How to repair cracks in plaster and render walls

Step-by-step procedure

1. 1

   Classify the crack

   Measure the width and note the pattern. Map cracks are shrinkage. Straight vertical cracks at joints are differential movement. Diagonal cracks from window corners, stepped cracks in masonry, or cracks that pass through structural elements need engineering assessment first.

2. 2

   Monitor if in doubt

   Fix a paper or glass telltale across the crack, or mark and date the ends. Re-check after two to four weeks. A live, moving crack must be treated as a joint, not filled rigidly.

3. 3

   Open the crack

   Cut a shallow V or U groove 5–10 mm wide along the crack with a crack chaser. Remove all loose, hollow and debonded plaster back to sound material and vacuum out dust.

4. 4

   Prime

   Apply a bonding primer into the groove so the filler does not lose water to the substrate and shrink back.

5. 5

   Fill in layers

   Press flexible filler firmly into the groove in layers no thicker than the data sheet allows, letting each layer set. Overfilling in one pass causes shrinkage and a visible line.

6. 6

   Reinforce wider cracks

   For cracks over 1 mm or across dissimilar materials, bed fibreglass mesh tape into a wet coat spanning at least 100 mm each side, then feather out.

7. 7

   Skim and feather

   Apply a skim coat over the repair, feathering 200–300 mm beyond the tape so the patch does not read as a raised band under grazing light.

8. 8

   Sand, spot-prime and finish

   Sand flush when dry, spot-prime the repair to equalise porosity, then paint or coat the full wall plane — never just the patch — to avoid flashing.

When to convert a crack into a joint

Cracks that reopen after repair are telling you the wall moves. Cut them out and install a proper sealant-filled movement joint with a bond breaker rather than repeating a rigid repair.