Chimney DPC - a difficult case

[MrDuck]

There is loads of information on the web about best practice for lead DPCs for single flue chimneys, and it all makes sense. However, I have a two flue chimney build into the inside cavity wall with the second flue closer to the ridge line, together with an upstairs room through which the chimney passes; and damp in this room is a significant problem which I have to fix.

I have spoken to various builders and they seem to have different views as to how the job should be done, which concerns me greatly as there should be a single view. As I understand it there should be a high level lead DPC (that's easy to understand) but also a low level lead DPC which prevents any damp in the remaining exposed part of the chimney penetrating the room space. But how is this implemented? A flat tray at apron level outside would end up part way down the wall on the inside of the house which would leave the wall very damp. Equally a flat tray at back gutter level outside would leave a lot of chimney exposed allowing damp to continue to penetrate. Should the tray be stepped in some way by hand crafting it on site? I can't afford to experiment with solutions to this problem and hope that there is a proper solution that builders can implement.

This photo shows my chimney.

View media item 70275
Any advice much appreciated.

 

[shane12345]

Are you sure the lead work isn't leaking?

 

[MrDuck]

Thanks for your suggestion, but yes the lead work has been checked by supposed experts. I think this is a longstanding problem, addressed unsuccessfully by the previous owners (new flashing, repointing etc.), and more drastic measures are now needed. Certainly there are no lead DPCs. I am just uncomfortable with different builders suggesting different approaches. That suggests to me that some at least of them are on the wrong track.

 

[Nige F]

Not difficult for Sussex builders to be wrong I would be debating your job with my M8 and we both started on the building in early `70 . I did a stepped , through the chimney flashing after the storm of `87 took down a chimney taller than yours in Seaford . But I wouldn`t necessarily say your damp in the room is from the part of the chimney above the roofline - it could be Hygroscopic salts from the flue

 

[MrDuck]

Thanks Nige.

Do I get it right if I understand that you shaped a lead DPC all the way across the chimney footprint generally following the stepping of the external flashing? That makes sense to me, but is seems quite difficult to do, as upstands are still needed to catch water coming down. I suppose these would have to be welded in.

 

[Nige F]

It was all cut and welded - then the chimney rebuilt above

 

[MrDuck]

Interesting Nige.

Just one of the roofers I have spoken to talked about cutting and welding lead work on site. Others wanted to put in a flat DPC tray at apron level, which would have involved rebuilding work inside the house(!!) and so far as I can see would have left a damp spot inside, which would need to be covered up.

Sounds like too much trouble to do the job properly, or lack of skilled personnel, or perhaps both...