Damp Chimney Stack after rain (ed.)

[theboldbadger]

I was wondering if anyone could point me in the right direction of what type of damp we might be dealing with and any possible causes that we haven’t already considered.



When we moved into our house a couple of years ago we noticed significant plaster cracking all over one of the bedrooms. As you will see in the first image, we have coombed ceilings and it became clear that there was a damp patch created right where the old chimney stack sits.



Earlier this year we thought the most obvious way to fix this was to replace the lead work, which we did and the obvious damp staining dried up within the space of a few weeks.



We thought we had the issue sorted but it recently reappeared after a wet patch in November with significant rain for 2+ weeks. Now I’m thinking the lead didn’t make a difference and it was just coincidence that the work was done as we were coming into the drier spring/summer months.



As we are about to decorate the room anyway, we decided to remove the old lath and have a closer look. One tap with a hammer and the old plaster quite literally crumbled. It was absolutely sodden and the lath boards were soaking too.



As you will see from the images, the old fireplace has been bricked up and the chimney now has no use whatsoever on either stack. We made a hole in the brick section to see how damp the internal chimney was… a few dead birds and nests were pulled out but interestingly all of it is bone dry. The fireplace was vented at the bottom where the skirting used to sit. Interestingly it does seem very dry so only seems to be wet further up, where the chimney stack sits.



On the roof please ignore the fact the lead doesn’t look too clever, this was pulled back for inspection last week and we are convinced the issue isn’t this.



The chimney stacks rarely get natural sunlight and to me they look quite damp. Our neighbour has more mortar attaching his chimney pots to the stacks than we do and his mortar is built to a kind of peak to allow water to better run off - so this is one area we have identified to sort. Other than that there are no clear holes in the pointing work but we are wondering if it might be penetrating damp and we should apply either a paint or clear breathable product to seal the brickwork on the chimney (although I’m not sure how effective these types of product are?)



I don’t have any experience of chimney related damp so I guess I’m wondering if I’m along the right lines by thinking this is likely penetrating damp? It just seems like a lot of dampness and it would be more obvious that it was coming from one hole or point of entry. I’m also surprised the inside of the chimney seemed dry, as you read a lot about unvented chimney breasts causing problems.



Any nuggets of information much appreciated.

 

[foxhole]

Unvented chimneys cause lots of problems , the fact you didn’t see damp doesn’t mean it’s not there .

 

[tell80]

OP,
As above.
You will need to have both flues opened up at the fireplaces, and swept.
Terminal/cowls could have been dropped into the pot - no mortar needed.
You will need to provide vents where the fireplace's are being blocked off.
Have you been into the loft and examined the loft chimney breast and trimming around the stack?
Or is it an external chimney breast?

The exposed back gutter should be about 150mm - 200mm min. wide, and it should have a slight downhill slope to it.
The side cover flashing appears to be wrong - you should have individual soakers under each slate, and the cover flashing should come down to stop a min. of, say, 40mm - 50mm above the slate.
The back gutter is like a large soaker with the cover flashing dropping down over the b/g upstand to stop about 40mm above the "flat".
On the outside corner where your side flashing meets the back gutter cover it also looks as though its been done wrong.
An angle grinder should have cut chases for the cover flashings to be turned into - your cover flashings appear to be sealed to the stone - not chased in?

FWIW: its great that you've not disturbed the coving or rail. If you want to know how to deal with plaster laths then come back here.