Render cracks for six distinct reasons — shrinkage, thermal movement, structural movement, frost damage, poor substrate preparation and the wrong specification for the wall. It blows, meaning it debonds, almost always because water got behind it or because the substrate was never prepared to take it. Identifying which cause you have decides whether the repair holds.
- Crack width, direction and position tell you the cause. The pattern is the diagnosis.
- Blown render is a bond failure, not a surface failure, and cannot be filled.
- Hard cement render on a solid wall is a specification error, and it causes both symptoms at once.
- Most failures we attend are detailing failures rather than material failures.
Six reasons render cracks
1. Shrinkage
Cement-based render contains water, and as it cures it loses some of that volume. The result is fine, map-like crazing spread across a panel, usually appearing within the first months. It is almost always cosmetic. It becomes a problem only if it lets enough water in to start a different mechanism.
2. Thermal movement
Render expands in sun and contracts in cold, and a large unjointed elevation has to release that movement somewhere. Without movement joints it releases as a crack, usually straight, often vertical, and frequently in the same position each year. This is why movement joints are specified on large elevations rather than being an optional extra.
3. Structural movement
The wall itself is moving, and the render is simply reporting it. Diagonal or stepped cracking, wider at one end, often near openings, and visible inside and outside in the same place. This is the one that needs investigating before anything is re-rendered, because re-rendering over live movement produces a new crack in the same place.
4. Frost damage
Render applied too late in the year, or when the substrate was frozen, never cures properly. Ice forming inside the uncured material breaks its structure and its bond. The damage is invisible at the time and shows up a year or two later as crumbling and hollow areas.
5. Substrate movement telegraphing through
A straight crack following a mortar joint, or a line exactly where two materials meet, is the wall behind showing through. Common where an opening has been blocked up, where a lintel sits, or where blockwork meets brick. The fix involves reinforcement across the junction rather than just filling.
6. The wrong specification
A hard, dense render on a wall that needs to breathe, or a rigid system on a substrate that moves. This produces cracking and debonding together, across the whole elevation, and it is the only one of the six where the answer is to remove and start again.
Reading the crack
| Pattern | Likely cause | Serious? |
|---|---|---|
| Fine map-like crazing across a panel | Shrinkage during curing | No — cosmetic |
| Straight vertical crack on a long elevation | Thermal movement, no joint | Low — needs a joint, not a fill |
| Crack following a mortar bed line | Substrate telegraphing through | Low to moderate |
| Diagonal or stepped, over about 2mm | Structural movement | Yes — investigate first |
| Wider at one end | Rotational movement | Yes — structural opinion |
| Matching crack inside and out | Movement in the wall itself | Yes |
| Crack with hollow render alongside | Debonding; the crack is the symptom | Moderate — cut out and rebuild |
| Cracking round an opening | Lintel deflection or missing bead | Moderate — check the lintel |
Width is the quickest filter. Hairline is a surface matter. Anything over about 2mm, diagonal, and near an opening deserves an opinion before a trowel.
Why render blows
Blown render has lost its bond with the wall. It may be perfectly intact as a material and still be held on only by its edges. Tap it and the note is hollow and dull.
There are essentially two causes, and they are not equally common.
Water behind it. By far the usual one. Water enters at a crack, at a failed seal round a window, over an unprotected top edge, or up from a bridged damp-proof course. It then sits between render and masonry, working at the bond and, in winter, freezing and pushing the render off mechanically. This is why the hollow areas so often map onto the path water has been taking.
The bond was never good. A dusty or friable substrate, a wall with suction so high it pulled the water out of the basecoat before it cured, a painted or sealed surface the render could not key into, or a missing primer. The render looks fine for a year or two and then lets go in sheets.
Either way, blown render cannot be filled or sealed. It has to be cut back to a sound edge and rebuilt, and the cause has to be dealt with first or the new work goes the same way.
The cause behind the cause
Almost every render failure we attend has a water story, and in most cases the water is arriving from somewhere entirely fixable and often quite cheap.
- A leaking gutter joint keeping one strip of wall permanently wet. The single most common underlying cause, and the cheapest to fix.
- Render carried down over the damp-proof course, bridging it so ground moisture travels up behind the render continuously.
- No bellcast or drip bead at the base, so water running down the face wicks straight round the bottom edge.
- Failed sealant at window and door frames, giving water a direct route behind.
- Raised ground levels — a new path, patio or flower bed built up above the damp-proof course.
- A blocked or overflowing downpipe discharging against the wall rather than into a gully.
This is why we survey the gutters and the ground levels on a rendering job. Re-rendering a wall that is still being soaked is an expensive way to buy two years.
When the render is not the problem
Two cases are worth separating out, because in both the render is reporting a fault rather than being one.
Structural movement. If the wall is moving, the render will crack wherever it is told to. Repointing, filling or re-rendering changes nothing about the movement, and the new work cracks in the same line. Subsidence, a failing lintel and foundation movement all need establishing first, usually with monitoring and a structural engineer, and often through an insurer.
Solid walls with a breathability problem. On a pre-1930s solid wall, a dense cement render stops the wall drying through its surface. The trapped moisture produces internal damp and pushes the render off from behind, so you get both symptoms simultaneously across whole elevations. This is a specification problem, and patching it is futile: the answer is removal and a vapour-permeable system.
What fixing it involves
- Tap the whole elevation and mark every hollow area. The scope is set here, not by what is visible.
- Establish where water is getting in and fix that first — gutter, seal, bead, ground level, damp-proof course.
- Where cracking is structural, get an opinion and allow the movement to be resolved before re-rendering.
- Cut blown areas back to a sound, undercut edge, past where the tapping goes solid.
- Make good the substrate: replace spalled brick, repoint where needed, let a wet wall dry.
- Prepare for suction with the right primer or stabiliser for what the wall actually is.
- Rebuild in a compatible material, with mesh reinforcement over larger areas and across junctions.
- Fit the beads that were missing — bellcast at the base, stop beads at edges, movement joints on long runs.
- Finish to match, and accept that on an aged through-coloured render a patch will show.
What it costs
These are the rendering figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Rendering | £40–£70 per m² | System & access dependent |
| Three-bed semi | £2,500–£5,000 | Full elevation render |
| Detached house | £9,000–£12,000 | Larger area & scaffolding |
| External wall insulation | £90–£140 per m² | Insulated render system |
A localised repair sits well below these. The expensive outcome is the one where an incompatible system on a solid wall has to come off entirely, which is why specification matters more than any other single decision.
We tap the elevation, read the crack pattern and trace the water before recommending anything. Free survey, fixed written quote.
Frequently asked questions
Are hairline cracks in render a problem?
Usually not in themselves. Fine map-like crazing is shrinkage from curing and is cosmetic. It matters only if it starts letting enough water through to begin a different mechanism, such as frost damage or debonding. The cracks to take seriously are diagonal, stepped, wider than about 2mm, or near openings.
How do I know if render has blown?
Tap it. Sound render rings solid; debonded render sounds hollow and slightly dead. Work across the whole elevation in a grid rather than only around the visible damage, because the hollow area is nearly always much larger than the crack suggests. It needs nothing but your knuckles.
Can cracked render be filled?
A crack in render that is still bonded can be opened out, reinforced and filled. A crack in render that sounds hollow cannot — filling the surface of something no longer attached to the wall achieves nothing beyond hiding it for a season. Tap before deciding.
Why does render keep failing in the same spot?
Because the cause was never addressed. Repeat failure in one place almost always marks a persistent water source — a gutter joint, a sill, a bridged damp-proof course, a missing drip bead — or an incompatible repair material concentrating stress at its edges. The render is the symptom.
Is cracked render a sign of subsidence?
Occasionally, and the pattern distinguishes it. Subsidence tends to produce diagonal or stepped cracks, wider at one end, often near openings, visible both inside and outside in the same position, and progressing over time. Most cracking is nothing of the kind. If yours matches that description, get a structural opinion before any rendering work.
Should I re-render the whole house or just repair?
Repair, unless more than roughly a quarter of an elevation sounds hollow, the same area has already failed once, or the system is wrong for the wall. On an aged through-coloured render a patch will show, so where appearance matters the usual middle course is to repair the defect and re-render that elevation to a natural break.
Get the cause diagnosed, not just the crack filled
We tap the whole elevation, read the crack pattern and find where the water is coming from before quoting. Free survey, fixed written quote.