The fastest response to blown render is to hack off the visibly detached patch and skim a fresh mix over it. If the new mix is the same hard, impermeable type that caused the original failure, and the water source that got behind it hasn't been addressed, the new patch will fail in more or less the same way, on more or less the same timescale.
A repair that actually lasts treats the render system and the water source as the two things that have to be right before a trowel goes anywhere near the wall.
The sequence a proper repair follows
- Cut back well beyond the visibly hollow area to genuinely sound, well-bonded render, since blown sections are often larger than what's already detached.
- Establish whether the wall is solid or cavity construction, and specify a render mix matched to that wall's need to breathe or not.
- Identify and resolve the source of moisture getting behind the render — a leaking gutter, damaged flashing, or a bridged damp-proof course — before any new render goes on, not afterwards.
- Prepare the background properly, ensuring it's clean, sound and correctly keyed for the new coat.
- Reinstate proper bell-cast and drip details at cills, openings and the base of the wall, so water is thrown clear rather than running back into the face.
- Apply the new render in the correct number of coats for the system specified, allowing proper curing time between coats rather than rushing the finish.
Why matching the repair to the wall's breathability matters so much
This is the single most important decision in a render repair, and it's the one most often skipped in a quick patch job. Applying a hard cement-based repair onto a traditionally built solid wall that needs to breathe recreates exactly the mechanism that caused the original failure — moisture gets trapped behind the new patch just as it did behind the old render, and the bond fails again. Matching a permeable, lime-based system to a solid wall, or a standard modern system to a cavity wall, is not a stylistic choice, it's the thing that determines whether the repair holds.
Why fixing the water source has to come first
Rendering over a wall that's still being fed water from above — a leaking gutter, a failed flashing, a poorly detailed cill — treats the visible render defect while leaving its actual cause running. New render applied under those conditions can fail within a single wet season, because the water that broke the bond in the first place hasn't stopped arriving.
Interfaces with other trades
Guttering, flashings and cill details often need attention alongside the render itself, since they're frequently the actual source of the water causing the problem — that work is best coordinated as part of the same job rather than treated as separate, since re-rendering under a still-faulty gutter wastes the render work.
Where render has been trapping moisture against timber framing or lintels, that timber needs checking for rot before the wall is closed back up with new render — covering damp or decayed timber with fresh render simply hides a problem that continues underneath.
How you know the repair has worked
A properly repaired area sounds solid when tapped, not hollow, and — over subsequent wet seasons — shows no fresh cracking or detachment at the repair's edges. Because the repair matches the render system to the wall and removes the water source, it should behave like the surrounding sound render rather than needing revisiting within a year or two.
Common questions
Can I patch just the blown section, or does the whole wall need re-rendering?+
It depends on how widespread the hollow areas are once the whole elevation is tap-tested. A single isolated patch on an otherwise sound wall can often be repaired locally; more extensive hollowness usually means a wider area needs attention to avoid repeated patch failures.
Why does render matter so much on older houses specifically?+
Older solid-wall houses were built to manage moisture by letting walls breathe. A modern, impermeable render or paint system fights against that, trapping moisture that the wall is otherwise designed to release, which is why breathability matters more on these properties than on standard modern cavity walls.
What happens if the water source isn't fixed before re-rendering?+
The new render is very likely to fail again in a similar way, because the moisture that broke the original bond is still getting in. Fixing the water source — a gutter, flashing or cill detail — is not optional if the repair is meant to last.
Request a written quote
We identify the water source and the right render system for your wall before quoting, so the repair is built to hold.