How do I know if my plaster has blown?

Tap it. A hollow, drum-like sound over an area larger than a hand means the backing has separated from the wall behind. Small isolated hollows can be cut out and patched, but widespread hollowness means the wall needs stripping.

Can a skim fix a wavy wall?

No. A skim is two to three millimetres thick and follows the shape underneath. Dot and dab boarding over the top, or a float coat before the skim, is what gives you a flat and plumb wall.

Is it worth stripping a wall just to add insulation?

On a cold solid external wall in an older terrace, insulated laminate board is often worth doing while the wall is open, because the labour is already being spent. On an internal partition there is no case for it.

How much more does a full replaster cost than a skim?

Roughly double per room in Teesside, £500 to £1,000 against £300 to £600, because of the strip-out, the boarding or backing coat, the waste and the extra drying time.

Does my wall need a skim or a full replaster?

A skim is enough when the existing plaster is firmly stuck, dry and reasonably flat, which covers most decorating-driven jobs at £300 to £600 a room. Strip back and board or re-render when the wall sounds hollow, is bulging, is crumbling, or is still damp, which typically doubles the cost but is the only fix that lasts.

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Four tests you can run yourself

You do not need a trade to work out which category your wall is in. Five minutes with a knuckle, a torch and a straight edge gets you most of the way, and it means you can read a quote rather than take it on trust.

The knuckle test: tap across the wall in a grid. A dull, solid note means the backing coat is stuck to the masonry. A hollow, drummy note means it has let go, and skimming over it only bridges the gap.
The damp check: look at the bottom 600mm and around chimney breasts, window reveals and external corners. Tide marks, salty white bloom, soft plaster or a musty smell mean water first, plaster second.
The level check: hold a long straight edge or spirit level against the wall in a few places. Bows and hollows of more than a few millimetres will still be there after a skim, because a skim follows what is underneath.
The finger test: rub a bare patch. Old lime or gypsum that powders away, or a surface that has been painted, sealed and painted again until nothing will bond to it, both point to a bigger scope.

When a skim is the right call

The everyday case. Woodchip has finally come off a Billingham semi and left a scarred but solid wall. A room has been papered for thirty years and the owner wants paint. A kitchen has been ripped out and the walls behind it are patchy but sound. A textured ceiling needs to go flat. In all of these the substrate is stuck and dry, so two coats of finish plaster at £300 to £600 a room is exactly the right amount of money to spend.

When the wall needs stripping

Hollow areas covering more than a small patch, bulges you can see from the doorway, crumbling backing coat, or plaster that is still carrying damp and salts. On the older solid-wall stock across central Middlesbrough, the Hartlepool Headland and the Saltburn streets, gypsum plaster applied over the years onto walls that want to breathe is a recurring cause. The two routes from there are hacking off and re-rendering in the traditional way, or hacking off and dry lining, which is faster, drier and lets insulation go in at the same time. Either way you are into £500 to £1,000 a room rather than £300 to £600.

The middle ground nobody mentions

Most rooms are mixed. One wall is failing, three are fine. The sensible quote treats them differently: strip and board the bad one, skim the rest, finish everything in the same visit so the whole room ends up level and consistent. Beware of both extremes, the quote that skims everything regardless of the knuckle test, and the quote that strips a whole house because one chimney breast is damp. Small isolated defects belong on the repairs page, and the cost guide has the figures for each route.