Why Is My Render Cracking? Causes and Fixes

How to read a crack

Before worrying about any crack, take a close look at four things.

Width. Hairline cracks you can barely see are very different from cracks you can slide a coin into. Width is one of the simplest guides to seriousness.

Pattern. A random web of fine cracks (“crazing”), a single diagonal line from a window corner, a horizontal line along a floor level, and a stepped crack following brick joints all have different typical causes.

Location. Cracks at openings, at junctions between old and new construction, near the base of the wall, below gutters or across a whole elevation each suggest something specific.

Change. Is it growing, or has it been the same for years? Has it come back after being filled? Cracks that grow or return point to an ongoing cause.

Change is the one you can’t judge in a single look, which is why monitoring (covered below) is so useful.

Type 1: fine hairline cracks and crazing

What it looks like. Very fine cracks, often in a random, map-like or spider’s-web pattern, sometimes only visible when the wall is wet or the light is low.

Usual cause. Shrinkage as the render cured. Cement-based renders shrink as they dry, and if a coat dries too fast — in hot sun, wind, or over a very absorbent wall — or the mix is too strong, the surface can craze. It’s common on older sand and cement renders and can occur on one-coat renders such as monocouche.

How serious. Usually cosmetic only. Fine crazing rarely affects the render’s performance, though it can collect dirt and, over many years, allow small amounts of water in.

How it’s fixed. Often nothing more than monitoring. On painted render, a flexible masonry paint or crack-bridging treatment at the next redecoration can help. On through-coloured render, cleaning and, if needed, a new finish coat can refresh the appearance.

Type 2: diagonal cracks from windows and doors

What it looks like. Cracks running diagonally from the corners of windows, doors and other openings, often at around 45 degrees.

Usual cause. Stress concentrates at the corners of openings as the building moves slightly with temperature and moisture. Modern render systems handle this with extra reinforcing mesh — diagonal patches at each corner — bedded into the base coat. Where that reinforcement is missing or poorly placed, cracks follow. Movement of the lintel above an opening can also contribute.

How serious. Usually localised and repairable, but they will let water in if left.

How it’s fixed. The crack is cut out, reinforcement mesh is bedded across the corner in a new base coat, and the finish is rebuilt and blended in. If the lintel is failing, that needs addressing first.

Type 3: cracks from water and frost

What it looks like. Cracks with staining around them, often accompanied by hollow-sounding render, bulging, or pieces coming away. Often worst below gutters, sills and downpipes, at the base of walls, or on the most exposed elevation.

Usual cause. Water getting behind the render — through an existing crack, a defective sill, a leaking gutter or downpipe, or render taken down into the ground — then freezing. Ice expands, widening cracks and pushing render away from the wall. Each winter makes it worse, which is why water-related cracking can deteriorate quickly once it starts.

How serious. Moderately to very serious, because it’s progressive and can damage the wall behind.

How it’s fixed. Fix the water source first. Then cut out hollow and damaged render back to sound material and rebuild. If the problem is widespread, re-rendering the elevation may be better. Our guide on whether to repair or replace render helps you decide.

Type 4: cracks from movement in the building

What it looks like. Wider cracks, often more than a few millimetres, stepped cracks following brick joints beneath the render, cracks that taper (wider at one end), horizontal cracks along floor levels, or cracks at the junction between the original house and an extension. Cracks that return after repair.

Usual causes. Settlement, subsidence or heave in the ground; failing lintels; roof spread; differential movement between an extension and the main house; or long-term movement in older buildings. Trees close to the house, leaking drains and changes in soil moisture can all play a part.

How serious. Potentially serious. Not every movement crack means structural trouble — many old buildings have moved slightly and stabilised — but wide, growing or tapered cracks should be assessed.

How it’s fixed. Certainly not by simply rendering over it. The cause needs investigating, sometimes by a structural engineer, and dealing with if it’s ongoing. Once the building is stable, the render can be repaired, often with movement joints or extra reinforcement at junctions. Rendering over active movement just hides it until the new render cracks too.

If you also have cracks inside. Cracks in external render combined with cracks in internal plaster in the same area — particularly diagonal cracks above doors and windows, or cracks where walls meet ceilings — strengthen the case that the building itself has moved. Sticking doors and windows can be another sign. That doesn’t automatically mean serious structural problems; many houses have minor, historic movement. But it does mean the cause should be understood before any render repair, and a structural engineer is usually the right person to assess it. Your home insurer may also want to know if subsidence is suspected.

Type 5: cracks from the wrong render for the wall

What it looks like. Widespread cracking across an elevation, often with hollow areas and damp inside, typically on older houses with dense cement render.

Usual cause. A render that’s too rigid or impermeable for the wall. The classic case is hard cement render on a soft, solid, lime-built wall. The render can’t move with the wall, and it traps moisture behind it; cracks form, water gets in, and the cycle continues.

How serious. Serious over time, because it damages the wall and keeps recurring.

How it’s fixed. Usually by removing the unsuitable render and replacing it with a breathable, flexible system, often lime on older buildings. Patching in more cement continues the problem. See our lime vs cement render guide.

Other crack patterns you may see

Cracks at the base of the wall. Often where render has been taken down to or below ground level and is wicking up moisture, or where there’s no base bead. The fix is cutting the render back to a bead above ground.

Cracks along beads. Straight cracks following corner or stop beads can point to rusting old metal beads, render applied too thinly over them, or poor fixing.

Cracks at junctions. Where render runs over two different materials — brick and block, or an old wall and a new extension — without mesh or a movement joint, a crack along the junction is likely.

Horizontal cracks at floor levels. Can reflect movement of floor joists or the junction between storeys in some constructions.

Cracks around fixings. Where satellite dishes, brackets or downpipes are fixed, water can get in around the fixing and crack the render locally.

When to worry — and when not to

Usually nothing to worry about: fine hairline crazing that isn’t changing, with no hollow areas or damp inside.

Worth a repair soon: isolated diagonal cracks at openings, cracks at junctions or beads, and any crack that’s letting water in.

Get it assessed promptly: cracks wider than a few millimetres, stepped or tapered cracks, cracks that are growing or have returned after repair, cracks with hollow or bulging render, cracks with damp inside, and cracks around an extension junction. If you’re worried about structural movement, a structural engineer is the right first call.

Whatever the type, don’t leave cracks open through winter if you can help it, even if the full repair has to wait until spring. Water and frost are what turn small cracks into big repairs.

What the timing of a crack tells you

Within weeks of rendering. Fine crazing soon after application usually points to curing conditions — hot sun, wind, frost — or a mix that was too strong. Diagonal cracks at openings appearing in the first months suggest missing reinforcement. Both are installation issues worth raising with the renderer under the workmanship guarantee.

After the first winter or two. Cracks that open up after the first cold, wet seasons often reflect water getting in and freezing, an unsound substrate, or hollow areas that were rendered over. Again, raise them early.

Years later. On an older render, cracking is often age-related: the render has reached the end of its life, water has been getting in for some time, or the building has moved. Cracks appearing suddenly in an old render, especially after a dry summer or near trees, can point to ground movement and are worth having assessed.

Our render guarantees guide explains how to make a claim on a recent job.

How to monitor a crack

If a crack doesn’t look urgent, monitoring it for a few months tells you a lot. Mark the ends of the crack with a pencil and the date. Photograph it alongside a ruler or coin for scale, from the same position each time. Note the width at its widest point. Take a wider photo too, showing where the crack sits on the elevation, as the location is often as telling as the crack itself. Repeat every month or two, and after any spell of very wet, dry or cold weather. That record is useful to any specialist you call in.

If nothing changes over several months, the crack is likely historic and stable, and a straightforward repair is usually enough. If it grows, lengthens past your marks, or new cracks appear nearby, the cause is ongoing and needs investigating before any repair. Simple crack-monitoring gauges are also available if you want more precise readings; a specialist or structural engineer can fit one if needed.

What crack repairs typically cost

Crack repairs are usually priced per job rather than per square metre, because setting up, cutting out, rebuilding and blending in take much the same time for a small crack as a slightly larger one. As indicative guides:

indicative Access at height and colour matching on through-coloured render can add to the cost.

Where cracks come with hollow-sounding areas, the size of the hollow patch usually decides the price more than the crack itself; our guide to blown render explains how those areas are found and cut out. Our render repair cost guide covers repair pricing in more detail.

How a proper crack repair is done

A lasting crack repair follows a few steps. The renderer finds and fixes the cause where possible. They tap around the crack to find any hollow render, then cut out the damaged area back to sound, well-bonded material, usually with clean edges. Any damage to the wall underneath is repaired. The render is then rebuilt in layers matching the original system — base coat with mesh bedded across the crack line and any stress points, primer where required, and finish — and blended in.

A quick fill with exterior filler or sealant isn’t a render repair. It may keep water out for a while and is fine as a temporary measure, but it can also trap water behind it and usually fails as the crack moves.

Will the repair show?

On painted render, repairs are usually hidden once the wall is repainted, provided the texture is matched. On through-coloured render — silicone, acrylic, monocouche — they can be harder to hide, because the original has weathered and new material from a new batch rarely matches exactly. Good renderers minimise this by matching the colour as closely as possible, taking the repair to a natural break such as a corner or bead line, or re-coating the whole elevation. Ask how visible a repair is likely to be before work starts.

Cracking by render type

Sand and cement is rigid and shrinks as it cures, so it’s more prone to cracking than flexible systems. Crazing and movement cracks are common on older cement renders.

Monocouche can develop fine hairline crazing, especially if applied too thinly or in poor conditions.

Silicone and acrylic thin-coat systems are flexible and use full mesh, so they resist cracking well. When they do crack, it’s usually down to missing mesh, an unsound substrate or building movement.

Lime is soft and flexible and accommodates movement; fine cracks can partly self-heal. Repairs must be in lime.

Whatever the type, a lasting fix follows the same render repair principles: cause first, then cut back to sound material and rebuild in the original system.

Our K-Rend cracking and repairs guide covers cracking in branded through-coloured systems specifically.

Temporary measures until a proper repair

If a proper repair has to wait — perhaps until better weather or until a renderer is available — there are sensible temporary steps. Fix any obvious water sources first: clear blocked gutters, reattach loose downpipes, and redirect water that’s pouring down the wall. For cracks that are letting water in, a flexible exterior sealant applied neatly along the crack can keep rain out for a while, as long as the render around it is sound. Don’t seal over hollow or loose render, which can trap water behind it, and don’t use a pressure washer near cracked render.

Treat these as stopgaps, not repairs. Note what you’ve done and tell the renderer, as sealant will need removing before a proper repair.

Questions to ask a specialist

What do you think caused these cracks? Is the render around them sound, or are there hollow areas? Is any of this movement-related, and should I get a structural opinion first? Is my render the right type for my wall? Will the repair include mesh across the crack line? How visible will the repair be? Would re-rendering the elevation be better value? A specialist who answers with reference to your walls — and shows you what they’ve found — is giving you advice you can rely on.

How to prevent render cracking

For a new render, the best prevention is a properly specified system, of the kind our exterior rendering guide describes: full reinforcing mesh with extra at openings, movement joints where needed, a sound and properly prepared substrate, the right render for the wall, application at the correct thickness, and suitable weather with protection while it cures. Our questions to ask a renderer help you check these.

For an existing render, keep water away from it: clear gutters and downpipes, make sure sills throw water clear, keep render above ground level, and fill small cracks before winter. An annual walk round the house catches most problems early.

When cracking means it’s time to re-render

Repairing cracks makes sense while the render around them is sound. There comes a point, though, where repair stops being good value. The usual signs are cracks across most of an elevation rather than a few isolated ones; hollow areas found in several places when the wall is tapped; cracks that have been repaired before and returned; and an old render that is the wrong type for the wall, such as dense cement on soft solid masonry.

At that stage, a specialist will often suggest re-rendering the worst elevation or the whole house rather than chasing individual cracks. Re-rendering is priced as new work. As an indicative guide, the wall-count bands are indicative £3,500–£5,000 for one wall, £5,000–£8,000 for two, £8,000–£12,000 for three, and £12,000–£17,000 or more for four or more. Where the old render has to be stripped first, the rate is typically around indicative £100–£170 per m². Our house rendering cost guide explains how those figures are built up.

A sensible middle route on some houses is to repair the sheltered elevations and re-render the most exposed or most cracked one. Ask any specialist to explain which elevations they would repair, which they would replace, and why.

If a survey has flagged cracked render

Cracked render is one of the things a buyer’s surveyor commonly notes, and the wording in a report can sound alarming. A recommendation for “further investigation” usually means the surveyor couldn’t judge the cause from a visual inspection, not that something serious has been found.

If you’re buying, ask a render specialist to look at the cracks and give a written view and quote before you commit, and, where the report mentions possible movement, follow its advice on a structural engineer. The aim is to know whether you’re facing a small repair or a re-render, so you can factor it in.

If you’re selling, getting cracks diagnosed and repaired before you list removes an easy point for renegotiation. Keep the invoice and any guarantee to show buyers. If you suspect movement, deal with that first; a fresh repair over an active crack will simply reopen.

When the problem isn’t really the render

Sometimes cracking render is a symptom of the movement or water problems covered above, and no amount of render repair will help until they’re dealt with. If cracks are wide, growing or tapered, or you see cracking inside the house too, get the building assessed first.

When the render itself is the issue, SmartMatch™ weighs render-type experience, verified reviews, online reputation, insurance and accreditation to introduce you to one vetted local specialist for a free, no-obligation survey. Most of our specialists take on projects from around £3,500, so they’re best suited to larger repairs and re-renders; for a single small crack, a local builder or plasterer may be quicker. Our render repair page explains more.