Blown plaster is plaster that’s lost its grip on the surface underneath (brick, block, old plaster, lath, or plasterboard). Instead of being firmly bonded, it can lift, bulge, or crumble because there’s a gap behind it. A common giveaway is that it no longer feels solid under your hand.
This matters because what you can see on the surface (bubbling, flaking, cracking) is often the end result of something else going on behind the plaster, most often moisture, but sometimes movement or poor prep.
How To Spot Blown Plaster?
You don’t need special tools to find early plaster damage. A few simple checks can tell you a lot:
- The tap test : Gently tap the wall. If it sounds hollow when tapped, that patch may have detached. Solid plaster sounds dull and firm.
- Bulges, bubbles, and ripples : Areas that look slightly raised or uneven can be plaster lifting away behind the paint.
- Flaking paint or wallpaper : If finishes peel away in strips or keep failing in the same place, the plaster underneath may be unstable.
- Powdery surfaces and white staining : A dusty, chalky feel or white deposits can point to salts being carried through by moisture, which can weaken plaster over time.
If you’re seeing repeated cracking (where plaster to crack keeps coming back after you fill and repaint), treat that as a clue to look deeper rather than endlessly patching.
Why Moisture Causes Plaster To Blow?
Moisture is the big one. When water gets into a wall, it can break down the bond between plaster and the surface beneath, leaving weakened plaster that starts to lift and fail.
Common moisture routes include:
- Rising damp : Moisture travels up from the ground through porous masonry, often showing as damp patches and tide marks low on the wall. Over time, salts can build up in the plaster and finishes fail.
- Penetrating damp : Rainwater getting in through defects such as cracked render, missing pointing, or roof leaks can cause patches higher up or on ceilings.
- Condensation : Warm, moist air hits a colder surface and turns into water droplets. In modern, well-sealed homes, this can be more common if ventilation is poor.
A key reason damp-related plaster fails again and again is salt contamination. Some salts are hygroscopic, meaning they attract water from the air. Even after you “fix the damp”, those salts can keep trapping moisture and cause new blistering, powdering, or staining unless the contaminated plaster is removed and replaced properly.
Other Common Causes of Blown Plaster
Not all blown plaster is damp (though damp is common). Other causes usually come down to bonding and workmanship:
- Poor surface preparation : Plaster doesn’t like dusty, greasy, painted, or loose backgrounds. If the wall wasn’t cleaned, keyed, or sealed correctly, the new coat may not stick.
- Wrong products or timing : Replastering before the wall is dry, or using the wrong system where salts are present, can lead to repeat failure.
- Old, tired backgrounds : Older plaster walls can have areas where previous repairs, hairline cracks, or delamination have built up over decades, so sections finally give up.
If the failure is widespread or keeps spreading, treat it as more than a cosmetic job.
Underlying Structural and Environmental Contributors
Sometimes you’re dealing with more than just the plaster layer.
- Structural movement : Small shifts can open cracks that let moisture in or break the bond over time. This isn’t always dramatic. Minor settlement or seasonal movement can be enough to start problems.
- Ongoing external water exposure : Recurring rain penetration, defective gutters, and roof leaks can create a repeating wet/dry cycle that damages finishes and encourages further breakdown.
- Moisture in newer homes : Even new builds can have higher internal moisture while materials dry out, so ventilation becomes important to prevent condensation and mould.
Leaving damp and mould unmanaged can also create health risks, especially for people with asthma or allergies, so it’s worth acting early.
What To Do First: Assess The Underlying Cause
Before any plaster repair, work out why it’s failing, otherwise you can spend money twice.
- Map the area : Is it low down (think rising damp), around windows/chimneys (often penetrating damp), or in bathrooms/bedrooms (often condensation)?
- Look for patterns : Do the damp patches get worse after heavy rain (penetrating damp) or in winter when heating is on and windows are shut (condensation)?
- Check obvious sources : Leaking gutters, missing tiles, cracked pointing, plumbing leaks, anything that keeps feeding moisture into the wall.
If you suspect salts (powdering, repeat bubbling after “damp repairs”), remember: guidance commonly recommends removing contaminated plaster and replastering with a suitable system, not just skimming over it.
Practical Repair Options For Blown Plaster
The right fix depends on how much has failed and whether moisture is still present.
For small, localised areas
Scrape back to firm edges and remove anything loose. Ensure the wall is clean, sound, and dust-free. Seal the background so it doesn’t suck moisture out of your filler too fast. Then build up in layers and sand back.
For larger or spreading areas
If it’s wide, bulging, or powdery, you normally need to remove it back to a solid substrate. Let the background dry if damp is involved. Rushing replastering is a common reason patches fail again.
When you apply plaster, the surface must be properly prepared. That means stable, keyed, and suitable for the product you’re using. If salts are present, removing salt-contaminated plaster and using a salt-resistant replastering approach is often recommended to reduce repeat breakdown.
Long-Term Prevention Tips
Good prevention is mainly moisture control and basic upkeep.
- Keep gutters, downpipes, roof tiles, and pointing in good order to reduce penetrating damp and hidden roof leaks.
- Improve ventilation where condensation is common, especially after cooking and showering.
- Avoid sealing damp walls with the wrong finishes. Some coatings can make problems worse by trapping moisture in the wall fabric.
These steps protect walls in the long term and reduce repeat failure.
When To Call A Professional
Call in help if the area is large, spreading, or soft and unstable. Also get advice if you see tide marks, salts, or repeated bubbling after past repairs. If you suspect a structural issue, such as widening cracks, sticking doors, or obvious structural movement, it’s worth getting it checked before spending money on finishes. For damp problems, a competent surveyor or damp specialist can help confirm the cause and stop repeat repairs.
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Final Thoughts
Blown plaster means the plaster has detached and will not stay sound without proper repair. If it sounds hollow when tapped, it’s likely loose behind the surface. Damp is a leading cause. Rising damp, penetrating damp, and condensation can all lead to plaster damage.
Do not rush to skim or paint. Find the cause, then choose the right plaster repair and make sure surfaces are properly prepared before you apply plaster. If salts, repeated damp patches, or possible structural problems are present, professional advice can stop you paying twice.
