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Leaking Chimney – dodgy tray I think

Postby hoogerbooger » Tue Sep 07, 2021 5:07 pm

Chimney.jpg
I think I know the cause, but suggestions on my envisaged fix sought. Anyone good on lead working?

I attach a picture of my chimney rebuilt a few years ago. A lead tray was included, but I think it can’t have an upstand in the lead where it fits over the last section of the 8” clay liner.

Having spent some time investigating, by being in the loft or on the roof during and after rain, I am now pretty sure that the route for water to my sitting room chimney breast is via the outside of the clay liners, that stands proud above the tray, and then via the cracks in the liners, blocks and mortar.

As to water routes into the stack:

The pot cover in the picture is one I’ve just put on to replace the one that had wire bird guard sides. This will have been one route for ingress. New cover has gap to allow chimney to breath.

I also notice that water running down the outside of the stack wicks inwards when it reaches a crack/gap where the mortar sits on top of the lead tray. Not sure how significant this is as a route. Looking at the lead levels on the shoulders of the skirt, I don’t think it can be bashed to a form where water will run away from the crack. I tried filling it with a durable exterior silicone 24 months ago, which I think helped (hard to say really), but that has now perished.

The brickwork mortar looked crumbly and had moss growing on it. But having just brushed this off, the remaining mortar seems solid and not a route for ingress.

The Flaunching is shallow crumbly and a bit cracked, but I’m not convinced it’s leaking yet.

I haven’t managed to get any roofers out. All seem too busy & this is not likely to change any time soon, for a small job like this. I am not a good enough DIY’er to do tidy brickwork but should be able to redo the flaunching ( having watched load of youtube videos). It’s a bungalow with a low angle roof so access is easy (obviously H&S is for me to address) SO:

I am thinking if I take the pot off I can physically lean in and reach the lead tray. If, as I presume, there is no upstand, maybe I can with some form of hooked edge and a bar leaver up a degree of upstand in the lead against the clay liner. What I don’t want is it to rip. So any views or suggestions on this before I commit. If I can get the tray to flow to the weep holes I may not need to get the chimney rebuilt.

The current flaunching seems shallow to me, so I’m intending to increase the thickness, ensure it comes out to the edges of the brickwork ( it’s a cm short currently) and have a two phase slope with a sharp angle near the edge so there is no thin section to crack and fall away.

Originally the chimney had a capstone. I won’t sensibly be able to man-handle one safely, so won’t be replacing. So sealing the gap above the lead tray that seems to allow water to wick-in it, will be helpful, particularly if I can’t work re-work an upstand into the track. Any suggestions on a sealer between mortar and lead that will last?

Comments and suggestions welcomed.
hoogerbooger
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Postby stoneyboy » Thu Sep 09, 2021 9:10 pm

Hi hoogerbooger
Your suggestion of creating a raised lip in the chimney flue is worth trying, only investigation will determine whether there will be enough of a lead flange in the flue to do this.
Suggest if you are going to improve the flaunching that you cast a capstone on the top - fix timber flush with the top edge and form a box so the casting protrudes outside the bricks.
Chimneys built in engineering bricks (which it looks like yours is) are difficult because the only absorbency for rain is the mortar which quickly becomes saturated.
Sealing the mortar lead joint with silicone may lead to additional water ingress because the silicone forms a dam directing water inwards.
You may find regular soaking of all mortar joints with silicone liquid may improve things but the presence of water stains part way down the front lead face is an indication that something is wrong.
Regards S
stoneyboy
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