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Chimneys & Fireplaces

Damp patches at top of chimney breast in victorian terrace

I would seriously appreciate any help with this!

- London terraced Victorian house c 1890, 5 floors, of which I have top two
- Top floor is in the eves, and large dormer windows front and back were built about 20 years ago
- Flat roof was done about 20 years ago, but still looks fine according to roofers. Pointing also looks fine.

- Damp patches on 3 of the 4 chimney breasts, in the top 30cm from the ceiling, and also some dampness on the ceiling where it meets the the chimney breast. One room is much worse, the other two are minor.
- Damp appears after heavy rain, but takes weeks of dry weather to dry out.
- Similar in winter and summer
- The property otherwise does not feel damp.
- I cut away a small ceiling section to find the bricks above the plaster are slightly damp as measured with damp meter. They never dried out.
- I never actually see any drops of water.
- The damp patches have been there as long as I have (15 years!)

- roof checked by roofers
- Roof coated with a water proof sealant twice (acquapol or something?)
- Put wide flashband over the flashings (which look fine anyway) and other areas
- Top half of chimney rendered (with waterproof additive)
- Builder went over the roof and sealed any vaguely dubious areas
- check chimney pointing
- Covered entire chimney with a huge plastic sheet, nailed in with batons!!
- leave the window slightly open for a year.

- 2 of the 5 flues are vented / still in use
- The worst damp is over the vented/used flues

- the chimney breast is plastered with BONDING plaster. Not ideal. But it doesn't explain why the exposed bricks above the ceiling are damp.
- There is no ventilation in the flat roof – no roofer seems to worry about this, but I am a little surprised.

Does anyone have any ideas?? Am thinking of putting a dehumidifier in the bedroom to see if it makes a difference. I would change the bonding plaster except it doesn't explain why the exposed bricks just above the plaster are damp?

3 Answers from MyBuilder Chimney & Fireplace Specialists

Best Answer

it really impossible to say exactly what is causing your problems without looking at chimneys although their are a few concerns raised, flash band is really only a short term solution as a flashing as it fails most of the time, the fact the chimney has been rendered has it also been painted ? . the age of the property tells me it was originally built using lime mortar,lime mortar allows your chimney
to breath anything that stops the chimney breathing and holding in water is not good cement does not allow your chimney to breath if its been re-pointed using a cement based pointing this will also retain damp, lime mortar does not contain any cement , lime mortar and cement do not work well to-gether
water may be getting into the chimney from the top, from cracks in the render from exposed brick,down the edge of the chimney pot or venting cap,under the flash band. the inside walls would have been plasteres using lime plaster again this allows your building to breath
you really need a specailist that deals in period property repairs .

good luck alex


Answered 9th Oct 2012

The bricks are wet above the ceiling due to condensation forming and this is where the problem begins. They form the coldest surface above the ceiling so create a dew point low enough to create moisture, this then absorbs into the ceiling, the flat roof void should be ventilated but with a passyfier vent so that moisture is also removed . That is only the basic information, I would advise you get a damp analysis company in to give you a full report and schedule of works. There are plenty of companies on Mybuilder that can deal with this problem so why not post the job.


Answered 11th Jul 2013

Hi there
You should not put any sheeting over the chimneys as you are stopping the airflow which helps dry and vent the flues, you do not say if you have vented caps on top of the chimney pots if not this could be the problem ,it does sound like a ventilation problem possibly combined with water ingressing down chimney if not covered, it all arises because of the soot inside the flue which sweats and helps condensation, so it also will help to have the flues swept.


Answered 9th Oct 2012

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