chimney damp problem

[speedycar53]

As a result of having an extension built, the lead tray in the chimney is now below the roof level and is channelling damp into the internal wall and plasterwork.

This has been managed by treating the entire chimney with silicone damp-proofer but this only lasts about 5 years and is not 100% effective.

This is a large chimney and I don't want to have it rebuilt with a new lead tray so was wondering what alternatives are open to me. My thoughts are:

- using high tech fluorocarbon water repellent treatment on the chimney instead of silicone
- using an injectable damp proof course - effectively working in reverse (falling not rising damp!!)
- cutting in a damp proof membrane somehow.

Any advice would be most welcome.

 

[dann09]

1. What do you mean by "chimney plate"? The very top of the c/stack that has a c/pot? Surely, not a chimney register plate?

2. Perhaps you can post some pics, and do a little research about your Q. on this forum.
Pics of the c/stack or c/breast outside and inside(ie. the damp areas) the house if possible?

3. At face value, i dont think that any of your three suggestions will do the trick. More info. and perhaps people here might help you.

 

[Nige F]

I wondered if OP was talking about a lead tray

 

[speedycar53]

Yes, I did mean lead tray. Sorry for confusion. Will edit original post to clarify.

Have also included a sketch to illustrate the problem (click on thumbnail). The chimney was originally built into an external wall which has now become an internal wall in a double height atrium space with a flat roof. Unfortunately the flat roof is above the level of the lead tray so strictly speaking I suppose the chimney should be rebuilt with a new lead tray at the correct height but this would be an expensive exercise which I would prefer to avoid if there is a simpler alternative. The silicone treatment was effective initially but after 5 years is no longer doing the job.

 

[dann09]

1. How is your flat roof constructed?

2. Does it have a fall?

3. Are the chimney flues in the stack active or dormant?

4. The damp area is indicated on the former external face of the c/breast - what about the space below the pitched roof, any damp or discolouring signs on that side of the c/breast?

5. If the flat roof has falls, and the c/stack flashing is sound, then suspicion goes to:
Pointing, and brickwork.
Flaunching.
Terminals.
Inside the flues - possible sulpherisation/ degredation.

6. I've seen plenty of stacks built without lead trays ( or wrongly positioned Dpc's ) or any other DPC in-situ, and they were problem free, so i should have the stack inspected for the above possibilities.

7. If the silicone "worked", fair enough, but it's a poor professional approach to dealing with things.

 

[speedycar53]

Thanks for your input, dann09. In answer to your questions...

1. Flat roof is a flexible plastic coating on membrane on timber decking. It is all perfectly sound.

2. Fall is away from chimney stack.

3. One flue dormant, chimney capped; one flue active from ground floor fireplace. Flaunching is sound.

4. There is some slight evidence of damp on the other side of the chimney breast.

5. I think probably its down to brickwork and pointing - chimney was built in the 1930s, mortar is fairly soft and bricks fairly absorbent. The fact that the silicone was fairly effective suggests this is likely the case. The damp is most evident after periods of driving rain from a direction that affects the most exposed side of the chimney breast, i.e. the side opposite the pitched roof which is also the side where the damp is most evident.

6. Interesting you should say this. I suppose it depends on the nature of the bricks and mortar used.