Blown or Hollow Render: What It Means and What to Do

What “blown” render actually means

Render works by bonding to the wall behind it. That bond might be mechanical — the render keying into rough brick, raked joints or a scratch coat — or partly chemical, as with modern bonding primers and adhesive base coats. When the bond fails across an area, the render is still in place but no longer properly attached. That’s what renderers mean by blown, hollow or debonded render.

Sometimes the render has separated from the wall itself. Sometimes one coat has separated from another — a finish coat lifting off a base coat, or a newer render detaching from an older one it was applied over. Either way, there’s a thin gap behind the render, and that gap is what makes it sound hollow.

Blown render isn’t the same as cracked render, though the two often go together. A crack can let water in that then blows the render around it; equally, a blown area can crack as it flexes. If cracking is your main concern, our why is my render cracking guide covers the crack patterns in detail.

How to do a tap test

The simplest way to find blown render is to tap it. You don’t need special tools — a knuckle, the handle of a screwdriver or a coin will do, and renderers often use a small hammer with light taps.

Sound render gives a solid, dull knock that feels firm through your hand. Blown render sounds hollow, papery or drummy, a little like tapping on a box. The difference is usually obvious once you’ve heard both.

Work methodically. Start at a corner at a height you can safely reach and tap every hand-width or so in a grid, moving along and down. Pay extra attention to the areas where blowing typically starts: below windowsills, around downpipes and gutters, at the base of the wall, near the corners of openings, alongside cracks, and on the most exposed elevation.

Tap gently. The point is to listen, not to test strength, and hard blows on badly blown render can dislodge it. Don’t lean ladders against hollow areas, and leave anything above the height you can safely reach from the ground to a specialist with proper access.

Other signs of blown render

Tapping is the clearest test, but blown render often gives itself away in other ways:

• Bulging — areas that stand slightly proud of the wall, visible when you look along the surface with low light.
• Cracks with lifted edges, where one side of the crack sits higher than the other.
• Staining or damp patches that don’t dry out, often in a tide-mark pattern.
• Pieces missing at the base, edges or around fixings, with the edge of the remaining render loose.
• Movement when you press gently on an area.
• Damp inside on the corresponding internal wall.

Any of these alongside a hollow sound tells you the problem is active rather than historic.

What causes render to blow

Water getting in. A very common cause. Water gets behind the render through cracks, failed sealant around windows, defective or missing sills, leaking gutters and downpipes, or from render taken down into the ground. Once behind, it weakens the bond.

Frost. Water trapped behind render freezes and expands in cold weather, pushing the render away from the wall. Each freeze–thaw cycle widens the gap, which is why blown areas often grow after a hard winter.

Poor bond or preparation. Render applied over dust, dirt, algae, loose material or a smooth surface without a proper key or bonding coat may never have bonded fully. These areas can sound hollow within a year or two of the work.

Incompatible substrate. Some backgrounds are difficult to render: dense engineering brick, very smooth blocks, or mixed materials that move differently. Without the right preparation or primer, adhesion is weak.

Render over painted surfaces. Rendering over old paint relies on the paint’s grip on the wall. If the paint fails, the render comes with it.

Age. Older renders, particularly cement renders several decades old, gradually lose their bond as moisture, temperature cycles and small cracks take their toll.

Cement on soft brick. Hard, dense cement render on soft brick or stone is a classic mismatch. The render is stronger and less flexible than the wall, it traps moisture, and the face of the soft masonry can fail behind it, taking the render with it.

How to map blown areas

Before anyone quotes for a repair, it helps to know how much of the wall is affected. A simple map makes that clear and gives you a baseline to compare against later.

Take a photo of each elevation from straight on. As you tap, mark hollow areas lightly with chalk or masking tape on the wall, then mark them on a printed copy of the photo. Note anything else you find: cracks, bulges, staining, missing pieces. Photograph the marked wall again.

Then estimate the proportion. Is it one or two hand-sized spots? A patch a metre or so across? Several patches scattered over the elevation? Or a large share of the wall? That rough proportion is one of the most useful things you can tell a specialist, and it’s the main thing that decides between a repair and a re-render.

Repeat the exercise every few months, or after a cold spell, if you’re monitoring. Growing areas mean an active problem.

How serious is blown render?

Low concern: small, isolated hollow spots that are flat, uncracked, dry and not changing. These can often be monitored and repaired when convenient, ideally before winter.

Repair soon: hollow patches with cracks, staining or damp, patches beside obvious water sources, and areas that have grown since you last checked. Water is getting in, and frost will make them worse.

Act promptly: bulging render, loose or moving sections, pieces already falling, hollow areas above doors, paths, drives or a neighbour’s property, and widespread hollowness across an elevation. Blown render at height is heavy, and a section coming away can injure someone or damage property. If you’re worried about something falling, keep people clear and get it made safe. Our render falling off guide explains what to do.

The location matters as much as the size. A small blown area over the front door is more urgent than a larger one at low level behind a shed.

What to do while you wait for a repair

Good renderers are often booked up, and blown render shouldn’t be rebuilt in frost or heavy rain, so there may be a gap before a proper repair. Use it sensibly.

• Stop the water. Clear and repair gutters and downpipes, reseal obvious gaps around windows, and redirect anything dripping onto the blown area. This is the single most useful thing you can do.
• Keep people clear of risky areas. If render over a path, door or drive is bulging or loose, keep people away from underneath and arrange for loose pieces to be taken down safely by someone with proper access.
• Don’t seal over hollow render. Sealant or paint over a blown area can trap water behind it and hide how far the problem extends.
• Don’t pressure wash. High-pressure water can drive straight behind cracked or hollow render.
• Keep your records. Dated photos and your map help the renderer price accurately and show whether it’s changing.

How blown render is repaired properly

A lasting repair follows the same broad steps whatever the render type.

1. Find and fix the cause. If water from a sill, gutter or failed sealant is the reason, that has to be dealt with, or the repair will blow again.

2. Cut back to sound render. The renderer taps out the full extent of the hollow area, then cuts it out with clean edges, continuing until they reach render that is firmly bonded all round. The hole is often bigger than the area you mapped, because blowing extends beyond what’s obvious.

3. Repair and prepare the wall. Loose or damaged masonry, failed paint and old debris are removed, defects in the wall are repaired, and the background is prepared with a key, bonding coat or primer suited to it.

4. Rebuild in layers. The render is rebuilt to match the original system — base coat with reinforcing mesh lapped onto the sound render around the edge, then primer and finish where the system requires them.

5. Blend in. The finish is matched as closely as possible to the surrounding render. On painted render, the wall is then redecorated.

What a repair should not be is filling a hollow area with expanding foam, injecting adhesive behind it without assessment, or skimming over the top. Some specialist injection methods exist for particular situations, but for most blown render on houses, cutting out and rebuilding is the reliable approach.

When re-rendering is the better answer

Patch repairs make sense when the hollow areas are limited and the rest of the render is sound. They become poor value when:

• A large share of the elevation sounds hollow, so you’d be replacing most of it piecemeal anyway.
• New hollow areas keep appearing after each repair.
• The render is the wrong type for the wall — for example hard cement on soft brick — so patching with the same material repeats the problem.
• The render is old and nearing the end of its life everywhere.
• Colour matching on a through-coloured finish would leave an obvious patchwork.

In those cases, stripping and re-rendering the affected elevation can give a better, longer-lasting result than a series of repairs; ask what guarantee the renderer offers on the new work. Our repair or re-render guide sets out the decision in steps.

What blown render repairs typically cost

Repairs are usually priced per job, because setting up, cutting out, rebuilding and blending take much the same effort for a small patch as a slightly larger one. Access at height often adds to the cost. As indicative guides:

indicative Access, scaffolding, wall repairs and colour matching can add to these figures. Ask for scaffolding to be itemised. Not a quote.

Our render repair cost guide covers repair pricing in more detail, and re-rendering cost covers the bigger option. For whole-house budgets by number of walls, see our house rendering cost guide.

What happens if you leave blown render

Blown render rarely gets better on its own. Left alone, the usual pattern is:

Water gets in more easily. Hollow areas crack as they flex, and every crack lets more rain behind the render.

Frost widens the gap. Each hard winter pushes the render further from the wall, and the blown area spreads into render that was sound.

Damp reaches the wall and the inside. Water held behind render can soak the masonry, show up as damp patches indoors, and damage brick or block faces.

Sections fall. Eventually pieces come away. Render is heavy, and falling sections from upper storeys are a genuine safety risk to people below, as well as to cars, windows and conservatories.

Repairs get bigger. What could have been a modest patch repair turns into a larger repair or a full re-render, and wall damage behind it can add to the bill.

Will insurance cover blown render?

Generally, home insurance is designed to cover sudden, unforeseen events rather than gradual deterioration. Blown render caused by age, wear and tear, frost over time or lack of maintenance is usually treated as a maintenance matter and not covered. Damage caused by a specific insured event may be treated differently, depending on your policy.

Every policy is different, so check your own wording or ask your insurer if you think an event caused the damage. If the render is recent and blowing because of how it was installed, the workmanship guarantee is the more likely route; see our render guarantees guide. Either way, keep dated photos and your tap-test map.

Blown render on a recent job

Hollow areas appearing within the first year or two after rendering usually point to installation: render applied over a dusty or unprimed background, over old paint, in frost or very hot weather, or too thinly over beads. They can also follow a water leak that was present when the work was done.

Raise it with the renderer promptly and in writing, with photos and your map, and ask them to inspect. A good contractor will want to see it and put it right under their guarantee. Don’t let anyone else cut into the area first, as that can complicate a guarantee claim.

Blown render by render type

Sand and cement. Commonly found blown, particularly on older houses. Its rigidity and tendency to crack let water in, and it’s frequently found on soft brick it doesn’t suit.

Pebbledash and dry dash. The dash coat often sits on a cement base that blows in patches, and repairs are hard to blend. Widespread blowing is a common reason for stripping it during a full re-render; see our rendering over pebbledash guide for when overcoating is and isn’t possible.

Monocouche. Can blow where it was applied over an unsuitable or unprepared background or where water has got behind it.

Silicone and acrylic thin-coat systems. Less prone to blowing when installed correctly, because the system includes bonding base coats and mesh, but they rely on the base coat being sound and the background properly prepared.

Lime. Lime is softer and more forgiving, but it can still lose its key over time. Repairs should be in lime, not cement.

Can you repair blown render yourself?

A small, low-level blown patch on a painted sand and cement render is within reach of a competent DIYer: cut back to sound render, prepare the background, and rebuild in coats. But there are real limits.

Anything above ground-floor level needs safe access, and working on blown render from a ladder is risky because the render can come away. Through-coloured renders are hard to match. Lime needs lime. And if you don’t find and fix the cause, the repair will blow again. For anything beyond a small patch you can reach safely from the ground, a specialist is usually the better call.

How to prevent render blowing

For new render: make sure the background is properly prepared — clean, sound, keyed or primed — and that the system suits the wall, breathable where needed. Old paint and loose material should be removed. Beads should stop the render above ground level, sills should project and drip clear of the wall, and work should only happen in suitable weather. Our what to expect during a rendering job guide shows what good preparation looks like.

For existing render: keep water off and out. Clear gutters and downpipes, keep sealant around windows in good order, fill cracks before winter, make sure soil and paving are below the bottom of the render, and seal around any new fixings. A tap test once a year, especially on the exposed side, catches blowing while it’s still a small job.

Questions to ask a specialist

How much of the wall is hollow, and can you show me? What do you think caused it? Does the cause need fixing first, and who does that? Will you cut back to sound render, and what if it extends further than expected? Will you use mesh, and lap it onto the sound render? How closely can you match the finish? At what point would re-rendering the elevation be better value? What guarantee comes with the repair?

Ask how they’ll price it if the hollow area turns out larger once they start cutting. A clear answer — a rate for extra area, or a call before going further — avoids surprises.

When the problem isn’t really the render

Sometimes blown render is the symptom of something else. A leaking gutter, a missing sill, a cracked downpipe, ground levels piled up against the wall, or a failing wall behind the render will keep blowing any new render applied over it. Occasionally, the masonry itself is the issue — spalled brick faces or decayed stone that can’t hold render. In those cases the first job is a builder, roofer or surveyor, and the render comes after.

When the render itself needs attention, 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 job, a local builder or plasterer may be quicker. Our render repair page explains more, and our exterior rendering guide covers systems and choices if a re-render is on the cards.