Is my cracked render causing the damp inside?

Cement render on a solid wall is one of the commonest causes of persistent damp in older UK housing, and the mechanism is not obvious. Intact, it keeps rain off. Cracked, it becomes a funnel: water enters at the crack, runs down the back of the render in the gap between render and masonry, and is held against the wall by the impermeable coat in front of it. The wall cannot dry outwards, so it dries inwards, into the room.

What you'll need

• Something to tap with — knuckles or a small hammer
• Binoculars
• Pencil and a camera

Step by step

1. Map the cracks and photograph them.Note whether they are fine crazing across the surface, straight lines following a joint or a lintel, or diagonal cracks at the corners of openings. The pattern indicates the cause, and diagonal cracking at openings needs a structural opinion rather than a filler.
2. Tap the render everywhere you can reach.Sound render rings solid. Render that has debonded sounds hollow and drummy. Work systematically and chalk the hollow areas — the result is usually much larger than the visible cracking.
3. Look at the bottom edge.Render should stop above the damp proof course with a bell cast bead so water drips clear. Render taken down to the ground is a bridge, and it will also be the wettest part of the coat.
4. Check the top and the details.Where render meets a sill, a verge, a soffit or a downpipe bracket, there should be a sealed or throated junction. Open joints at those details take in far more water than a hairline crack in a flat panel.
5. Work out whether the wall behind is solid or cavity.It changes the answer. On a cavity wall, cracked render is a nuisance. On a solid wall it is a mechanism, because the wall's only route to dry outwards is through the render.
6. Decide between patching and removing.Isolated fine cracks in sound, well-bonded render can be cut out and repaired. Widespread hollowness means the coat has failed and patching it just moves the problem, because water still gets behind the parts you left.
7. Choose the right replacement material.On an older solid wall, a lime render lets the wall evaporate. A dense cement render on the same wall recreates the problem you are fixing. On a modern wall, a proprietary through-coloured or silicone render system is normal.
8. Let the wall dry before recovering it.A wall that has been wet behind failed render for years must dry before anything goes back on it. Months, and preferably over a summer.

Tips

• Hairline crazing across a render face is usually shrinkage and is often cosmetic. Single straight cracks and cracks at the corners of openings are the ones that let real volumes of water in.
• Painting cracked render with masonry paint bridges the crack for a season and then splits along the same line, having reduced the wall's ability to dry in the meantime.
• Removing render from an old wall is dusty, heavy work, frequently reveals poor masonry underneath, and is a scaffold job above ground floor. Cost it honestly before starting.

Common mistakes

• Filling cracks with sealant or filler and painting over — The crack is a symptom of movement or of a debonded coat. Sealing the visible line leaves the water route behind the render open at every other junction, and the repair splits again.
• Patch-rendering a hollow area with cement onto an old solid wall — The new patch is harder and less permeable than what is around it, so it debonds at its edges and concentrates water at the join.

Questions people ask