Cracked Render: What the Crack Pattern Tells You

Fine crazing across the surface

A web of hairline cracks, usually less than a millimetre wide and not following any line, is shrinkage in the render coat. It comes from a mix that dried too fast, was applied in hot or windy weather, or was over-trowelled.

Crazing is cosmetic but it lets water in slowly. On a sand-and-cement wall the usual answer is a breathable masonry coating or a thin-coat silicone topcoat over a stabilised surface, rather than removing sound render.

Straight vertical or horizontal cracks

Straight cracks that stop and start at a consistent line normally follow something behind the render: a change of material, a lintel, a joint between an extension and the original wall, or a missing movement joint on a long elevation.

These need cutting out, reinforcing with mesh across the joint, and rebuilding locally. If the underlying junction is one that genuinely moves, a purpose-formed movement joint is the durable answer — the crack will simply reappear through any rigid repair.

Stepped or diagonal cracks

Cracks that step diagonally across a wall, widen towards one end, or appear at the corners of openings are following the mortar joints of the masonry behind. That is a structural movement pattern and it is the one case where render repair should wait.

The right order is: get a structural engineer's opinion, deal with the cause — commonly drainage, tree roots or a foundation issue — allow any monitoring period they recommend, then repair the render once the movement has stopped.

Hollow render is a separate problem

Tap the elevation with a light hammer or the back of a scraper. Hollow areas have debonded from the substrate and will come off in sheets given a hard frost, regardless of how narrow the visible crack is. Debonded render around a crack always widens the repair area beyond the crack itself.

Check below sills and around downpipes first — the wettest zones fail earliest · Look for render carried below the damp-proof course, which draws ground water up into the coat · Check bell-cast beads at the base of the wall are present and clear · Note any hollow render that sounds different above and below an obvious crack

Frequently asked questions

Can cracked render just be painted over?

Masonry paint bridges very fine crazing but it cannot span a moving or open crack, and it will not re-bond hollow render. Painting an unsound wall usually means the coating peels off with the render underneath.

How do I know if render cracks are structural?

Diagonal or stepped cracks wider than about 2mm, cracks that continue through mortar joints inside the property, and cracks that are visibly wider at one end are the ones that warrant a structural engineer rather than a renderer.

Is it worth patching render or should the whole wall be redone?

Patching is fine where the surrounding render is well-bonded and the finish can be matched — which is realistic on textured and painted finishes, and difficult on through-coloured render. Where more than a third of an elevation is hollow, re-rendering the full elevation gives a better and more predictable result.

Will re-rendering cure damp inside the house?

Only if the render was the route the water was taking. Cracked or hollow render is a common cause of penetrating damp, but so are failed gutters, blocked cavities and bridged damp-proof courses, so the diagnosis has to come first.