How do I repair cracked or blown render?
Finding the full extent of the failure by tapping, cutting back to sound material, and matching a patch in — plus the honest test for whether patching is worth doing at all.
- Difficulty
- advanced
- Time
- 3 hr
- Read
- 5 min
- Safety
- warning
Short answer
Tap the whole area with your knuckles or a light hammer — hollow means it has debonded, and the hollow area is always larger than the visible damage. Cut back to sound render with a square edge, prepare and prime the background, and build up in coats no stronger than the background, finishing to match. Repair the water that caused it first.
Do this first
A render patch fails for one of two reasons: the failed area was not fully removed, or the cause was not dealt with. Render very rarely fails on its own — it fails where a wall gets wet, where the background moves, or where the original render was stronger than what it was applied to. Repairing the symptom without finding that out simply moves the problem along by a couple of years.
Safety
What you'll need
- Proper access — a tower or scaffold, not a ladder
- Eye protection, gloves and a dust mask
- A club hammer and a bolster, and a brick hammer
- Render matched to the existing system and background
- An SBR or proprietary bonding agent and a brush
- A hawk, a rendering trowel, a straight edge and a scratching comb or float
- Beads and stainless fixings if edges or stop details are involved
Step by step
- Find the water first.A leaking gutter, a blocked downpipe, a failed sill or a missing drip, ground level above the damp proof course, or a cracked coping. Render almost never fails in a dry, well-detailed location. Repairing without dealing with this books the repair again.
- Tap the whole elevation, not just the damage.Knuckles or the handle of a screwdriver, systematically across the wall. Sound render rings solid; debonded render sounds hollow and drummy. Mark the hollow areas with chalk. They are almost always considerably larger than what looks damaged.
- Decide honestly between patching and replacing.A small number of discrete hollow areas is a patching job. Hollow across the majority of an elevation, or render that is stronger than a soft background and coming away in sheets, means the elevation needs taking off. Patching a failing whole is a waste of money.
- Cut back to sound material with square edges.Hack off all the hollow render and cut the perimeter square with a bolster or a shallow cut, rather than feathering the edge out. A feathered edge is thin, weak and always cracks. Undercut very slightly if you can, so the patch is keyed in.
- Look at what is behind it.This is when you find out what the wall really is and whether the original render was the wrong material for it. Check the background is sound, that the joints behind are not decayed, and that the masonry is dry.
- Let a wet wall dry before covering it.If the render came off a saturated wall, the wall has to dry before it is covered again, which can take months. Rendering over a wet wall seals the water in and the patch fails.
- Prepare and prime the background.Brush out and rake back any loose joints, remove dust, wet the background, and apply a bonding coat — a cement and SBR slurry brushed in, a spatterdash coat, or the proprietary primer for the system. Apply the base coat while the bonding coat is still tacky.
- Build up in coats, weakest at the front.Base coat, scratched to key with a comb in a diagonal pattern once it has firmed, allowed to set, then the finish coat no stronger than the base. Do not try to make up the full thickness in one pass — it slumps, shrinks and cracks.
- Match the material, not just the look.Lime render is patched in lime. A monocouche or thin-coat system is patched with that manufacturer's products. Patching a lime render in cement produces a hard spot that damages what is around it.
- Repair a crack by cutting it out, not filling it.Cut the crack out to a square-sided channel, prime it, and fill it with the render material in layers, finishing to match. For a crack from movement rather than shrinkage, a reinforcing mesh bedded across the repair in the base coat is what stops it reopening.
- Accept that the patch will show.A patch in a textured or coloured render is very difficult to hide, because texture and colour both vary. On a visible elevation, taking the render back to a natural break — a corner, a downpipe, a bead — and redoing the whole panel gives a far better result.
- Protect it while it cures.Damp, shaded and covered, held clear of the surface, for several days on a cement render and considerably longer on lime. Render that dries too fast crazes and debonds, which is how the problem started.
If it doesn't work
The patch cracked round the edge within months
Cause: The edge was feathered rather than cut square, or the failed area was not fully removed — Fix: Cut back further, square the perimeter, and check by tapping well beyond the visible damage before starting again.
The new patch rings hollow
Cause: No bonding coat, a dusty background, or render applied to a background that had dried out and sucked the water out of it — Fix: Take it off, wet the background, apply an SBR slurry and render onto it while it is tacky.
The crack has reopened in exactly the same line
Cause: It is a movement crack rather than a shrinkage crack, so filling it does nothing — Fix: Establish what is moving. A reinforcing mesh across the repair handles small movement; anything structural needs assessing first.
The render is coming off a soft old wall in large sheets
Cause: Cement render on a background much weaker than itself — Fix: This is a specification failure, not a repair. The elevation needs the render removing and a lime render, or no render, in its place.
The patch is a different colour and texture
Cause: Unavoidable to some degree with textured and through-coloured renders — Fix: Take the repair to a natural break and redo a whole panel, or accept it and plan to paint the elevation if the system allows it.