DIY Doctor

Dark Patches on Newly Boarded and Skimmed Walls

Postby olivia1985 » Fri Oct 28, 2016 1:43 pm

Hi All,

I’m hoping for some advice regarding a newly dot-and-dabbed and skimmed wall in my living room.
Old stone cottage with narrow cavity, brick inner walls. External walls felt damp and cold (north and east facing). Had the room moisture tested by a damp ‘specialist’ a year ago when I moved in, he said there was no damp, but a condensation problem, he prescribed a dehumidifier/heat/ventilation. Nevertheless, I installed a DPC on the north wall ‘just in case’ (I was doing the kitchen and living room anyway as they were damp, and had the stuff leftover). When I striped the wallpaper off in the living room, it looked like the plaster had been painted with some sort of sealant by the previous owner, which I think we causing the room to sweat. When the dehumidifier was on, the plaster was dry. Turn it off and walls were wet again within a day. I then decided to remove all the plaster to see what was going on underneath, and the brickwork appeared dry, whether the dehumidifier was on or not. No salt deposits on the brick. Also found an airbrick which had been blocked up, so I unblocked that for ventilation. I scrubbed the outside (north and east walls) with biocide as there was moss growth at the bottom, filled any small gaps in the mortar and then used a StormDry masonry paint as a breathable waterproofing for the outside. The east wall (gable end) has been re-pointed by previous owner and looks in good nick.
The 2 radiators in that room were removed about 6 months so there has been no heat in there, however it’s not been particular cold (weather-wise).
2 weeks ago we boarded the room with dot and dab, and one week later the plasterer came and skimmed. Plaster appears dry (light pink and even) so last night (4 days after it was skimmed) did the first mist coat (60% water 40% trade emulsion). This morning, the while mist coats has big dark spots where the dot is, only on the north and east walls (south side, also external, is fine, as is the internal wall). The whole of the walls are affected, you can see every area where adhesive was applied, from floor to ceiling. Could this be from damp in the brick tracking through the adhesive? Can this happen after just 4 days? Or maybe is the plasterboard adhesive not dry? Is there a solution or am I going to have to remove the skim and plasterboard and baton the wall instead? I feel like if the issue is caused by damp, it would have taken longer than 13 days to come through, and it would be more patchy, not so regimented.
I’d be really grateful for any advice! (Sorry for the long post too).
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Postby welsh brickie » Sat Oct 29, 2016 4:58 pm

it sounds like damp to me, But give it a few more days to see if it dries out.
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