewal, specified to suit the diagnosed cause. Categories 4–5 call for engineered structural repair, and at that point the assessment typically escalates into a full movement investigation so the repair follows evidence. In every case the sequencing rule holds: stabilise or remove the cause first, repair the symptom second — money spent the other way round is spent twice.

Fees and timescales

Fixed-fee inspection and written report, typically within 5–7 working days — faster during a transaction. If wider movement is suspected, the assessment rolls into a full subsidence investigation with credit for the initial visit.

Send photos of the cracks — get a fixed quote →