Which cracks matter and which do not

Comparisons of cracks in buildings usually pick a winner. This one picks the circumstances, which is more useful.

Buildings move constantly

Materials expand and contract with temperature and moisture, and different materials do so at different rates. Where two materials meet — plaster and timber, an extension and the original wall — a crack is the predictable result. Almost every older building therefore has cracks, and their existence proves nothing on its own.

The useful questions are how wide, in what direction, where, and is it changing.

Direction is informative

Vertical cracks at a junction between two materials, or in plaster along a joint, are usually shrinkage or thermal movement. Diagonal cracks that run from the corners of windows and doors follow lines of stress and are the ones surveyors examine closely.

Horizontal cracks in masonry can indicate wall tie failure or lateral movement and are treated seriously. Stepped cracks following the mortar in brickwork indicate movement; cracks passing straight through bricks indicate more of it.

Width is the standard measure

Surveyors use classification systems based on crack width to describe severity, because width correlates with the amount of movement. Hairline cracks in plaster are routine; cracks wide enough to insert a coin are not.

Length is much less informative, since a long hairline crack may be a plaster joint and a short wide one may not be. Report language reflects these categories, which is why the exact wording repays reading rather than skimming.

Is it moving

A crack that has been filled and has reopened is telling you the movement continued. Monitoring with tell-tale gauges or repeated measurements over several months distinguishes historic from active movement. Seasonal cracking that opens in dry weather and closes in wet suggests clay soil behaviour rather than progressive failure.

Time is the only instrument that answers this question, which is why the answer takes months rather than a viewing.

Look at the whole building

Distorted door frames, sloping floors, gaps between skirting and floor, and doors that stick only in one season all form part of the picture. Movement affecting one corner or one elevation points at a localised cause such as a drain or a tree. Look outside as well as in, since the external brickwork shows movement more clearly than decorated plaster.

The useful part is this: fresh filler and paint in one small area of an otherwise tired room deserves a question.

What to do with a finding

A survey noting cracks and recommending investigation should be followed with a structural engineer report rather than a builder quote. The engineer establishes cause; the builder prices the remedy, and doing it in that order avoids paying for the wrong repair.

Where the cause is drainage or vegetation, the remedy may be inexpensive and the damage cosmetic. This is general information; a qualified surveyor or structural engineer should assess any specific crack.

Side by side

The takeaway

Measure the width, note the direction, and find out whether it is still moving before anyone quotes.

Pick the one that costs you least, and let the rest wait.