Cracks in walls: which ones to worry about

Almost every house has cracks. Most are harmless and caused by normal movement in the building. A few are signs of a structural problem that needs attention. Knowing the difference saves both worry and money.

Why buildings crack

Buildings move. Materials expand and contract with temperature and moisture, new plaster shrinks as it dries, and the ground under a house moves with the seasons. Cracks appear where this movement is concentrated: at corners of windows and doors, where different materials meet, and where new work joins old.

Usually not serious

• Hairline cracks in plaster, less than about a millimetre wide, often in a random pattern (“map cracking”). These are usually shrinkage cracks in the plaster itself.
• Cracks at the junction between walls and ceilings, or between old and new work. These follow the joint and are caused by different materials moving differently.
• Fine cracks above door and window frames that do not grow over time.
• Cracks in screeds and cement floors that are fine and stable (see our post on cement floors).

These can usually be raked out, filled with a flexible filler and painted. Some may reappear and need a flexible treatment or a crack-bridging paint.

Worth getting checked

• Cracks wider than about 3 to 5 mm, or cracks you can fit a coin into
• Diagonal or stepped cracks running from the corners of doors and windows, especially when wider at one end
• Cracks that are growing, getting longer or wider over months
• Cracks visible on both sides of the same wall
• Doors and windows that start sticking for no other reason
• Floors that slope or have separated from the skirting
• Horizontal cracks in retaining walls, or walls that lean or bulge

These can be signs of foundation movement, structural overloading, a failed lintel or a problem with a retaining wall. They need an engineer.

Common causes in Cape Town

• Clay soils that swell when wet and shrink when dry, particularly in parts of the northern suburbs and the Cape Flats
• Leaking drains or pipes that soften the ground under foundations
• Tree roots close to the house, drawing moisture from the soil
• Stormwater discharged against the house
• Alterations where walls were removed without proper support
• Old houses with shallow foundations or no foundations at all

How to monitor a crack

If you are not sure whether a crack is moving, monitor it. Mark the ends of the crack with a pencil line and the date, and measure its width. Check every few weeks. A crack that grows is telling you something. Crack monitors, small plastic gauges fixed across the crack, make this easier.

What to do

1. Fix obvious causes: leaking pipes, blocked drains and stormwater pouring against the house.
2. Monitor any crack you are unsure of.
3. For significant or growing cracks, call a structural engineer, not a painter. The engineer will identify the cause and specify the repair.
4. Only once the cause is fixed, repair the crack and repaint.

Painting over a structural crack without fixing the cause just hides it for a while. If you have cracks you are worried about, talk to us and we can recommend an engineer and do the repairs.