The knuckle test
Tap the wall with a knuckle or the back of a screwdriver handle. Sound render gives a dull, solid note. Blown render rings hollow, because the bond between the render and the masonry behind it has broken and there is now a void.
Work across the whole elevation, not just the area that looks wrong. The patch you can see is almost never the full extent of it — we routinely find that a visible failure the size of a dinner plate has two square metres of hollow render around it.
What to look for
- Hairline cracking in a map or crazed pattern rather than a single straight line
- Bulging, most visible when you look along the wall from one side rather than straight at it
- Staining or green growth below a sill, a gutter joint or a downpipe
- Paint blistering or lifting in patches
- Damp patches appearing inside the house on the same wall
Why it happens
Water. It gets behind the render through a cracked sill, a failed gutter or downpipe joint, a gap at the verge or roof edge, or around a window frame where the sealant has gone. Once behind, it freezes and thaws through winter and breaks the bond. Older properties with cement render on soft brick are particularly prone, because the render is harder than what it is stuck to.
This is why a repair that does not deal with the source is a waste of money. The same patch fails again within a few winters.
What a proper repair involves
Cut back until the render sounds solid all the way round the edge of the opening. Deal with the cause. Build the render back up in coats to the original thickness. Float it to match the surrounding texture — this is the part that decides whether the repair is visible from the pavement for the next twenty years. Let it cure, typically three to four weeks. Then paint the whole elevation, not just the repair.
The two shortcuts to refuse
Filling a hollow area rather than cutting it out: the void is still there and so is the water. And painting only the repaired patch: even a perfect texture match shows as a different shade, because the rest of the wall has weathered.
Questions we get asked about this
Can blown render be repaired, or does the whole wall need doing?
Usually repaired. Whether it is a patch or a full elevation depends on how much sounds hollow, and that can only be established by tapping the whole wall.
Will a repair be visible afterwards?
Not if it is floated to match the existing texture and the whole elevation is repainted. Those two steps are what separate a repair from a patch.
How urgent is it?
Once render is hollow, water is already getting behind it. It will not improve on its own and it gets worse over a winter. It is not an emergency, but it is not something to leave for two years either.