Damp patch on chimneybreast

[Helzo79]

Any ideas what is causing this wet/damp patches in our lounge and dining room please? It’s a 3 bed 1930s semi. This is all downstairs.

3 patches are either side of chimney breasts but one is on the floor near where a door has been blocked.

They fade completely in hot weather but come back get darker when it’s has rained. They only get darker not bigger. They have not spread so I don’t think it’s a leak. We don’t have the same issue upstairs.

A previous owner moved the lounge wall, the door and blocked the chimney.

Chimney recapped recently and roof checked - all ok but it’s still happening. See photos please

 

[blup]

Condensation inside chimney due to lack of ventilation.

Blup

 

[mrrusty]

Chimney + dark "damp" spots that appear when the air is humid = salt contamination. It'll be in the brick of the chimney as a result of combustion gases and moisture reacting with lime-based building products. When the walls are plastered the evaporation of the water in the drying plaster pulls salts to the surface. In damp weather these hygroscopic salts absorb moisture from the air. I have it in my house. 80% fix has been to paint the wall with a damp seal paint to stop room air getting to the salts. 100% fix has been to strip back to brick, tank the brick with SBR slurry (only the affected area) and fix plasterboard with foam adhesive which isolates - unlike "wet" dab adhesive.

 

[ahow35]

My parents had a similar issue with theirs. It turns out that the lead on the outside of the chimney was damaged and had slowly been letting in water.

I think it would be best to get a roofer to go up and take a proper look before the wetter weather comes. I know you said it's been checked, but chances are a small dribble is still getting in somewhere!

 

[Helzo79]

Chimney + dark "damp" spots that appear when the air is humid = salt contamination. It'll be in the brick of the chimney as a result of combustion gases and moisture reacting with lime-based building products. When the walls are plastered the evaporation of the water in the drying plaster pulls salts to the surface. In damp weather these hygroscopic salts absorb moisture from the air. I have it in my house. 80% fix has been to paint the wall with a damp seal paint to stop room air getting to the salts. 100% fix has been to strip back to brick, tank the brick with SBR slurry (only the affected area) and fix plasterboard with foam adhesive which isolates - unlike "wet" dab adhesive.

Huge thanks this is really helpful. It’s probably been there ages as they blocked but didn’t vent the chimney back in 2002 ish. We have just repainted ( over egg shell) so I wonder if that was hiding it.

 

[Helzo79]

My parents had a similar issue with theirs. It turns out that the lead on the outside of the chimney was damaged and had slowly been letting in water.

I think it would be best to get a roofer to go up and take a proper look before the wetter weather comes. I know you said it's been checked, but chances are a small dribble is still getting in somewhere!

Good idea. Thanks for your time - really helpful

 

[Helzo79]

Condensation inside chimney due to lack of ventilation.

Blup

Thanks - makes sense

 

[blup]

Properly ventilating the flue will reduce the source of water vapour to the chimney salts. Give it time to dry out and see the effect over a year. Try a solvent based seal such as zinsser cover first before progressing to more costly solutions. If you're in more of a hurry board over it with appropriate fixings in plastic wall plugs.

Blup

 

[Helzo79]

Thanks -.really helpful

 

[jeds]

As Blup's advice. Salt contamination gets into the plaster, which then absorbs atmospheric moisture. During wetter periods the absorption is greater, which gives the appearance of a dampness, when it is really just condensation. It's not a roof leak though so save your money there. The only 100% remedy is to re-plaster with a renovation plaster.

 

[Helzo79]

Thanks - that makes sense as we have had the roof checked and chimneys capped. Do we need to vent it too?

 

[mrrusty]

The only 100% remedy is to re-plaster with a renovation plaster

@jeds or, as I have done, sbr slurry the brick and then use plasterboard fixed with foam adhesive. Both methods isolate anything in the wall. This has also worked 100% for me with quite severe salt contamination

 

[JohnD]

Thanks - that makes sense as we have had the roof checked and chimneys capped. Do we need to vent it too?

Start by venting it as this is essential anyway.

Top and bottom, both flues

It will take months to dry out. Then see what else (if anything) you need to do.

 

[jeds]

plasterboard fixed with foam adhesive.

That will work also, but not so practical to patch a piece in - you'd be looking at lining the whole chimney breast.

 

[opps]

A customer recently had a similar problem with hygroscopic salts.

I had redecorated the room earlier in the year. I was under the impression that the stains were old water stains. I applied SBR to the walls prior to filling and lining the walls. The stains reappeared later in the year.

He contacted a specialist. The (free) advice that he was given was to go back to the brick work and render the chimney breast.