Damp & Re-plastering

Why does plaster blow after damp — and why can't you just skim it?

Because the problem is in the plaster, not on it. Rising moisture carries hygroscopic salts up into the wall, where they stay after the water is stopped. Those salts pull moisture from the air forever, so salt-loaded plaster reads damp for good — and skimming over it ruins the new coat too.

That is why "blown" plaster around old damp is so distinctive: it sounds hollow when tapped, the surface crumbles back to powder, paint lifts in sheets, and a fluffy white bloom of salt crystals pushes through anything applied over it. The gypsum in standard plaster is itself moisture-sensitive, so once damp and salts get in, it breaks down mechanically — no sealer or stain block rescues it.

The only honest fix is removal: the contaminated plaster comes off back to the masonry, past the visible damage, and the wall is rebuilt with a system designed to hold residual salts back while the masonry finishes drying. It is messier and dearer than a skim — and it is the difference between a wall fixed once and a wall redecorated annually.

What happens after a damp-proof course: the re-plastering spec

The damp specialist who installs or injects the DPC issues a re-plastering specification — typically: hack off to one metre, or at least 300mm above the highest tide mark; rebuild in a salt-resistant system; keep the new plaster clear of the floor. Their guarantee usually depends on that spec being followed. We build to it.

This division of labour protects you. The DPC and the survey behind it are the damp-proofing company's product, often with a long guarantee attached — and that guarantee is conditional on the wall being reinstated their way. If a builder re-plasters casually in standard gypsum straight off the brick, the guarantee is compromised the day it is written. We work from the specialist's written spec and note the materials used, so the paper trail stays intact.

The same logic applies to other moisture sources. Penetrating damp through a tired external wall is often a job for fresh render outside before anything is rebuilt inside, and a one-off soaking — a leak from above, pipework failure, a flood — is a different animal again: see our water-damaged plaster service for that. Diagnosis first, plaster second, always.

Renovating plaster, sand & cement or lime: which system?

Renovating plaster is the usual choice: a purpose-made, salt-resisting backing that tolerates residual moisture while the wall dries. Sand and cement with an added salt inhibitor is the harder-wearing traditional alternative many DPC specs call for. Lime is the right answer in older solid-wall buildings where breathability matters more than speed.

For most post-DPC work in the 1930s and later stock around Molesey, Walton and Hersham, the first two do the job: renovating plaster where the spec allows it, sand and cement with inhibitor where the specialist demands it, each finished with Thistle MultiFinish once the backing has set. Both hold salts back at the backing layer so the finish and your decoration stay clean.

The older, solid-wall stock is a different conversation. Victorian terraces toward Surbiton and Kingston — and especially the listed cottages out toward Guildford, Farnham and Godalming — were built to breathe, and sealing a solid wall behind dense cement can push moisture somewhere worse. There, breathable lime plaster is often the honest specification even when it is the slower one, and we will say so even when the cheaper system would have won us the quote.

What does damp re-plastering cost in Surrey?

Re-plastering a wall after damp treatment typically costs £400–£900 per wall with us, covering hacking off the contaminated plaster, disposal, the specified backing system — renovating plaster, sand and cement with salt inhibitor, or lime — and a Thistle MultiFinish top coat. Price depends on wall size, system and access.

The spread is mostly about what comes off and what goes on. Hacking off is slow, heavy, dusty work — we sheet out thoroughly and take the rubble away — and lime systems cost more in material and programme than renovating plaster. Several walls done together price better than one visit at a time, and re-skimming the room's unaffected walls in the same visit gets everything to one uniform surface for less than a separate booking.

What the figure does not include is the damp treatment itself — the survey, the DPC, any drainage or ground-level corrections — which belongs to the damp specialist and their guarantee. Beware of any single quote that bundles diagnosis, injection and re-plastering into one suspiciously tidy number: the trades are different, and the accountability should be too.

How long before a damp wall can be decorated?

Longer than anyone wants. The masonry itself dries at roughly one month per 25mm of wall thickness — so a 225mm solid brick wall can take nine months to dry fully. The new plaster surface is paintable far sooner: once the finish coat has dried evenly, a mist coat of watered-down emulsion can go on.

The practical sequence: let the fresh skim dry for its usual 2–5 days, mist-coat, then use breathable emulsion in thin coats — and hold off on vinyl papers, washable paints and anything film-forming until the wall has had its full drying time. A breathable finish lets residual moisture keep escaping; an impermeable one traps it, blisters or shadows, and then the re-plastering gets blamed for physics.

Help the wall along with ventilation and steady background warmth rather than blasting it — fast, fierce drying can drag more salts to the surface. Expect the odd faint bloom of salt on the mist coat, which brushes off dry. We put the realistic dates for your actual wall thickness in writing with the quote, because the schedule is the part of this job nobody else seems willing to be honest about.