Filling Cracks in Walls

1. 1

   Check the crack: its width, direction and length, and whether it also shows outside.

2. 2

   Rake out: the crack is opened into a V shape so the filler can grip, and loose material is removed.

3. 3

   Clean and prepare: dust is brushed out and the crack is dampened or primed, as the filler maker recommends.

4. 4

   Fill: the filler is pressed firmly into the crack and left slightly proud. Deep cracks are filled in layers so the filler does not slump or shrink.

5. 5

   Tape recurring cracks: a crack that has opened before is bridged with scrim or fibreglass mesh tape and skimmed over, so small movements do not show through.

6. 6

   Sand and seal: once dry, the repair is sanded flush and spot-primed, so it does not show as a dull patch under the painting.

There is no single best filler. The right one depends on the crack:

A crack that reappears is usually still moving, or was filled with the wrong product. Common reasons are:

• New plaster drying out: Shrinkage cracks are common in new builds and extensions, and often settle once the plaster and timber have dried.
• Seasonal movement: Timber, plaster and masonry expand and contract with heating, temperature and humidity.
• Clay soils: Many parts of London sit on clay, which shrinks in dry summers and swells in wet winters, so fine cracks can open and close through the year.
• Untaped joints: Cracks along plasterboard joints return if the joint was never taped.
• Rigid filler in a moving crack: A hard filler cannot move, so it cracks again at the edges.

If a crack keeps widening, filling it again is the wrong answer.

Usually not. Most cracks are cosmetic. These signs, however, suggest movement that should be checked before anything is filled:

• Diagonal cracks running from the corners of doors and windows, especially if they are wider at the top.
• Cracks wider than a few millimetres, or cracks that reopen and grow after filling.
• Cracks that also show on the outside wall, often stepped through the brickwork.
• Doors and windows that start to stick for no obvious reason.

To monitor a crack, mark its ends in pencil with the date and check it over the following weeks. Only a structural engineer or surveyor can confirm subsidence, and you should tell your buildings insurer if it is suspected. If foundation work is recommended, it is carried out by our underpinning contractors; see our piling and underpinning page for the options.

Homes built before about 1960 may have lead-based paint under later layers. It must not be dry-sanded, so where we suspect it, we test first and use wet, dust-controlled methods. Artex and other textured wall coatings applied before 2000 may contain asbestos, so we do not sand or scrape a textured wall until a sample has been tested.