Part of the guide: The Construction Defect Field Guide: 40 Defects by Trade, With Photos
Walk a new house a few months after handover and there will be a crack leaving the corner of a door frame at about forty-five degrees. The buyer has photographed it; the site manager has called it shrinkage; someone has said the word "subsidence" out loud. Nearly always the site manager is right, and occasionally the other person is, and the job of the inspection is to say which, with evidence, not reassurance.
What does a diagonal crack above a door look like?
A single crack starting at the top corner of a door or window opening and running up and away from it at roughly 45°, usually 150–400 mm long, in plaster, plasterboard or blockwork. Sometimes a matching crack leaves the other corner. In plaster on blockwork it is a fine, slightly ragged line; in plasterboard it is straighter and often follows a board joint away from the corner of the frame.
Three things to notice before you write anything down: whether it is on one face of the wall or both, whether the crack is the same width along its length or wider at one end, and whether the edges are level with each other or one side stands proud. Those three observations do most of the diagnostic work below.
Why do cracks start at the corners of openings?
An opening is a hole in a wall that wants to be solid. Every stress in the wall, drying shrinkage, thermal movement, the load from above, the wall settling onto its foundation, has to go around the hole, and it concentrates at the corners. The corner is also where the lintel bears on the wall, where the frame is fixed, and where the plasterer had the most awkward junction to finish. It is the weakest point on the wall by design, so it is where the wall lets go first.
In a new building the dominant stress is shrinkage. Concrete blocks, mortar, plaster and screed all lose water for a year or more after they are laid, and they shrink as they do. A long wall shrinking by a millimetre has to put that millimetre somewhere; it puts it at the openings. Heating the building for the first winter accelerates it, which is why the crack that was not there in September is there in February.
Movement produces the same crack from a different cause: a foundation settling unevenly, a lintel deflecting under load, or ground shrinking under a corner of the building in a dry summer. The wall rotates slightly and the opening corners tear.
Shrinkage or settlement: how do you tell?
No single check settles it; six together usually do.
| Check | Points to shrinkage | Points to movement |
|---|---|---|
| Width | Under 1 mm, occasionally up to 2 mm | Over 3 mm, or still widening at re-measure |
| Taper | Same width along its length, or fading out | Clearly wider at one end, usually wider at the top when a corner of the building is dropping |
| Both faces? | One face only; nothing on the other side of the wall | Matching crack on the other face, or visible outside |
| Path | Straight or slightly ragged through plaster | Stepped through mortar joints in the masonry behind |
| Displacement | Edges level; a fingernail runs across without catching | One side proud of the other, or the crack has opened in shear |
| Company | A single crack, or a few small ones at other openings | Doors binding, gaps opening at skirtings, cracks that line up across floors, a sloping floor |
Shrinkage cracks also have a timetable: they appear in the first six to eighteen months, grow a little through the first heating season, and stop. Movement cracks do not stop, and the ones caused by clay shrinkage under foundations are seasonal: wider in late summer, narrower in winter, which is why a single visit cannot always answer the question and monitoring exists.
How wide is too wide?
UK surveyors use the BRE crack classification, and it is a useful vocabulary anywhere: hairline (under 0.1 mm), fine (up to 1 mm), slight (1–5 mm), moderate (5–15 mm), severe (15–25 mm) and very severe (over 25 mm). Hairline and fine cracks at openings in a new building are decoration. Slight cracks are repairable once static but need watching. Anything moderate or above, or any width with a step across it, is an engineer's question and the report should say so without hedging.
Measure with a crack width gauge, a plastic card with printed lines, costs a few pounds, lives in every inspector's pocket: and photograph the gauge on the crack at its widest point. A coin or a ruler is the fallback; "about a millimetre" from memory is not a measurement.
How do you report it?
The item has to survive being read by a contractor who wants to fill it tomorrow and a client who has just searched for "subsidence" on their phone. It does that by recording observations and saying what would change the conclusion, not by delivering a verdict the inspector is not qualified to sign.
- Location: room, which opening, which corner, which face of the wall.
- Observation: length, width at the widest point with the gauge, direction, taper, whether the edges are level, and what the other face of the wall shows.
- Context: the doors in the room operate normally; no other cracks in the house line up with it; or the opposite, if that is what was found.
- Severity: medium for a fine or slight crack with no other signs, it is not cosmetic until it has been shown to be static, and high for anything wider, stepped, displaced or accompanied.
- Action: monitor and re-measure at a stated interval; then either fill and decorate once static, or refer to a structural engineer. Write both branches in the item so nobody has to come back to you to ask.
The box at the end of this article is the full line. It is deliberately not "monitor" on its own: monitoring without a date, a measurement and a threshold is a way of not deciding.
How to monitor a crack properly
- Draw a short pencil line across each end of the crack and write the date beside it. If the crack grows past the line, it has moved; no equipment needed.
- Photograph the gauge on the widest point. Note the reading in the item, not just in the photograph.
- Re-measure at four weeks, and again at twelve if there is any doubt. For a suspected clay-shrinkage problem, the honest interval is a full year, because the crack will open and close with the seasons.
- Check the other face of the wall and the outside each time, not just the first time.
- If it needs more than pencil marks, a crack over 5 mm, a dispute, or a warranty claim, fit a proper tell-tale (a two-part calibrated gauge screwed either side of the crack) and let the engineer read it.
The pencil-and-date method is on the site inspection checklist, and the field guide entry for this crack has the short version of everything above, with its cousins, stepped cracking in masonry and cracks at the wall-to-ceiling junction: immediately after it.
What the fix should be
For a shrinkage crack, once it has stopped: rake it out to a small V, fill with a flexible filler (or scrim tape and filler on plasterboard), sand and redecorate. Do it after the first heating season, not before. Filling a moving crack is a way of hiding the evidence for a few months.
For a crack that has moved: nothing cosmetic until an engineer has seen it. The repair might be as simple as re-bedding a lintel or as involved as underpinning a corner, and the filler is the last step of either. A report that lets a contractor decorate over a movement crack has failed the client twice: once now, and once when the crack comes back through the new paint after the defects period has closed.
If you photograph and write these up for a living, the snagging inspectors' page shows how the measurement, the two photographs and the two-branch recommendation go into the report from the landing, while you are still standing in front of the door.
Questions people ask
Are diagonal cracks always serious?
No. In a new building most diagonal cracks at openings are drying shrinkage in plaster or blockwork, appear in the first year, stop at about a millimetre and never move again. The serious minority are the ones that keep widening, step through the masonry, show on both faces of the wall, or come with doors that have started to bind.
How wide does a crack have to be before it is structural?
Width alone does not decide it, but it sets the alarm level. Under 1 mm is fine; 1–5 mm is slight and repairable with filler once it has stopped moving; over 5 mm, or any crack with a step across it, needs a structural engineer's opinion before repair. That is the scale UK surveyors use (the BRE categories), and it is a sensible one anywhere.
Should I fill a crack in a new build straight away?
Not in the first months. A crack filled while the building is still drying re-opens through the filler and you have lost the record of whether it moved. Mark the ends, date it, wait through a heating season if you can, then fill once it is static. A crack that is still widening is telling you something the filler would hide.
Are shrinkage cracks covered by the new-home warranty?
Usually not, if they are small. UK warranty providers treat fine drying-shrinkage cracks, typically up to about 2 mm, as normal and the homeowner's to redecorate; wider cracks, or any caused by movement, are the builder's during the defects period. A dated, measured, photographed record from a snagging inspection is what turns a borderline crack into a claim rather than an argument. When to book one covers the timing.