Cracked render: which cracks matter and which do not
Most of them are the coating, not the house. The pattern of the crack tells you which one you are looking at, and you can read it yourself.
Most render cracks are shrinkage in the render itself and are cosmetic. The pattern tells you which one you have: fine map-like crazing is the render drying, straight cracks on a regular line are the background moving, and a stepped diagonal crack following the mortar joints is the one that may be structural.
The short version
- Read the pattern before the width. Random crazing across a panel is the coating. A crack that runs dead straight, or steps through the mortar joints, is following something behind it.
- Tap around the crack. A hollow note means the render has let go of the wall, and filling the crack does nothing about that.
- Mark both ends in pencil, date it, photograph it with a coin in shot. Movement over a season is the test, not the width on the day.
- A stepped diagonal crack that is widening, shows in the brickwork too, and comes with doors starting to bind is a structural engineer’s job, not a renderer’s.
- Re-rendering over live movement pays for the same wall twice and hides the evidence somebody will need later.
Start here
The pattern is the diagnosis, not the width
Render is a thin coat stuck to something else, so a crack in it is one of two stories: the coating has failed on its own, or the wall behind has moved and the coating has telegraphed it through. Random and branching, with nothing displaced, is the coating. A crack that lines up, repeats at a regular spacing, or has one side standing proud of the other is the background.
BRE’s starting position is worth having in your head before anybody prices anything: cracks are inevitable in virtually all types of construction, whether one is a cause for concern depends on the circumstances, and what to do about it depends on getting the diagnosis right first.
Six patterns
What each crack pattern usually means
| Pattern | The check | What it usually means |
|---|---|---|
| Fine map-like crazing | Low sunlight across the panel. Shallow, random, no step between the two sides? | The render shrank as it cured. Cosmetic, though on an exposed wall it lets water in over the years. |
| Straight vertical cracks at regular intervals | Does the spacing match block joints, a change of material or a floor level? | Shrinkage or movement in the background, telegraphing through a coating bonded across it. |
| A stepped diagonal crack following the mortar joints | Is the same line in the brickwork, or on the inside face? Is it widening? | Movement in the masonry. The one pattern that may be structural. |
| Cracks radiating from window and door corners | Short, diagonal, out of the corner. The same at several openings? | Stress concentrating at the weakest part of the wall. Common, usually not serious, worth watching. |
| A horizontal crack at one consistent height | Does it match a floor, a wall plate, the damp proof course or the top of a bay? | A bed joint, a wall plate, or something bridging at that line. Trace it before you fill it. |
| Any of the above, with a hollow note round it | Knuckle-tap outwards from the crack and chalk where the note changes. | The render has lost its bond. The crack is a symptom of that, not of movement. |
Crazing is the pattern people fear most and it matters least. It is the top coat shrinking as it dried, and it goes no deeper. Watch it for water rather than structure: fine cracks on the elevation that takes the weather admit driven rain slowly, and BS 8104 treats exposure to wind-driven rain as a measurable quantity, not an opinion. Damp inside that only shows up in a storm is a driving-rain leak.
Regular straight cracks are the background talking. Blockwork shrinks at its joints as it dries, so a rigid coat bonded across them has to crack somewhere, and the same happens where brickwork meets a lintel or an old wall meets an extension. Cracks at the corners of openings are related: a window is a hole, and the corners of a hole are where stress collects. Two or three short diagonals not getting longer are ordinary.
A horizontal crack at one height is the least discussed and often the most useful, so look at what sits at that level. Low down, with the render carried on past into the soil, the coating has been taken over the damp proof course, which makes it damp at the bottom of a wall before it is a cracking problem. Higher up it may be a floor, a wall plate or a failed bed joint. Where a strip of wall stays wet under a leaking gutter joint, cracking concentrates there, so check why a gutter overflows in heavy rain before you blame the render.
Tap it
Tap the render before you read anything into the crack
The most useful twenty minutes you can spend on a rendered wall costs nothing. Work along the elevation in a grid, tapping with a knuckle or the edge of a coin. Sound render gives a hard, dull knock. Render that has parted from the wall gives a hollow, drummy note, and the difference is unmistakable. Chalk round every hollow area.
That is the overlap that changes what a crack means. If it sounds hollow around the crack, the coating has lost its bond, and the crack is a symptom of debonding rather than of movement behind. Filling it wastes the money, because the hollow area governs the repair and is always bigger than it looks. The same test indoors answers a different question, what a hollow-sounding wall inside is telling you, and there a hollow note is often normal. Do it before anybody prices the job: two chalked patches the size of a tea tray is a different conversation about rendering work from a whole elevation.
Do it from the ground. Loose render comes off in slabs and it is heavy, so do not stand under a hollow area on a ladder and tap it. The HSE rules on work at height exist because falls from low heights cause serious injuries too.
The one that matters
The stepped diagonal crack, and when it needs a structural engineer
A crack that steps diagonally along the mortar joints comes from the masonry rather than the coating. Mortar is weaker than brick, so a wall being pulled apart cracks along the joints, and render over it copies the pattern.
Which way the ground is moving
Two features carry the information. Where it is: these cracks surface at openings first, because a window or a door is the weakest line through a wall. How it tapers: the rule of thumb a surveyor applies is that a crack narrows towards whatever is moving, so wider at the top points at ground shrinking away beneath the foundation and wider at the bottom points at ground swelling and lifting. Neither is a finding on its own. Both are read alongside what is nearby: a large tree, a leaking drain, a run of dry summers on clay.
Then check whether it goes all the way through. A crack in the coating alone is a render problem. The same crack in render, masonry and the plaster inside is a wall problem.
What the width does and does not tell you
Width gets quoted at homeowners as though it settled the question. BRE assessed 130 properties that had suffered subsidence and produced six damage categories linking width and number to the appropriate repair. As a broad guide, BRE describes categories 0 to 2 as aesthetic issues requiring only redecoration, categories 3 and 4 as serviceability issues affecting weather-tightness and the operation of doors and windows, and category 5 as a stability issue likely to need structural intervention. BRE then adds the caveat the trade leaves off: those comments are a simplification, and several factors, including whether the widths are increasing with time, can affect the classification. It is a guide applied with the building in front of you, not a threshold you can settle with a tape.
Four things that mean stop and get an engineer
- A stepped diagonal crack measurably wider than when you first marked it.
- A crack of more than a few millimetres running through the masonry as well as the render, and showing inside in the same place.
- Doors and windows binding in frames that used to work, or new gaps at a skirting.
- Movement in the ground: a large tree close to the wall, a leaking drain, a sunken path, ground dug out or built up against the house.
That is a structural engineer’s work rather than a renderer’s. They are the ones qualified to say whether the movement is live and what the foundation needs, and structural repair is building work judged against Part A of the Building Regulations.
Do not re-render over movement that is still going on. New render on a moving wall cracks again within a year, in the same line, and it hides the only evidence of what the wall was doing. Fix the cause, establish that the movement has stopped, then make the surface good.
A specification problem
When the render is stronger than the wall it is on
Some cracked render was never going to work, and that is a specification symptom rather than a workmanship failure. A hard sand and cement render on a soft solid wall of old brick, cob or rubble stone is stronger and less forgiving than what it is stuck to, and the weaker of the two suffers.
That is the reasoning behind Historic Environment Scotland’s guidance on lime and cement in traditional buildings: on an older wall the coating should be the weaker partner, so it takes the movement and the wear instead of passing them into the masonry.
You can spot it. The cracking runs in long straight lines, not a random web, the render is harder than any mortar behind it, and it lets go in slabs. Patching with more of the same buys a year. On a solid pre-1919 wall the answer is usually a lime-based system that lets the wall dry, the same argument as lime mortar against cement mortar. On a modern cavity wall the choice is about maintenance and drying, which is silicone render against sand and cement. If the joints behind are the real problem, that is repointing and brickwork.
The test that settles it
Mark it, date it, photograph it, look again after a season
Monitoring a render crack over a season is what a surveyor would do first, and it is free. Movement over time is the test, not width on the day.
- Draw a short pencil tick across each end of the crack, so you can see later whether it has travelled past the mark.
- Write the date beside it in pencil, on the render, not on a note you will lose.
- Photograph it twice: from across the road so the pattern shows, and close up with a ruler or a coin flat against the wall for scale.
- Note the season. Cracks that open in a dry summer and close in a wet winter are usually clay ground shrinking and swelling.
- Look again after a season from the same spot, then after a year.
A crack that has not passed its pencil marks in a year, with no binding doors and nothing matching inside, is one you can fill and forget. One that has moved comes with dated evidence attached, which is worth having.
Next
What to do next, in order
- Photograph the elevation from a distance, work out which pattern you have, then tap the wall in a grid from the ground and chalk every hollow area.
- Check the same line inside, whether the doors and windows still work, and what is near that wall: a tree, a drain, ground raised or dug out.
- Mark and date the crack, then price the specific defect, not the whole elevation.
For a good share of readers the answer is a pencil mark and another look in the spring, and saying so is more use to you than a quote would be. Where the render has genuinely let go, or the coating is wrong for the wall, that is plastering and rendering work, and usually a smaller job than a full re-render once the hollow areas are mapped.
Thomas comes out, taps the wall himself and puts in writing what he found. If the crack is one to watch rather than one to fix, he will say so. You can ask for a written itemised quote or send him a photograph first, with something in shot for scale.
Questions
Common questions
Are hairline cracks in render serious?
Usually not, fine, shallow, random cracking spread across a whole panel is the render shrinking as it cured, and it goes no deeper than the coat it is in. On a sheltered elevation it needs nothing. On the wall that takes the driving rain it is worth closing up in time, because a network of fine cracks lets water in slowly.
How do I know if a crack in my render is structural?
Look for a stepped diagonal line following the mortar joints instead of a random web, check whether the same crack appears in the brickwork and on the inside face, and check whether doors and windows in that wall have started to bind. A stepped crack that is measurably widening, with movement showing inside as well, is one for a structural engineer.
Why does my render sound hollow near the crack?
Because it has lost its bond with the wall behind. Tap outwards from the crack in every direction and chalk where the note changes from a hard knock to a drummy one. Where the render is debonded, the crack is a symptom of that and not of anything moving, and filling the crack achieves nothing.
Should I fill the crack or wait?
Wait, unless water is getting in. Mark both ends of the crack in pencil, write the date beside it and photograph it with a coin in shot for scale, then look again after a season. Movement over time is the test, not the width on the day, and a crack that has not passed its marks in a year is one you can fill and forget.
Can I just re-render over the cracks?
Not while anything is still moving. New render over a wall that is still moving cracks again in the same line, usually within a year, and it covers up the only record of what the wall was doing. It also does nothing about a coating that is too hard for the wall it sits on, which will crack the same way a second time.
Read next
Related advice
The guides below pick up where this one leaves off, and the plastering and rendering page covers the work itself and the places we cover, if you would rather have the job looked at.
- Water coming in only during driving rain
- Blown plaster and the hollow sounding wall
- Damp at the bottom of a wall: rising or penetrating damp
- Silicone render or sand and cement render
Plastering and rendering, and where we cover All advice articles
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
Free quote
Not sure whether that crack needs anything doing?
Thomas comes out, taps the render, looks at what is behind it and puts in writing what he found and what it would take to put right, itemised. If the answer is a pencil mark and another look in the spring, he will say so.