Not every crack or dodgy patch means ripping the wall back to bare brick. Knowing the difference between a wall that needs a skim or full replaster and one that just needs a bit of filler can save you hundreds of pounds. Here is how to work it out yourself before you call anyone in.
The single most useful check is to knock gently across the wall with your knuckles. Sound, well-bonded plaster gives a solid, dull thud. If you hit a patch that sounds hollow, drummy or papery, the plaster has 'blown', meaning it has come away from the wall behind it. Filler will not fix a blown area, because there is nothing solid holding it in place.
In older Stirling properties, particularly stone-built terraces and Victorian tenements, blown plaster is common on external walls where damp has crept in over the years. Map out the hollow areas with a pencil. If they cover more than the odd patch, you are into replastering territory rather than filling.
Plenty of tired walls look worse than they are. If the plaster underneath is solid and only the surface is damaged, filler and a bit of patience will do the job. These are jobs a competent homeowner can often tackle over a weekend.
Some problems point to replastering or a fresh skim coat over the whole wall. Cracks wider than a couple of millimetres, or ones that keep coming back after filling, usually mean movement or a bond failure underneath. A wall that bulges, feels soft, or crumbles to dust when you scrape it has failed and needs taking off.
Watch for damp too. Brown tide marks, salty white deposits (efflorescence) or bubbling paint mean moisture is getting in, and in Central Scotland that is often driving rain on exposed gable walls or an old chimney breast. Filling over damp plaster just traps the problem and it returns within months. The damp source has to be sorted first, then the affected plaster replaced.
Even when replastering is needed, it is rarely all or nothing. If most of the wall is sound but the surface is uneven or covered in old artex and patches, a skim coat of roughly 2 to 3mm over the whole wall gives a fresh, flat finish. If only one section has blown, a local patch and reskim of that wall keeps costs down.
As a rough guide, patching a small area might be a couple of hundred pounds, while skimming a full room tends to run into several hundred depending on size, ceiling height and access. Bare-back replastering, where old plaster comes off and fresh coats go on, costs more again. Any honest plasterer will happily tell you when a job is genuinely a filler and roller weekend rather than a paid one.
As a rough rule, hairline cracks up to about 2mm that stay still after filling are fine to fill and paint. Anything wider, or a crack that reopens repeatedly, suggests movement or blown plaster and needs a proper look.
Often yes, if the existing plaster is solid and well bonded a skim coat can go straight over it. If it sounds hollow when tapped or shows damp, it needs removing first, otherwise the new finish will fail too.
Small blown patches can be cut out and repaired by a confident DIYer, but larger areas are tricky to get flat and are usually better skimmed by a plasterer. The key is removing all the loose material first, as filling over it never holds.
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