You notice it by tapping the wall — a section gives a dull, hollow note rather than the solid sound of render still bonded to the substrate, and sooner or later that patch cracks at the edges and detaches, often around a window reveal or corner where the render was thinnest to begin with. Once one patch has gone, it's rarely the only one; the same underlying cause is usually present across a wider area than what's already visibly failed.
Render fails by losing its bond to the wall behind it, and in the vast majority of cases the reason is moisture that got trapped where it shouldn't have been, rather than the render simply wearing out with age.
What the hollow sound and blown patches actually mean
A solid tap means the render is still adhered to the wall. A hollow tap means an air gap has formed behind it — the bond has failed even though the render surface might still look intact. That gap almost always started as moisture sitting behind the render, either because it got in and couldn't dry out, or because it was trapped there from the moment the render went on.
The likely causes
Cement render applied to a solid wall that needs to breathe is one of the most common causes on older properties. Traditional solid brick or stone walls manage moisture by letting it pass through and evaporate; a hard, impermeable cement render sitting on that kind of wall traps moisture behind it instead of letting it out, and the trapped moisture eventually breaks the bond.
Trapped moisture behind an impermeable coating more broadly covers any situation where a modern paint or render system that doesn't allow the wall to breathe has been applied over an older, more permeable wall — the mechanism is the same even where the render itself isn't cement-based.
Poor background preparation at the time of application — render going onto a dusty, wet, or otherwise poorly keyed surface — means the bond was never fully sound to begin with, and failure is really just a matter of time and weather exposure.
Failed or missing bell-cast and drip details are a specific and often overlooked cause: without a proper drip edge, particularly at the base of a wall or under windows, water runs back into the wall face rather than being thrown clear, and that concentrated wetting accelerates local bond failure right at those points.
Checks that separate one cause from another
- Establish whether the wall is solid or has a cavity, since that changes which render systems were ever appropriate for it.
- Trace where damp is likely entering above the render — a leaking gutter or damaged flashing feeding water down behind a render section is a very common and fixable contributing cause.
- Compare the hardness and finish of any patch repairs against the surrounding original render — a harder mix used to patch an older, softer original render is itself a cause of localised failure around the patch.
- Tap systematically across the whole elevation rather than just around visibly failed patches, to establish how widespread the hollow sections actually are before deciding on a repair scope.
Why this shouldn't just be left
Blown render is a physical hazard on its own — sections can and do detach and fall, particularly after wet weather followed by frost. Beyond that immediate risk, the trapped moisture behind failed render keeps working on the wall itself for as long as it's left, which on a solid wall can eventually affect the masonry or timber frame behind it, not just the render finish.
Common questions
Is hollow-sounding render always going to fall off?+
Not immediately, but a lost bond doesn't repair itself — the affected area will continue to deteriorate and detach eventually, usually accelerated by frost and wet weather, so it's worth addressing rather than waiting for it to fall.
Why does render fail more on old solid-wall houses?+
Because those walls were designed to manage moisture by breathing, and a hard, impermeable render or paint system stops that happening — the trapped moisture then works to break the bond from behind rather than evaporating through the surface as it should.
Can I just patch the blown sections myself?+
You can, but if the same cause — the wrong render type, a missing drip detail, or an ongoing water source above — isn't addressed, the patch is very likely to fail again in the same way, sometimes within a single season.
Book a free render survey
We tap-test the full elevation, trace where moisture is entering, and give you a written quote matched to the actual cause.