Can cracked render just be painted over?

No. Paint bridges a crack for a season and then splits along it, and a waterproof coating seals moisture behind the render instead of keeping it out. Cracks are raked out and filled, and debonded areas are removed and re-rendered.

How do I know if render has blown?

Tap it. Sound render gives a solid note, debonded render sounds hollow and drummy, and the hollow area normally has a clear edge. Cracking plus a hollow note over the same patch means the render is holding on by its edges.

Why did damp appear inside after the house was rendered?

Usually because the wall lost its ability to dry outward. A dense cement render slows evaporation, and any water entering at a crack or over the top edge then leaves through the inside face instead, showing as a damp patch or blown plaster.

Should render go all the way down to the ground?

No. Render taken down past the damp proof course bridges it, so ground moisture rises behind the render into the wall. It should stop above that line, with a bell cast or drip so water is thrown clear rather than running down the face.

Caution

Updated 30 August 2026

Hollow Or Cracked Render Outside

External render sounds hollow when tapped, is cracked, or is coming away in patches from the wall.

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What's happening?

Render is a weather coat, and it only works while it is continuous and stuck down. Blown render has debonded from the masonry, so there is a void behind it; cracked render has an entry point. Either way, water gets in behind the render, and once it is there a cement render is very good at stopping it getting out again. That is why a rendered wall can be wetter than a bare one: the wall absorbs water through the cracks, then dries slowly because the render is less porous than the brick or stone it covers. The tell is the same as inside — tap it. A hollow drummy note over a patch, cracks that run vertically or in a map, and staining or moss below a crack all point at the same thing. Approved Document C is the reference for how an external wall is expected to resist moisture.

What usually causes it?

A render mix stronger than the wall behind it, which cracks rather than moving with the masonry
Applied over a dusty, painted or unprepared background, or without a proper key, so it never bonded
Water entering at a crack, a cill, a bell cast or the top edge, then freezing and pushing the render off
Render carried down over the damp proof course or to the ground, bridging it and staying permanently wet
Cement render on an older solid wall that needs to breathe, trapping moisture behind an impermeable skin
Movement in the wall — settlement, a lintel, or a junction with an extension — cracking a rigid coating
Failed masonry paint or a waterproof coating over the render, blistering and holding water underneath

Is it dangerous?

Two risks, and the first one surprises people. Debonded render falls, and it falls in sheets from height onto paths, doorways and cars; a hollow patch above a front door is a reason to act rather than monitor. The second is damp. Water held behind render wets the wall for far longer than rain on bare masonry, which shows up as internal damp patches, blown plaster, and wet embedded timber. On a solid wall, the Property Care Association's penetrating damp guidance is the starting point, and the render is usually the mechanism rather than the victim. Removal is also work at height on a brittle material, which is why access is planned rather than improvised off a ladder.

Can I fix it myself?

Surveying it, from the ground and from a safe platform only. Tap what you can reach, mark the hollow areas, and follow each crack to see where water is entering above it. Check whether the render runs down to the ground, because that single detail explains a great many damp walls. Small cracks can be raked out and filled with a compatible repair mortar. What does not work is painting a cracked render with a waterproof coating — that seals the surface, leaves the water behind it and blisters — and neither does patching a hollow area without removing it. Working at height on render is where DIY on this fault goes wrong.

When should you call a plasterer?

Call a plasterer or renderer where areas sound hollow, where cracks are opening, where render has already fallen, or where damp has appeared inside on the same wall. Ask specifically about the mix on an older solid-walled house, because a lime render and a cement render behave differently and one of them will trap moisture. Call a builder or engineer where the cracking is structural — stepped, tapering or opening at an extension junction — since the render is only reporting it.

What will the plasterer actually do?

Tap out and mark the debonded areas, which are usually larger than the cracked or stained ones
Establish where water is entering and why the render failed before removing anything
Hack off failed render to sound masonry with dust control and safe access rather than patching over voids
Re-render in coats and a mix suited to that wall, lime-based where the wall needs to breathe
Reinstate the details that shed water — bell cast or drip at the base, stop above the damp proof course, cills and edges
Refer structural cracking, rainwater goods or ground level faults to the trade that owns them

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Blown Or Hollow Plaster On A WallCaution Crumbling Lime Plaster In An Old HouseCaution Ground Level Above The Damp Proof CourseCaution Rain Coming Through A Solid Brick WallCaution

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