Cracked render is repaired by opening the crack to find out whether it is surface crazing or a sign of movement, then filling and reinforcing it. Blown render cannot be filled at all — it has lost its bond with the wall and has to be cut back to a sound edge and rebuilt. In both cases the cause is dealt with first, or the repair fails in the same place.
- Tap the wall before you do anything. A hollow note means the render has debonded and no amount of filler will hold it.
- Hairline crazing is cosmetic. Diagonal or stepped cracks wider than about 2mm near an opening need investigating before any repair.
- A blown area is almost always larger than the visible damage, which is why repairs are priced after tapping rather than from a photograph.
- On a thin-coat silicone system a patch will usually show. Re-rendering a whole elevation to a natural break is often the more honest answer.
First work out which problem you have
Cracked and blown render get talked about together because they often appear on the same wall, but they are different failures and they need different work. Getting this wrong is the single most common reason a render repair does not last.
Cracked render is still stuck to the wall. Something has moved, shrunk or expanded and the surface has split. Depending on the crack, the repair can be as simple as opening it out and filling it, or it can mean investigating the structure behind.
Blown render has come away from the wall. It may look perfect from the pavement, but it is being held on by its edges and by habit. Filling a crack in blown render is pointless: you are sealing the surface of something that is no longer attached to anything.
The tap test
Tap across the elevation with your knuckles or the handle of a screwdriver. Sound render gives a sharp, solid note. Blown render sounds hollow and slightly dull, like tapping a cardboard box. It is crude and it is the single most useful thing you can do before calling anyone, because it tells you roughly how much of the wall is affected.
- Work in a grid rather than tapping only around the visible crack — debonded areas spread well beyond what you can see.
- Mark the hollow areas with chalk as you go. The shape usually tells you something about the cause.
- Hollow patches that follow a horizontal line often sit above a damp-proof course or a lintel. Hollow areas at the base usually mean water.
Reading the cracks
Not every crack is a problem, and treating them all the same way wastes money on some walls and hides something serious on others.
| What you see | What it usually means | What it needs |
|---|---|---|
| Fine map-like crazing across a panel | Shrinkage as the render cured, or thermal movement | Cosmetic. A breathable coating or simply leaving it |
| A straight crack following a mortar joint | Movement in the wall behind, telegraphing through | Open out, reinforce and fill; monitor |
| Cracking in a line above a window or door | Lintel movement or deflection | Investigate the lintel before repairing |
| Diagonal or stepped cracks wider than about 2mm | Possible structural movement | Structural opinion first. Do not re-render over it |
| Cracks at the junction of two materials | Differential movement between materials | A movement joint, not a filled crack |
| A crack with render sounding hollow beside it | Debonding, with the crack as the symptom | Cut out and rebuild the area |
A crack that is wider at one end, visible inside and outside in the same position, or getting measurably longer is a different conversation. That is one to have with a structural engineer before anyone mixes render.
Repairing a crack that is still bonded
Where the render around a crack is sound, the repair is straightforward in principle and fiddly in practice. The aim is to give the filler something to grip and to stop the crack simply reappearing through the repair.
- Rake the crack open. Counter-intuitive, but a hairline crack has nowhere for material to go. It is cut out to a consistent width and depth so the repair has body.
- Clean out dust and loose material, and damp the edges on cement-based systems so they do not pull water out of the fresh material.
- Reinforce where the crack is anything more than cosmetic. A strip of mesh bedded over the line spreads the next bit of movement instead of concentrating it.
- Fill in a material compatible with what is already there. A hard cement repair in a soft or flexible render will crack around its own edges within a season.
- Finish to match the surrounding texture, which on a scratched or dashed render is the part that takes the skill.
- Let it cure before any coating goes on. Rushing this is why repairs show as a pale patch later.
Repairing blown render
There is no shortcut here. Blown render is removed, not repaired.
The area is cut back with a disc or a bolster until the edge rings solid when tapped. That edge is almost always further out than the hollow sounded, because the bond thins out gradually rather than stopping at a line. Cutting back to a neat rectangle that happens to be where the damage looked like it ended is how a repair fails at its own perimeter within a year.
Once back to sound material, the exposed substrate is checked. If the wall underneath is wet, the repair waits until the source has been dealt with and the wall has dried. Rendering over a saturated substrate traps the water and guarantees the new patch blows too.
The edge of the existing render is undercut slightly so the new material keys into it rather than butting against a straight face. The substrate is prepared for suction, the basecoat is built up in layers to match the existing thickness, mesh is bedded in where the area is large, and the topcoat is applied and textured to match.
Why the repair is bigger than the damage
Clients are often surprised that a patch the size of a dinner plate becomes a repair the size of a door. It is not padding. The bond fails progressively outwards from wherever the water got in, and leaving marginally attached render at the edge of a repair means the new work is keyed into something that is itself about to let go.
Dealing with the cause, not just the damage
Almost every render failure we are called to has a water story behind it. Repairing the render without finding that story produces a repair with a shelf life.
- A leaking gutter or a missing drip detail keeping one strip of wall permanently wet. The cheapest fix on this list and the most commonly skipped.
- Render bridging the damp-proof course, carried down past it so ground moisture tracks up into the wall above.
- Failed sealant at window and door frames, letting water behind the render at exactly the point where it has an easy path.
- No bellcast or drip bead at the base, so water runs off the face and straight behind the bottom edge.
- Frost damage where render was applied too late in the year and froze before it cured.
- An impermeable render on a solid wall, trapping moisture that the wall needs to release through its surface.
Will the repair be visible?
Usually, to some degree, and anyone promising otherwise is overselling. This is worth being clear about before work starts rather than after.
Render weathers. A ten-year-old elevation has been bleached by sun, washed by rain and colonised by a thin film of algae, and new material will not match it even if the mix and the colour are identical on paper. On a through-coloured silicone or monocouche system the difference is more obvious than on a painted sand and cement render, because with paint you can simply coat the whole elevation afterwards and the patch disappears.
Where appearance matters, the usual recommendation is to repair the defect and then re-coat or re-render the whole elevation to a natural break — a corner, a downpipe, a change of plane — rather than stopping mid-wall. It costs more than a patch and it looks like a wall rather than a wall with a repair on it.
What render repair costs
Render repairs are priced per job rather than per square metre, because the variables are access, how much has to be cut out once tapping is done, and whether scaffold is needed. For context, 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 |
Those are the new-render figures. A localised repair sits well below them; a repair that turns into re-rendering one elevation sits somewhere in between. Every job gets a free survey and a fixed written quote before anything starts.
When repair stops being the right answer
There is a point where patching is throwing money at a wall. The honest signals are cumulative rather than any single one:
- More than roughly a quarter of an elevation sounds hollow.
- You have already had the same area repaired once and it has gone again.
- The render is a hard cement system on a solid wall that needs to breathe.
- Cracking is appearing in new places each year rather than staying put.
- The detailing is wrong throughout — no beads, no movement joints, render over the damp-proof course — so there is no sound starting point to repair from.
We tap the whole elevation, trace where the water is getting in and tell you plainly whether a repair will hold. Free survey, fixed written quote.
Frequently asked questions
Can I repair cracked render myself?
A short hairline crack in sound, painted sand and cement render is within reach of a careful DIYer. Blown render is not, and neither is anything on a thin-coat silicone or monocouche system, where matching the texture and the through-colour is the whole job. The bigger risk is misreading the problem: filling a crack in render that has already debonded looks like a fix for about one winter.
How do I know if render is blown without specialist equipment?
Tap it. Sound render rings solid; debonded render sounds hollow and slightly dead. Work across the whole elevation rather than just around the visible damage, because the hollow area is usually much larger than the crack suggests. It is a genuinely reliable test and it needs nothing more than your knuckles.
Will filler from a DIY shop work on render cracks?
On fine cosmetic crazing in a painted render, a flexible exterior filler followed by a breathable masonry paint is a reasonable short-term answer. On anything structural, anything on a through-coloured system, or anything where the render sounds hollow, it is cosmetic at best. The filler will usually outlast its bond to the material around it, and the crack reappears at the edge of the repair.
How long does a render repair take?
A localised repair on an accessible ground-floor wall is typically a day or two, plus curing time before any coating. Anything above ground-floor level needs scaffold, which adds time at both ends. The part that cannot be compressed is curing between coats — rushing it is what causes patches to show up pale later.
Can render be repaired in winter?
Not in freezing conditions. Cement-based materials should not be applied when the substrate is frozen or when temperatures are heading below about 5°C, because frost in uncured render destroys the bond and you end up repairing the repair. A settled mild spell with sheeted scaffold is workable; a cold snap is not, and we would rather move the dates than gamble.
Should I repair the render or wait and do the whole house?
If water is getting in, repair now — the damage continues while you wait and it spreads into timber and plaster. If the problem is purely cosmetic and you are planning to re-render within a couple of years anyway, waiting is reasonable. We will tell you which of those two situations you are in rather than quoting for the larger job by default.
Get a straight answer on your render
We tap the elevation, find where the water is getting in and tell you whether a repair will hold or whether the wall needs more. Free survey, fixed written quote, no obligation.