Blown Plaster: How to Spot It and What to Do

You tap a wall to hang a picture and it sounds hollow. A crack has opened in a curve across the ceiling. A bit of plaster near the skirting has crumbled away. These are all signs of blown plaster: plaster that has come away from the wall or ceiling behind it.

We see it a lot in older Midlands homes. This guide shows you how to spot it, what causes it, and what to do about it. RCR Connections Ltd is Birmingham based, covering the whole of the Midlands, and our team has 62 years of combined experience fixing it.

What is blown plaster?

Plaster grips the wall behind it. On brick, it keys into the rough face and the joints. On old ceilings, it grips the thin wooden laths. Over time, that grip can fail. The plaster still looks fine from the front, but there is now a small gap behind it. That is what “blown” means.

Once it has let go, it does not stick back. It may stay put for years, or it may crack, bulge and fall.

How to spot it

The tap test

Gently tap the wall with your knuckles or the handle of a screwdriver, working across it in a grid. Sound plaster gives a dull, solid knock. Blown plaster sounds hollow. Mark the edges of the hollow area in pencil so you can see how big it is.

Other signs

• Bulges. Hold a long straight edge or a spirit level flat on the wall. A gap behind it can show a bulge you cannot see by eye.
• Cracks. Cracks that run in a curve or form a rough box shape often mark the edge of a loose patch.
• Crumbling. Soft, powdery plaster that breaks away, often low down on the wall.
• Bubbling paint or salts. Blistered paint or white, fluffy marks can mean damp is pushing the plaster off.
• A sagging ceiling. A dip or bulge in a ceiling, often with cracks around it.

What causes it?

• Age. Old plaster in Victorian and Edwardian houses can simply lose its grip after many years.
• Damp. A leak, a failed gutter, bad pointing, cracked render outside or rising damp can soften plaster and push it off. Salts in the water make it worse.
• Movement and knocks. Slammed doors, traffic or building work nearby can loosen it.
• Chasing in cables and pipes. Cutting into walls for a rewire or new heating can shake nearby plaster loose.
• Poor work in the past. Plaster put on a dusty, shiny or painted surface, or with no bonding coat, may never have gripped well.
• Hard patches on soft walls. Cement or gypsum patches on an old lime-plastered wall can trap damp and make the plaster around them fail.

Is it urgent?

A small hollow patch on a wall is not usually urgent, but it will not get better. Plan to fix it before you decorate, or the new paint or paper will crack.

Ceilings are different. Old lath and plaster ceilings can drop large, heavy pieces with little warning. If a ceiling is sagging, bulging or has a new crack in a curve, keep people out from under it and get it looked at soon.

What to do about it

1. Find and fix the cause

If damp is behind it, plastering first is a waste of money. Fix the leak, the gutter or the render outside, and let the wall dry. Then plaster.

2. Work out how much has gone

Tap the whole wall or ceiling. A small area can be patched. If large parts sound hollow, it is often better to re-plaster the whole wall so you get one flat, even finish.

3. Take off the loose plaster

The blown plaster is cut back to a firm, sound edge. Anything that sounds hollow comes off. Leaving loose bits at the edge is the main reason patch repairs crack again.

4. Prepare and re-plaster

The bare wall is cleaned down and given the right bonding or backing coat, then built back up and finished flush with the wall around it. On old solid walls that need to breathe, lime or a breathable plaster may be the right choice rather than gypsum.

5. Ceilings

For a failing lath and plaster ceiling, the usual options are to take it down and put up new plasterboard, or to fix new boards over the old ceiling once it is secured. Both are then plastered. Which is best depends on the ceiling and the room.

6. Let it dry before you decorate

New plaster needs time to dry and a thinned mist coat before paint. Your plasterer should tell you how long to leave it.

What about blown render outside?

Render on the outside of a house can blow in the same way, and the tap test works there too. Loose render lets rain in, which can then cause damp and blown plaster inside. Small areas can be cut out and patched. Where a lot has failed, re-rendering, for example with K-Rend monocouche or silicone render, is often the better long-term fix.

Need blown plaster fixed?

We fix blown plaster on walls and ceilings across Birmingham, Solihull and the whole of the Midlands, from small patches to full rooms. See plastering in Birmingham, ceiling plastering and repairs, plastering in Solihull and plastering in Shirley. Send a few photos and we will tell you what it needs.

Free plastering quote: WhatsApp me on 07453 614095 or message the page. We work Mon-Fri 7am-5pm and Sat 7am-1pm.