Cracks In Walls

What's happening?

Most cracks in a house are not structural. Plaster shrinks, timber dries, and a building moves a little with the seasons — all of which produce fine, straight cracks that stay the same width. Structural cracks look different. They tend to run diagonally, taper so they are wider at one end than the other, follow the mortar joints in a stepped line through brickwork, appear inside and outside at the same place, and get worse over months rather than settling down. Where a crack starts matters too: corners of windows and doors are where any movement in a wall concentrates, so a crack radiating out from an opening is telling you something about the wall, not about the plaster.

What usually causes it?

• Plaster shrinkage and thermal movement — fine, even-width cracks that do not change
• Lintel or arch above an opening deflecting or failing, producing a crack out of a window or door corner
• Foundation movement from clay shrinkage, a tree, a leaking drain or made-up ground — usually diagonal and tapering
• Wall tie corrosion in a cavity wall, which typically shows as horizontal cracking along the bed joints
• Loss of restraint where a wall is no longer properly tied to a floor or roof, so it bows and cracks
• A wall or chimney breast removed without adequate support, loading a beam or a wall that was never designed for it
• Sulphate attack or expansion in an old solid floor or hardcore pushing walls outwards near ground level

When should you call a builder?

Call a builder if the crack is wider than a coin edge, tapers, steps through the mortar joints, shows on both faces of the wall, or has grown since you first noticed it. Call urgently if it appeared suddenly, if there is a bulge or lean in the wall, if a lintel is visibly sagging, or if a floor has dropped at the same time. If your home insurance covers subsidence, tell your insurer before commissioning repairs, because they will normally want to investigate the cause first.

What will the builder actually do?

• Inspect the crack inside and out, tracing where it starts and finishes and what it passes through
• Open up locally where needed to see the lintel, the wall ties or the bearing that the crack points at
• Fit crack monitoring — tell-tales or studs measured over months — where the cause is not yet clear
• Bring in a structural engineer for a design where the wall or a support has genuinely failed
• Carry out the structural repair: replace or support the lintel, stitch the masonry with helical bars, renew wall ties or reinstate restraint
• Make good the brickwork, render and plaster only once the movement has been dealt with