Balanced flue through slates, leaking

[Tim W]

This is a job I wish I had done 2 yrs ago but I don't really know what the problem is so I am stuck

A single story lean to with slates at about 30deg pitch. The 'balanced' flue from the boiler goes vertically through the roof and water comes in around the flue when it rains hard. there are some pictures here:

There is no access to the void above the ceiling for reasons of fire regs.

You can see a massive lead square around the flue. It all appears to be in order and complete. It didn't always leak. I repaired a broken slate uphill of it and I have been inserting shed felt between the slates uphill of it too to make sure the leak isn't coming from elsewhere. I have no idea how the rainwater can be coming in. Do these flues have particular problems? Does anybody know where they tend to leak, or how I would fix the leak?. I don't want to have to pull the whole lot out just because I don't know what to fix. surely a bit of flash band in the right place would do it?

Much obliged,

Tim W

[harryagain]

I would have a look at the metal annular piece that is attached to the flue and overhangs the sleeve on the lead slate. Especially the joint where it fits round the chimney. This usually has some sort of clamp and sealant. Needs to be watertight. Quite often weathering, the sealant going hard and expansion etc causes it to crack and leak in heavy rain.

[Chris French]

In message , Tim W writes

I'd be looking at the bottom of the flue area, maybe wehre it meets the weather slate at the back, or of of the connections above. Say where the top gray bit is fixed to the lower grey bit (rivetted?)

Does it always leak, or only in some wind directions? I have a leak from that only happens under certain conditions, - though never quite worked out what, but it's needs to be heavy rain and windy.

I stopped water ingress by patching a small hole in the membrane under the slates (made by a piece of timber pushing against it ) but I never did work out how it came in (the bit of roof abuts a small bit of vertical wall going up above the roof, so it's probably related)