Moved into our Victorian house 18 months ago. The previous owners (good folk, who we trust) had had a new roof put on and the attic chimneys capped soon before we moved in. There was a damp patch near one of the two chimney breasts in the attic room when we moved in, but the roofers (who did a good job) had told the previous owners the patch should eventually dry out – maybe take six months. A thorough survey by our surveyor picked up no issues. Needless to say, the patch hasn’t dried yet, and looks as though it never will. A roofer friend of mine (who knows his stuff) went on the roof and said the workmanship is good, with no obvious sign of where the water is getting in. Any ideas for treating the wall and repainting? The damp isn’t bad really, and doesn’t seem to be worsening, but there is some powdering/flaking of the paint and the patch is always visible. I live in Calderdale, where it’s very wet. Ta for any tips.

Have you got a vent in the chimney?

Or has a big old wet chimney breast been sealed off?

I’ll keep an eye on this, cos I’ve got pretty much the same problem.

What’s your heating regime? Could well be condensation on a cold spot on the wall.

Try running a dehumidifier for a bit and see if it clears up, if it does it’s probably not an external source.

No amount of damp proof paint will cover it. You need to work out the actual cause.

Does the way face the prevailing wind? Checked the pointing?

We had a problem with old plaster that had been wet in the past and was full of salts from the original damp/wetness water. It would attract moisture form the air and make the patch damp, efflorescence. https://en.wikipedia.org/wiki/Efflorescence. This would then go mouldy..

We cleaned the salt off numerous times, and used stain block and painted over it, but even small amounts of salt attract water to the surface. A damp professional came out with his Infra red camera and showed us it happening as you breathed on the wall. He drilled into the plaster and took samples to test for salt and it was present in high doses.
After that I realised you could clean the patch off, let it dry and sneak look in through the door and it wouldn’t be there, walk and breath for 3 or 4 mins and it would bloom up damp!
In the end we had to knock the plaster off the wall (it was bone dry underneath) and replaster. This was over a period of 5 years or so until we go the bottom of it.

Thanks for these. I should have said – house is mid-terrace and the wall is internal, separating us from our neighbour. The chimneys were capped off by the roofers the previous owners used, I think to help solve the damp wall problem and because they served fires that weren’t being used. We have other chimneys to serve a wood burner and open fire on the ground floor. Not sure about vents to be honest. Roof is insulated. We use central heating Oct-March. Large attic room is the footprint of the entire house (was once two rooms – now huge single bedroom/office) tends to get warm, has four Velux windows. The room doesn’t get used much at all at the moment, but not because of the damp, which is confined to a corner and not too bad (just bad enough for me to think I need to do something, sometime). Cheers.

Can’t help with locating the source but can recommend Zinsser BIN or Cover Stain for overpainting.
Had problems with historic stains coming through new paint layers and these sorted them out.

houndlegs – Member
I’ll keep an eye on this, cos I’ve got pretty much the same problem.

+1

As for “treating” it, you can get an aerosol stain-blocker which you spray on then paint over.

5plusn8 – Member
We had a problem with old plaster that had been wet in the past and was full of salts from the original damp/wetness water. It would attract moisture form the air and make the patch damp, efflorescence. https://en.wikipedia.org/wiki/Efflorescence. This would then go mouldy..

We cleaned the salt off numerous times, and used stain block and painted over it, but even small amounts of salt attract water to the surface. A damp professional came out with his Infra red camera and showed us it happening as you breathed on the wall. He drilled into the plaster and took samples to test for salt and it was present in high doses.
After that I realised you could clean the patch off, let it dry and sneak look in through the door and it wouldn’t be there, walk and breath for 3 or 4 mins and it would bloom up damp!
In the end we had to knock the plaster off the wall (it was bone dry underneath) and replaster. This was over a period of 5 years or so until we go the bottom of it.

Now that IS an interesting take on it.

What’s behind the wall? Any chance the neighbour has a leak that is causing the issue?

hooli – I’ll ask. Think it’s their spare room/office. We get on very well with them and they’re decent folk, so shouldn’t be an issue.

We have a very similar sounding patch on one of our down stairs chimney breast. Our house was built in the 1880s and the patch is on a chimney on an outside wall.

Ours only really becomes apparent when its humid.

Basically our issue seems to have occurred from our chimney breast’s being bricked up with the vents becoming clogged with debris. The patch corresponds with a ledge in the chimney.

When we opened up the chimney, with hindsight we should have left them a while to breath before dropping a liner and backfilling as after 8 years the patch is still there 😆 Again only when its humid.

In one smaller place on the ledge of bricks creating the arch you can see the placement of the bricks through the plaster as moisture is drawn to the area. Very strange.

Absolutely no mould though.

Spoke to a builder friend recently who said the heat from the stove should “push” the moisture out but the fact its insulated doesn’t! Having the stove on does dry it out but it returns as though the top layer of moisture drys out but not the wet in the brick.

He suggested a dehumidifier or pulling off the plaster.

I’m going with a bigger mirror on the wall …. out of sight and all that 😆

Chimney wall sounds key, similar to 5+8s post, chimney carries soot deposits over time and often with old water leaks moisture moves salts and tars through masonry, eventually contaminating plaster walls and finishes. You will often see the pattern of a chimney flue on gable walls, usually darker colour. So if this is the case here, even if original leak fixed salts etc still in wall, eventually moist damp air, (it’s that time of year before we get full hearing on) is attracted to the salt and then stains can appear. So not always fresh damp but maybe signs of salts and staining.
Yes I too live in an old house and get this though the work I do. Depending on extent may be worth trying strain block, could test for damp and salts using a proper test (not timber moisture meter). Solution depends on extent and what you can live with.
If re plastering consider doing it in lime if that is what the original is, as it will act the same way add the other walls, I think the term is hydroscopic, i.e. The lime will take on moisture and evaporate in due course, hence buildings needing heat and ventilation.
Sorry for all the words, hope that helps.
C

Is your chimney capped off completely ? If so it shouldn’t be, it should be vented outside the roof and near the bottom to allow for air circulation. You get the same thing by filling your cavities with insulation, it stops air circulating from the airbricks to the roof.

I have / had the same problem on a house i’m selling.

I had just lived with it, but got a damp report done by a specialist firm before marketing it as I know buyers would question it.

It was as above identified as salts from the chimney breast, they quoted for removing plaster, applying a treatment / block and replastering. Wasn’t expensive, but I haven’t done it as it would then require redecorating. However the folk who bought the house just knocked the value of the quote off the price. Maybe worth looking at when you get round to decorating the room. It isn’t an issue to just live with it if it doesn’t annoy you.

EDIT – just read the title properly… I’m definitely not an expert.

Itll be hygroscopic salts leaching through the chimney walls from decades of burning fuel.

The hygroscopic chemicals from the smoke attract moisture in the air which appears as damp stains on the wall.

You can prove it by boiling a kettle in the affected room, the damp patch will get visibly worse as the steam is produced.

There are two ways of approaching this problem, the temporary solution is to use white vinegar to wipe the walls, do this 3 ir 4 times and the stain should disappear for 6-12 months,but will ultimately come back.

The proper solution is to hack the plaster off in the affected area and re plaster with waterproof plaster.

Ive been taking the vinegar approach for the last few years in my house,it works very well.

Julians answer is spot on. I’ve dealt with this several times, sometimes from uncapped chimneys, but also following flooding.
The hygroscopic salts will be drawing moisture in. These are caused by the combustion products in the flue.
You can buy salt neutraliser, or replaster, but treat the brick work as well.

BTW.. IAAE 🙂
ICIOB B.ENG (Hons)

@ dabaldie – that’s very good to know, what product would you treat the brickwork with?

I do love STW for this.

I presume the vinegar damages the paint but when dry you can just re paint over it?

We had a similar problem with a capped off chimney – permanent damp patch that no amount of dehumidifying etc seemed to shift. Our builder reckoned it was condensation caused by lack of venting. We had the chimney breast completely removed almost two years ago and (so far) have had no reoccurence of the problem.