In short
- Cracked or hollow render lets rain in behind, where it cannot easily dry out.
- Render carried down over the damp-proof course lets ground moisture bypass it; stop render at least 150 mm above ground on a base bead.
- Impermeable cement renders and paints on solid walls trap moisture; breathable renders let walls dry.
- Find and fix the cause of damp before re-rendering. Render is not a damp treatment on its own.
How render causes damp
- Cracks and hollow areas: water enters and is held behind the render.
- Bridging the damp-proof course: render or raised ground in contact with the wall below the DPC.
- Missing drips: sills and copings without drips channel water onto the wall.
- Impermeable finishes: hard cement render or plastic paint on a solid wall.
- Failed junctions: around windows, pipes and roof edges.
Penetrating damp affected 2% of English homes in 2024, double the 2019 share, according to the English Housing Survey. See rendering statistics for the figures and sources.
How render helps
A correctly designed render on a sound wall protects it from wind-driven rain, which is a common cause of damp on exposed elevations. Breathable systems such as silicone and lime keep rain out while letting internal moisture escape.
Check before you re-render
- Identify the damp type: penetrating, rising, condensation or leaks.
- Check ground levels against the DPC.
- Check gutters, downpipes and roof edges.
- Test the existing render for hollowness.
- On solid walls, consider whether a breathable system is needed.
Solid walls need special care
Older solid walls rely on drying out. Swapping hard cement render for lime can reduce damp. See rendering period and solid-wall homes.
Types of damp and what render can do
| Damp type | Typical cause | Can render help? |
|---|---|---|
| Penetrating | Rain through walls, cracks, failed details | Yes, if detailed correctly |
| Rising | Ground moisture past a failed or bridged DPC | Only by not bridging the DPC |
| Condensation | Indoor humidity on cold surfaces | Indirectly, via insulation |
| Leaks | Gutters, pipes, roofs | No; fix the leak |
Exposure in Surrey
Wind-driven rain mostly arrives from the south-west. Gable ends and elevations facing that way on exposed ground, such as the slopes of the North Downs, the Hog's Back and the hills around Haslemere and Dorking, take the most. These walls benefit most from a well-specified render and from sills and copings with good drips.
After re-rendering
Walls that were saturated behind failed render can take months to dry out after a breathable system is applied. Some temporary internal marks may appear as they do. If damp persists after a reasonable drying period, investigate other causes.
Questions people ask
Will rendering stop damp?
It can stop penetrating damp from rain if the wall is otherwise sound. It will not fix rising damp, leaks or condensation.
Can new render cause damp?
Yes, if it bridges the DPC, lacks drips or traps moisture in a solid wall.