Blown plaster means it has come away from the wall behind. Here is how to find out why, and why re-plastering over it is a waste of money.
"Blown" plaster means it has lost its bond with the wall behind and is now sitting loose. Tap it and it sounds hollow and drummy rather than solid. You will often see bubbling, crumbling, or a powdery white bloom on the surface first.
The plaster is the symptom. Something else caused it, and until you find out what, replacing the plaster is money down the drain.
In roughly the order we find them.
By far the most common. Water gets into the wall, the plaster stays wet, the bond fails. Find the water first, see damp and blown plaster.
Rising damp carries salts up through the wall. Those salts crystallise in the plaster, expand, and push it off. You will see a white powdery bloom on the surface.
Plaster applied to a dusty, unsealed or over-dry wall never bonds properly in the first place. It can hold for years and then let go all at once.
In older Exeter properties, the timber laths shrink and the plaster keys behind them snap. The ceiling or wall then hangs on very little.
Modern gypsum plaster on solid cob, stone or brick traps moisture in the wall. It will blow. Those walls need lime.
Structural movement, settlement or a knock will crack the plaster and break the bond locally.
Tap the wall with your knuckle and work across it. Sound plaster gives a solid, dull knock. Blown plaster sounds hollow and drummy, like tapping a cardboard box. Map out where the hollow sound starts and stops. That is the area that has to come off.
Then look for clues about why. Is the wall cold or damp to the touch? Is there a white powdery bloom? Is it worst at the bottom of the wall, which suggests rising damp, or in a patch near the ceiling, which suggests a leak above? Is it below a window, near a chimney, or on an external wall?
This is the important bit, and it is why we will always ask where the damp is coming from before quoting.
If you skim over blown plaster, you have put a thin new surface on top of a layer that is already detached from the wall. It has nothing to hold onto. If the underlying damp is still there, the new plaster gets wet too, the salts come through, and within a year you are looking at exactly the same bubbling and crumbling, except now you have paid for it twice.
The old material has to come off, the wall has to dry, and the cause has to be dealt with. Anyone offering to just plaster over it is either not looking properly or hoping you will not ring them back.
The ones we get asked most.
Blown plaster has lost its bond with the wall behind it. The most common cause is damp, followed by salt contamination from rising damp, poor preparation when it was originally applied, failed keys in old lath and plaster, and modern gypsum plaster used on solid cob or stone walls that need to breathe. The plaster is a symptom, something else caused it.
Tap the wall with your knuckle and work across it. Sound plaster gives a solid, dull knock. Blown plaster sounds hollow and drummy. You may also see bubbling, crumbling, cracking or a white powdery bloom on the surface.
No. Skimming over blown plaster puts a new surface on top of a layer already detached from the wall, so it has nothing to bond to. If the underlying damp remains, the new plaster will blow within a year. The old material must be removed, the wall must dry, and the cause of the damp must be fixed first.
A white powdery bloom on plaster is usually salt efflorescence. Rising damp carries salts up through the wall; as the moisture evaporates the salts crystallise in the plaster, expand, and push it away from the wall. It is a strong indicator of an ongoing damp problem rather than a one-off leak.
Solid cob, stone and old brick walls need to breathe. Modern gypsum plaster is relatively impermeable, so it traps moisture inside the wall. The moisture cannot escape, the bond fails, and the plaster blows. These walls need breathable lime plaster instead.
Send a photo on WhatsApp and we'll tell you straight, including if the answer is "leave it alone".