plastering sandstone building, persistent damp spots and ...

its amazing just how much you learn after the problem occurs....

I bought a vandalised property that had suffered due to having the lead ripped off the roof, combined with age... It's an old sandstone building with brick extensions.

After drying out the building as much as I can, I had it plastered. For a week.. it looked great, and then I noticed blotches on a couple of walls. The plasterer has dot'n'dabbed with basic plasterboard on sandstone, which I've since found out is sooo wrong. I understand that he should have used battens, or foil backed plasterboard with a chemical bonding. On a hot day the blotches disappear, which I can only assume is the heat making the moisture evaporate. On a cooler day the wet blotches re-appear, which I'm pretty sure is where they have dot'n'dabbed. I'm now of the understanding that the only suitable fix is to rip down the plasterboard and replace.

alternatives? (please!)

In the adjacent room part of the 'red brick' walls were re-plastered, but there are damp patches that wont dry regardless. I've tried dehumidifiers and oil radiators to no avail. Although the house was sodden with water, these patches persist with no explanation as the roof is now sound, and guttering has been replaced. Is there any infra-red super-duper dryers that I can use to vapourise the moisture and find the source?

thanks ...

Paul.

I think your problem sounds like salts/efflorescence. I would "definitely not" dot and dab any plasterboard onto your sandstone walls. I would batten them out, insulate, and then use 12.5mm foil backed plasterboard. Old red brick walls are notorious for salts/efflorescence when using gypsum plasters. If you are going to plaster onto them, it is always advisable to scratch coat the wall/s with render first, containing a waterproofer/salt inhibitor in the mix,,, or if you can,, batten and plasterboard them. It's worth checking for other sources that the damp might be coming from too,,, soil level outside to high,,, damaged pointing, leaks coming from gutter joints,, check all of these just in case, but my money's on efflorescence.

thnx... much appreciated.

You've confirmed my thinking on the sandstone. I think the only option is to take down and redo (with a different contractor!). Is it not possible to use a chemical adhesive on the sandstone with foil plasterboard, and skip the battens? Only ask to save room... Is it advisable to put vents in the plasterboard, so the walls can breathe?

Good idea on the waterproofer/salt inhibitor. I think I;ll take off the plaster back to the brick, and re-do.
The soil level is low, no visible marks/cracks on the external wall (which is rendered) so I think the moisture could be historical.

Much obliged!

Paul.

Out of interest, how long have you left the building to dry since sealing it against water ingress? Walls take 1 month per inch of thinckness to dry out as a rule of thumb.

have left it about 3+ months.... left doors/windows open when I can.
Had 2*dehumid's in there.
I understand salts/efflorescence can discolour the plaster, but it feels damp/cold in spots. I really should get a damp meter and take some readings to track it over time.

Leave it to dry out for a few more months then coat the plasterboard with oil based undercoat. It's far too early to go replacing the stuff as the walls are still drying. You should have used insulated plasterboard - way warmer.

thanks for all the info and detail.

I think what I'll do is drill more holes in the existing plasterboard, and then batten along the existing plasterboard as its smooth and level. Then I'll use some foil backed plasterboard on top.
I'm doing it this way as I'm trying to open the place as a business, and I'm running out of time.... (time is money!). Ideally I'd strip everything back and batten along the sandstone, however its quite uneven.
This way I'll lose a couple of inches (which is annoying) but it means I'll only have to wait a week for the plaster to dry.

For the plaster on the red brick, I'm going to take that out and put render in there with an additive.

thanks all.

Paul

To do what you suggest is going to cause problems, Sandstone, being very porous, needs to breathe and to double plasterboard,especially with second layer of foil backed will cause a damptrap within a short space of time.Is this building listed?It very much sounds like the walls need to be Lime plastered and lime washed to allow them to breathe, is the outside sand/cement rendered? if so it needs addressing, i.e knocking off and again Lime plastered or Lime re- pointed.If the walls can breathe you will solve the problem but the drying time, as posted, will take months.

fortunately its not listed, but I'm led to believe its 1700's.

The outside wall is pebbledashed with, I'm guessing, sand/cement - so I guess the point your making is that the outside wall wont breath either. Therefore, the indoor wall must?

Possibly it sounds like leaving what I have, as is, and making vent holes so there is a gap to breath, and to use an oil based paint (soak stain is it?) to stop the damp rising. At least this way I can get the business up and running.

Wow... never easy.

Paul.