Render protects your walls from the weather, so cracks in it are worth taking seriously. Most start small and cosmetic, but a crack that lets rain in behind the render can turn into damp walls and blown patches. Here’s why sand and cement render cracks, which cracks to worry about, and how it’s repaired.
Fine surface cracks (crazing)
A network of fine, hairline cracks across the surface is usually shrinkage. Sand and cement render shrinks slightly as it cures, and it cracks more if it dries too fast in hot sun or drying wind, or if the mix is richer in cement than the wall needs. Crazing like this is normally cosmetic, and a good masonry paint will usually cover it.
Cracks that let water in
Wider cracks are a different matter. Rain runs into them and soaks the wall behind the render. That water can show up as damp patches inside, and in winter it freezes and expands, pushing the render away from the wall. Look out for:
- Cracks you can fit a fingernail into.
- Staining or green streaks running down from a crack.
- Damp patches on the inside of the same wall.
- Render that sounds hollow when you tap it, or bulges away from the wall.
Common causes of cracked render
- Drying too fast. Render applied in hot sun or strong wind loses its water before it has cured properly.
- Too strong a mix. Render that’s harder than the wall behind it can’t move with the wall, so it cracks.
- Poor preparation. Dust, old paint or a very smooth surface stop the render bonding, so it cracks and comes away.
- Movement. Cracks often appear where different materials meet, such as over lintels, around windows and where an extension joins the house.
- Water from elsewhere. Leaking gutters, overflowing downpipes and failed window sills keep the render wet and speed up damage.
- Render bridging the damp-proof course. Render taken right down to the ground can draw moisture up into the wall.
When to get a crack checked first
Ask a surveyor or structural engineer to look before anyone repairs the render if a crack is wider than a few millimetres, keeps growing, runs diagonally from the corner of a window or door, or also shows on the inside of the wall. These can be signs of movement in the building itself, and that needs sorting before the render is repaired, or the new render will crack again.
How cracked render is repaired
- Fine, stable cracks: filled with a suitable exterior filler and painted over with masonry paint.
- Wider cracks and blown patches: the damaged render is cut back to sound render, the wall is prepared, and new sand and cement render is applied in coats and finished to match the surrounding wall.
- Widespread cracking or lots of hollow areas: it’s often better value to take the old render off and re-render the wall than to keep patching it.
Fixing the cause matters as much as the repair, so leaking gutters, downpipes and sills should be sorted at the same time.
Render repairs in Poole and Dorset
Four Four Finishes repairs and re-renders walls in traditional sand and cement across Poole, Bournemouth and the rest of Dorset. See our render repairs page, or send us a photo of the crack and we’ll tell you what we think it needs.
Frequently asked questions
Is cracked render a problem?
Fine surface cracks are usually cosmetic. Wider cracks matter because rain gets in behind the render, which can lead to damp inside, blown render and frost damage in winter. The sooner they’re repaired, the smaller the repair.
Can you just fill cracks in render?
Fine, stable cracks can be filled with a suitable exterior filler and painted over with masonry paint. Wider cracks, or cracks where the render sounds hollow, need the damaged render cutting out and replacing so the repair bonds properly.
Why does new render crack?
Usually because it dried too quickly in sun or wind, the mix was too strong for the wall, or the wall wasn’t prepared well enough for the render to bond. Fine crazing as new render cures is common and often cosmetic.
Can cracked render cause damp inside?
Yes. Water that gets in through cracks can soak into the wall behind and show up as damp patches inside, especially on walls that face the wind and rain.
Should render go down to the ground?
No. Render should stop above the damp-proof course, usually finished with a bellcast bead that throws water away from the wall. Render that bridges the damp-proof course can let ground moisture rise into the wall.