Why Is My Render Cracking, and Can It Cause Damp?

Render cracks for a handful of different reasons, and the pattern of the cracks usually tells you which. Fine hairlines and crazing across the surface are mostly shrinkage as the render cured. Cracks running out from the corners of windows and doors, or straight up the wall, usually come from movement in the wall behind. Long horizontal cracks at regular spacing on a cavity wall can mean the metal ties inside the wall are rusting. Patches that sound hollow when tapped have come away from the wall. And cracks that carry on through the brickwork, or keep getting wider, point to structural movement that needs an engineer rather than a plasterer.

Yes, cracked render can cause damp. GOV.UK's guidance on damp and mould lists cracks in render among the visible defects behind penetrating damp, where rain gets in from outside. How much it matters depends on the wall behind. On a modern cavity wall, most of the water that gets past the render is stopped by the cavity. On an older solid wall finished in hard cement render, even fine cracks let in water that then cannot dry out, and that is where cracked render does its real damage.

Hairline Cracks and Crazing

Almost all sand and cement render develops some fine cracking as it cures and shrinks. Sika's technical sheet on cracks in render describes these as hairline cracks under 0.2 millimetres wide, classed as fissure cracks, which are acceptable under the standard for external rendering and do not harm the way the render performs. Crazing, a fine web of these cracks across the face, comes from the same shrinkage spread over a wider area.

On a modern house with cavity walls, hairline cracks of this kind are largely cosmetic. They are worth keeping an eye on, but on their own they are not a reason to re-render.

Why the Same Crack Matters More on an Older House

Cavity walls have been built since the nineteenth century, but according to LABC Warranty they only became the standard way of building houses after 1950, so a great many older homes have solid walls. Solid walls of softer brick or stone were built to take in some moisture and dry it out again through the surface. That only works if the surface lets them. The Society for the Protection of Ancient Buildings explains in its advice note on why old buildings need to breathe that strong cement renders form hair cracks as they set, hardly visible to the eye, that moisture is drawn into the wall through them, and that once it is in the wall it is trapped, because the render stops it evaporating. The wall gets steadily wetter and the damp shows inside.

On these walls lime render is the material the house was built to wear, because it is softer and lets the wall dry. Both Folkestone and Hythe have plenty of houses old enough to have solid walls, so on an older house a cement render with fine cracks deserves a closer look than the same cracks on a modern one. If the bond of the brickwork is hidden by render, the thickness of the wall at a window or door reveal is a good clue to which kind you have. The same thinking applies to the joints in bare brick, where repointing a soft old wall in hard cement mortar causes damage in much the same way.

Cracks Around Windows, Doors and Corners

Render is rigid and bonded to the wall, so any movement in the wall shows through it. Sika's guidance is that cracks running diagonally from the corners of windows and doors, or following the courses of the blockwork, are usually caused by a lack of reinforcement in the mortar joints around those openings, and that straight vertical cracks are usually caused by a lack of movement joints. The corners of an opening are where stresses in a wall gather, which is why they crack first.

Cracks also appear where two different materials meet under the same render. A wall that is partly brick and partly block, or a rendered extension joined to an older house, moves differently on each side of the join, and the render cracks along the line between them. Sika's sheet calls for a reinforcing mesh in the render across joins like that. On a newer building, a crack that appears in the first year or two may be the structure settling, and once it has stopped moving it can be repaired. A crack that keeps reopening means the movement has not stopped.

Long Horizontal Cracks at Regular Spacing

On a cavity wall, the inner and outer walls are held together by metal ties built into the mortar joints. Older ties were often mild steel, and as LABC Warranty's technical guidance on wall tie replacement explains, steel expands as it rusts, forcing apart the bricks above and below the joints the ties sit in. The result is a pattern of horizontal cracks lining up with the tie positions, roughly every six courses of brick. On a rendered wall, the render tends to crack along the same lines.

That is a structural problem rather than a render problem, and patching the render over it hides the symptom while the ties go on corroding. The same guidance says that where corroded ties threaten the stability of a wall, a structural engineer should decide what remedial work is needed, and it notes that bulging in a cavity wall can be a sign of the same failure.

Hollow or Bulging Render

Tap the render gently with the handle of a screwdriver. Sound render gives a solid knock, while render that has lost its bond with the wall sounds hollow. This is usually called blown render. Water gets behind it through cracks, frost can then push it further from the wall each winter, and in time patches bulge and fall. Blown render often surrounds a crack that looks small from the ground, which is why it is worth tapping around any crack before deciding it is only cosmetic.

When the Crack Is in the Wall, Not the Render

Some cracks are not a render problem at all. A crack that steps through the brickwork as well as the render, one that is noticeably wider at one end than the other, one that keeps growing, or cracking that arrives along with doors and windows starting to stick, can all point to movement in the structure or the foundations. That is a question for a structural engineer before it is a job for anyone with a trowel. If that is what we find, we will tell you so rather than render over it, because new render on a wall that is still moving cracks again along the same line.

How Cracked Render Lets Damp In

Penetrating damp through cracked render follows the weather. It tends to appear on the inside of the walls that face the prevailing wind, which in Britain is south westerly, and it gets worse after wind-driven rain and through the winter. Near the coast that exposure is harder, because driven rain is forced into any gap by the wind rather than simply running down the face. The signs are damp patches that grow after storms from one direction, staining that follows the line of a crack, and plaster that stays cold and damp on one wall while the others are dry.

Check the gutters before blaming the render. A blocked or leaking gutter pours water down the same patch of wall every time it rains, and an overflowing gutter can soak the render beneath it until it stains, cracks and blows. Damp on a chimney breast can come from the stack itself rather than the wall.

Two other kinds of damp are easily confused with it. Condensation is the most common form of damp in homes, according to the same GOV.UK guidance, and it appears on cold mornings, in corners, behind furniture and in steamy rooms, with no link to the rain. Rising damp comes up from the ground, and the guidance lists a bridged damp proof course among the things to check for. Render carried right down to the ground, or soil and paving built up against the wall, can bridge it, which is why render is normally stopped above the damp proof course.

What Not to Do to a Cracked Wall

The quick fixes can make it worse. On an old solid wall, a coat of paint that does not let moisture through has the same effect as cement render, and the SPAB note describes water getting behind paint films like that, becoming trapped, and causing blistering in warm weather and frost damage in cold. Hacking off render can also do harm. On a listed building, removing or changing render needs listed building consent, so speak to the council first.

Patch Repair or Re-render

Small cracks and the odd blown patch can be cut back to sound render and repaired. Where a large part of a wall sounds hollow, patching tends to chase the problem around the wall, because the render next to each patch is usually the next to go. Stripping back and re-rendering is then the repair that lasts, in lime on an older solid wall and in a render suited to the wall on a modern one.

Re-rendering a large area can bring in the Building Regulations. Approved Document L, which covers energy efficiency, treats stripping a wall back and rebuilding it, or rendering its outer face, as renovating the wall. Where more than half of an elevation is renovated, the guidance is that the whole wall should be brought up to the current insulation standard, so far as that is technically, functionally and economically feasible. The 2026 edition, which takes effect on 24 March 2027, keeps the same rule. Building control will tell you whether your job is caught by it.

Replacing render like for like does not need planning permission. Changing the type or colour of render is likely to change how the house looks, and the Planning Portal says that in a conservation area rendering the outside of a house needs planning permission. A listed building needs listed building consent. Folkestone and Hythe District Council publishes a list of its conservation areas.

Finding the Cause First

Because we do general building work as well as roofing, we can look at a damp wall and the roof above it together, and tell you whether the water is coming through cracked render, an overflowing gutter, a chimney or the roof. We will also tell you when a crack is cosmetic and best left alone, and when it needs an engineer before it needs us.

If your render is cracking, or a wall is damp and you are not sure why, contact us and tell us where the cracks are and which way that wall faces. Hythe and Folkestone Roofers will tell you what the cracks mean and whether they need a repair, a re-render or someone else entirely.