damp on internal chimney breast

3 Oct 2010
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United Kingdom
got damp patches coming through plaster on chimney breast with gas fire (fire not used often). Only appeared since rainy season.

Got two chimney breasts with 4 pots - so 4 separate flues. The working flue (gas fire) has a cylinder pot on chimney stack (this is one with damp patches) - other 3 have bishops pots. Roofer whats to cap all pots, but says he needs to remove bishops pots as you can't cap these and replace them with cylinders - is this true - can you not cap bishop pots??

Pointing on chimney breast externally (end terrace) is really bad at roughly the height of the damp patches internally on plaster. Could this be a factor - Could the water cross the flue space?

No other damp in rest of house - some white powdery damp residue on internal chimney brickwork in loft

help much appreciated

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you can see on the external of the chimney breast pathces of damp I assume coming through from the inside of the flue?

These are at a smiliar level to the damp patches on teh plaster internally

any help appreciated

Looks like rain is running down the flue.

May be a problem with the pots or the flauncing on top of the chimney
what might the problem be with the pots? simply need capping or the joint to the chimney failing?

cheers for the help

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Misplaced or defective pots, or broken old flauching can allow rain to get straight to the top of the bricks in the stack and then run down the flue, or if lined, between the flue liners and the bricks. A sooty flue can also allow the water to run down the flue and not soak in further up

But this is only one possibility

The pattern staining of the flue/fireplace outline indicates excessive moisture within or around the flue. If may be that the pointing is defective, but it would be extreme for that moisture to get to the internal face of the breast, but not impossible - especially if that wall gets a lot of wind blown rain

Have the defective areas pointed first - as high up as needed. Or as an interim coat the external wall with something like thompsons water seal to see if water is repelled enough.

If you can investigate the top of the chimney easily then have a look, otherwise do the easiest things first
cheers for that woody.

That wall I would have thought is pretty proctected by the neighbours house from wind driven rain?

I can't get on roof,but roofer is going up this week. He reckons all the pots need capping, but that you can't cap the bishop top pots (like the one on the right) and that he needs to replace them to the cylindrical type (on the left)
Do you have any knowledge on that?

Many thanks for your time - many thanks
wondering as well, how the water could breach both sides of the chimney breast - internal and external?

I wonder if this may point to a blockage?
The pots should be OK if properly fixed and the flauncing on top of the chimney in good condition. Presumably the roofer's reference to 'capping' is not to block them completely, but merely to cover the top with a different pot to prevent rain falling vertically into the flue?

Having a covered pot can help with infrequently used flues, but as chimneys work well and stay dry with the pots like you already have, then it is hard to say if the actual pot design is the problem

Rain can track across the chimney or run down within a flue. Typically there are softer bricks used inside the chimney and/or the mortar becomes very porous

The way those stains are localised, would indicate that the flue is playing a part - either by channelling moisture downwards, or holding moisure in the area around the flue or any linings

Looking again at the pointing, it is quite degraded and the ledges of the bricks catch and hold water, and that granular mortar can soak it up quite well. This can quickly saturate the chimney internally and this then soaks across

Flue condensation is a possiblity due to infrequent fire use, but by the look of the staining in places where the in no flue, plus the outer damp, then condensation is less likely to be a factor

A blockage in the flue - soot build up, mortar or a a nest or something may be present, but again the location of the stains and the external damp make this less of a possibility.
cheers woody -

Roofer wants to put cowls on the top of the pots to stop the rain water going down - the gas fire flue is no issue (as such) cause the cylinder pot will take a cowl. Roofer says that the crown type pots don't fit cowls, so he would need to take the pots off and put cylinder types on. ££££££

Was wondering about the chinaman hat cowls for the crown tops? other pople in the streeet have simlar pots of both types without cowls and pressumibly they are ok, so I honestly don't know.

We have two chimeny breasts and 4 flues - (only 1 working flue) and we have no damp anywhere else i.e in the bedrooms or front room - so not sure cowls are the asnwer? just seems to be the gas fire flue - bit lost to be honest - seems a lot of damp to be the pointing?

Roofer is gonna look up there this week- - problem is he could say anything cause i can't see up there!

thanks fro your time mate - if anything else sprigs to mind please let me know.

many thanks

Woody has pointed you in the right direction.

Notice that the external "damp" patches move to the right following the one flue only, and stop at the approx. position of the next floor fireplace position. Has work been done there in the past? Are all your redundant flues vented.

You should bite the bullet and get a smoke test done on the gas flue in use ( observe at the second floor fire place for leaking smoke) and all being OK then have the flue(s) swept. This would eliminate blockage and help if sulpherisation is begginning, and also positively indicate which terminal is the gas terminal.

There's an enormous amount of info on this subject if you search.

does this picture help any? The upstairs fireplace in the back bedroom has been plastered over and there is no air vent! The damp does appear on the right hand side of the external chimney breast. the worst looking bit appears to be right behind the gas fire in the lounge

booked guy to come and serive fire and smoke bomb flue - was gonna employ a chimney sweep - but gas engineer says he can do it as he carries brushes to sort blockages - result?

thanks again for help - much appreciated

Be there and watch the smoke test if possible, sketch which terminal the smoke comes from. Note when the appliance is removed whats what and if excessive moisture appears on the flue wall. Write down all the gas guy tells you or you'll forget,

Sometimes the only remedy, without going into tech details, is to line the flue with a SS liner.

If you can do without the gas fire then you could wait and watch for staining with or without the flue in use.
Let us know what happens.
alas going to be at work!

Going to leave the missus with some instructions and leave a note for the engineer to ask him to look for excessive damp in the flue and behind the fire and to note which pot the smoke comes out of.

Can you think of any other pertinent questions?

When you say 'sketch which terminal' - do you mean see which pot the smoke comes out the top of n the roof to ascertain which flue we have the issue with?

We don't really use the gas fire - we've been debating whether to bother with at all, as we generally, when needs be, have the heating on.

will feedback - gas man coming a week on thursday. If you think of anything else, please let me know


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