Damp patches upstairs where chimney breast previously removed

[davidm1969]

Hi all

posting in case any of the experts here can help. Attached are photos of some damp patches upstairs in a bedroom - I painted the wallpaper about a year ago, the area affected is where a chimney breast used to be. Using one of those meters wiith electric prongs the readings are c. 10-15% from about 40cm either side of the lighter white rectangular patch from ceiling to a bit above the skirting board, and up to around 50% within the affected area [particularly but not only the darker patches within this rectangle. some higher reading in the lighter areas and either side up to 40cm]. Some small black marks now coming through [lines see pic ending 028 attached] as well.
No evidence of similar discolouration/damp patches either above in loft space or below in living room.


I have a roofer coming to take a look at the chimney and tiles above [when I bought the place a couple of years ago he replaced some cracked 40yr old concrete tiles and repaired flashing]. I'm aware my neighbour [other side of the party wall has had issues with damp]

My guess is that this is penetrating damp of some sort coming through the chimney stack or tiles above, but if that's the case I'm puzzled why there is no evidence of it in the attic above - except that it might be hidden by the [I think maybe newer] dry lining /dab and board in the attic room.

Any thoughts or ideas from others which much more expertise than myself very welcome

thanks in anticipation

 

[noseall]

Hi all

posting in case any of the experts here can help. Attached are photos of some damp patches upstairs in a bedroom - I painted the wallpaper about a year ago, the area affected is where a chimney breast used to be. Using one of those meters wiith electric prongs the readings are c. 10-15% from about 40cm either side of the lighter white rectangular patch from ceiling to a bit above the skirting board, and up to around 50% within the affected area [particularly but not only the darker patches within this rectangle. some higher reading in the lighter areas and either side up to 40cm]. Some small black marks now coming through [lines see pic ending 028 attached] as well.
No evidence of similar discolouration/damp patches either above in loft space or below in living room.

View attachment 305088View attachment 305089View attachment 305090View attachment 305091
I have a roofer coming to take a look at the chimney and tiles above [when I bought the place a couple of years ago he replaced some cracked 40yr old concrete tiles and repaired flashing]. I'm aware my neighbour [other side of the party wall has had issues with damp]

My guess is that this is penetrating damp of some sort coming through the chimney stack or tiles above, but if that's the case I'm puzzled why there is no evidence of it in the attic above - except that it might be hidden by the [I think maybe newer] dry lining /dab and board in the attic room.

Any thoughts or ideas from others which much more expertise than myself very welcome

thanks in anticipation

It's nothing to do with external damp. What you have there is classic hygroscopic manifestation. The damp is attracted from within. Masonry saturated with burnt fuel deposits are notorious for it. The plaster need hacking off and the offending area coated with a few coats of SBR slurry, before a prepared render backing is applied to the last tacky coat of SBR. Plaster can be applied to the top coat as normal.

 

[davidm1969]

Thanks, Noseall. Can you say more about what sort of 'SBR slurry' and 'prepared render backing' is needed?

If I ve understood you right it's moisture from inside the room, even tho I keep it well ventilated.

What about covering with a oil based paint primer? [suggested here https://surveyor.tips/hygroscopic/ ]. That will stop the moisture getting to the plaster and is a whole lot cheaper/easier? what's the benefit of slurry/new plastering exactly?

 

[noseall]

Thanks, Noseall. Can you say more about what sort of 'SBR slurry' and 'prepared render backing' is needed?

If I ve understood you right it's moisture from inside the room, even tho I keep it well ventilated.

What about covering with a oil based paint primer? [suggested here https://surveyor.tips/hygroscopic/ ]. That will stop the moisture getting to the plaster and is a whole lot cheaper/easier? what's the benefit of slurry/new plastering exactly?

Saturated masonry can be quite aggressive in terms of persistence. You may block it with a paint. Wouldn't hurt to give it a go, I guess.

SBR slurry creates a barrier/seal as does treating the immediate surface with an admixture (waterproof) render.

 

[JohnD]

Is the flue ventilated top and bottom? It needs to be. If the chimney served an upstairs and a downstairs fireplace, each will have its own flue.

The old fireplace may have had builders rubble hidden behind it. Dig this out.

 

[davidm1969]

"
Is the flue ventilated top and bottom? It needs to be. If the chimney served an upstairs and a downstairs fireplace, each will have its own flue.

The old fireplace may have had builders rubble hidden behind it. Dig this out."

thanks JohnD. No ventilation that I can see either in the bedroom or the living room below or attic room above. Might be below the skirtingboard/in the floor space . What is needed, an airbrick ?

Why would that cause the marks on the wall on 1st floor

thanks

 

[cdbe]

Last one I did took 4 coats of SBR slurry to stop the wet tar.

 

[JohnD]

An airbrick where the fireplace used to be before it was bricked up

At the top, depends if the chimney was sealed up, left open, or demolished down into the loft.

Lack of ventilation causes damp on the chimneybreast from internal condensation. If the damp is passing through soot if will leave yellow or brown stains, or even black tar. It can also be plain clean water.

 

[jeds]

noseall answered the issue. Chimney stacks, over many years, impregnate the surrounding masonry with hygroscopic salts. Those salts attract moisture from the atmosphere and appear as dampness, which will be highly visible in standard gypsum plaster finishes. An ideal solution would be to re-plaster using limelight plaster, which is what should have been used in the first place.

PS. if the chimney breast is no longer there, what will be ventilated? Are you talking about a stack in the loft?

 

[mrrusty]

Fix plasterboard with foam adhesive over the wall rather than dabs. Wet dabs pull the salt through to the surface as the water in the dabs evaporates. The hygroscopic surface salts attract moisture from the room causing damp patches. The foam adhesive avoids this, and the salts in the brick can't get through the foam. You could also SBR slurry as belt and braces to isolate the salts in the wall, but my own experience is that using foam is enough.