Damp

[LydiaEdwardianHouse]

We had our chimney capped and repointed during the summer, however have noticed a couple of damp patches at the front and to the side.

the two discoloured/ brown bits are reading high (about 37/8) but everywhere else seems fine.

anyone have any idea what else could be causing it or how to resolve? The chimney was already out of use (fireplace is decorative) so perhaps something stuck inside and we need to get it swept?

 

[blup]

Salt and other tarry deposits in the flue are the likely cause, it will be made worse through loss of ventilation of the chimney and fireplace.

You could chop the patches and fill with an appropriate plaster.

blup

 

[LydiaEdwardianHouse]

Thank you! We are about to have it skimmed. If we do that and add a vent, should that sort it? Or should we get a chimney sweep in and then add vent and plaster after do you think?

 

[^woody^]

If a chimney is capped, it should have an air brick fitted to the stack near the top.

Likewise an airbrick at the bottom if the fire opening is blocked up.

This is to prevent condensation in the flue and stains like that.

Any stain will need to be sealed on the bare brick before plastering over, else it will just bleed through again.

 

[LydiaEdwardianHouse]

An brilliant thanks v much.

how do we seal it? Sorry, probably stupid question..!!

 

[tell80]

OP,
Have you recently had the walls skimmed and painted? It looks like you may have covered previous hygroscopic surface damage with a skim finish?

I suspect that you might have to hack off back to brick all the plaster on three sides of the c/breast from skirting to ceiling - and same with the back wall on either side of the c/b from skirting to ceiling, & for about 600mm to 900mm sideways.

Then render with a 3:1 mix of sand and lime - dont use gypsum plaster and dont attempt to patch here and there.

Sweeping the flue is necessary.

Do any other c/breasts show similar damage?

 

[blup]

It is worth sweeping the chimney, modern methods can more effectively remove tar, hardened soot and birds nests.

Without knowing the state of the chimney it's difficult to say whether sealing the brick or re-covering with lime plaster is more effective. You could try a local repair to the area affected and keep it under observation for six months as the seasons and dampness change.

Blup

 

[LydiaEdwardianHouse]

We're due to have it skimmed next week but haven't had it done yet, no.

Can't see any similar damage on other chimneys in the house and it seems to be just these two little patches here.

We took out the cap on the bottom yesterday though that the previous owners added and A LOT of stuff fell out, included soot, bird skeletons etc.

Think we'll get it swept, add a vent and skim. Hopefully does the job. Thanks all.

 

[^woody^]

Get the flue vented, bottom to top as it should be.

Then treat the stained area of plaster plus at least 300mm all around with a product from Zinsser (there are several to choose from, have a look for suitability/compatibility) or another suitable stain block from other suppliers.

This will most likely resolve the problem with the minimum work. No need to sweep an unused flue, there is no point. No need for lime plasters, again no point.

The more extensive job would be to treat the bare brick wall before replastering, but that may not be necessary for such a small area if you vent the flue - which will control things.

The other caution is whether the chimney has actually been properly capped. You need to be sure that rain and excessive damp is not entering the old flue.

 

[tell80]

OP,
Using stain blockers is one of the worst things you can do when treating hygroscopic damage.
The blocker stops the wall plaster or brickwork from breathing. This will cause the hygroscopic solution to move up, down or sideways, away from the blocked surface.
Sealing the wall with blocker can attract condensation.

Sand and lime render allow the masonry to breathe, & can delay for about 20yrs the hygroscopic solution from presenting on the decorated surface. So dont block - breathe.

Venting the top & bottom of the flue is necessary, of course - but venting alone is not enough
All flues must be swept, redundant flues included.
Any soot (& other chemicals) remaining in a flue could eventually become hygroscopic and penetrate the chimney breast walls and perhaps the surrounding brickwork.

FWIW: lime plaster is not the same thing as sand & lime render.

 

[tell80]

OP,
From my limited experience I've often seen surfaces like the ones in your pics.
Hygroscopic damage presenting on the surface of the plaster or coming through the plaster has been covered over by accident or design.
It doesn't simply go away.
Good luck with the house.

 

[Deleted member 292770]

FWIW: lime plaster is not the same thing as sand & lime render.

would you be willing to explain the differences and their appropriate applications.
Also is it worth informing the op that neither lime render nor lime plaster is a miracle cure for rising or penetrating damp or the cure or prevention of hygroscopic salts.

 

[tell80]

Well I am sad that you dont know the difference but I shall certainly enlighten you.
Let us start slowly and apply ourselves to the spelling of the words plaster and render - are you still with me?
Perhaps you've not noticed that they are spelt differently? Please forgive me if I'm overloading you?

No, silly. You inform him, you said it. Thats a first for you.
Neither me nor anyone else, I should imagine, has ever said anything like that?
Perhaps you would be willing to quote me on that or any other miracles I've been associated with?

 

[Nige F]

It doesn't simply go away.
.

Like you

 

[Nige F]

Perhaps you would be willing to quote me on that or any other miracles I've been associated with?

You have been pontificating here for years under various names . You have more resurrections than The Bible. The miracle is you're still here