"Blown" is the word plasterers use for plaster that has lost its bond with the wall behind it. It might still look fine from a distance, but it is no longer properly stuck on. Left alone it tends to get worse, and anything you put over the top, whether paint, wallpaper or a new skim, is only as secure as the loose plaster underneath.
How to spot blown plaster
The tap test
Knock gently on the wall with your knuckles and work your way across it. Sound plaster gives a sharp, solid sound. Blown plaster sounds dull and hollow, almost like knocking on a box. Mark the hollow areas lightly with a pencil and you will soon see how big the problem is.
Bulges and bowing
Look along the wall with a torch held flat against it. Blown areas often bulge out slightly, and the light shows up the shape.
Cracks that keep coming back
A crack you have filled and painted that reappears in the same place, especially with a slight lip on one side, can be a sign the plaster behind it is loose.
Crumbling or powdery plaster
If the plaster turns to dust when you press it or scrape it, or comes away in chunks when you take off old wallpaper or a picture hook, it has broken down.
What causes it
- Age. Plaster on older solid-walled houses can simply reach the end of its life after many decades.
- Damp. Moisture getting into a wall, whether rising, penetrating or from a leak, weakens the bond and breaks down the plaster.
- Movement and knocks. Doors slamming, furniture hitting walls and general vibration all take their toll, particularly around door frames.
- Poor preparation. Plaster applied over a dusty, painted or unsuitable surface without the right preparation may never have bonded well in the first place.
Can blown plaster be patched?
It depends how much of the wall is affected.
If it is a small area, the usual fix is to cut the loose plaster back to a sound edge, build the patch back up and then finish it flush. Often the best result comes from then giving the whole wall a fresh skim so the repair does not show once it is painted.
If large parts of the wall sound hollow, patching becomes a false economy. You end up with more patch than original plaster, and the bits you left are likely to go next. In that case taking the wall back to brick and starting again is usually the better long-term answer. I go through how I decide in re-skim or back to brick.
On a plasterboard wall the problem looks different. Board does not "blow" in the same way, but it can be damaged by knocks or water and need a section cutting out and replacing. That is a plasterboard repair rather than a plastering job in the traditional sense.
Fix the cause first
If damp is behind it, re-plastering on its own will not last. The source of the damp needs to be found and dealt with before any new plaster goes on, otherwise the same thing will happen again. Signs that damp is involved include tide marks, white salty deposits on the surface, a musty smell or plaster that feels cold and soft.
Timing the work
Blown plaster can be dealt with at any time of year indoors. If the work involves stripping a wall back, allow longer for it to dry in the colder months. I cover that in my post on plastering in winter.
Frequently asked questions
Is blown plaster dangerous?
On walls it is mostly a cosmetic and practical problem, though large loose areas can come away. On a ceiling, loose or sagging plaster should be looked at sooner rather than later.
Can I just fill the cracks and paint over?
You can, but if the plaster behind is loose the cracks are likely to return. It is worth tapping the wall first.
Will stripping a wall damage the brickwork?
Hacking off plaster is done carefully to leave the brickwork ready for its new coats. Any loose mortar or damaged areas found underneath can be dealt with before replastering.
Get your walls checked
If you have hollow or crumbling plaster in Birstall or nearby, I can come and tap it out, tell you what it needs and give you a clear price. Request a free quote or call 07590 357963.
