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Slaying the demon chimney stack

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👋I've mentioned "the house up in Shetland" a few times on this forum. We bought it and moved up there in 2017, fairly quickly discovered water was leaking in through the chimney stack, and then failed to fix it, or get it fixed, in a whole variety of ways between then and now. We moved back down to Yorkshire a bit over a year ago, with it still unresolved (it was the least of the problems with the house at that point) - but this weekend I'm heading back up there to try to get it sorted once and for all. I've got a week 😅. Throwing words and plans (and, when I get there, progress) in here to keep myself organised - but feel free to chip in if you have ideas or see a question you want to answer 🙏.


The house itself is a suspended timber frame bungalow with an external block skin, harled, and now with a layer of corksol on the exterior too. The chimney stack is twin block (except for a bit that would have originally been the fireplace - that's a single block between the back of the fireplace and the outside world), effectively built into a cutout from the timber frame on the east gable end. The roof is Decra, and there's lead flashing around the junction between it and the chimney stack. The coping is cast concrete (now also given a coating of corksol), then there's one pot with a fairly rubbish bird screen on top of that.


At some point the fireplace was removed and a wood stove + flue was installed. When it rains, water seems to ingress from the flue, rather than from the roof. For ages we assumed it was the flashing's fault, but the last thing we tried was to take up and re-do all the flashing and surrounding roofing, which didn't improve matters. When the water does come, it appears inside at the junction of the roof and chimney stack and spreads out from there - but it also seems to travel down the outside(?) of the flue all the way to the bottom of the stack. 


One consistent datum is that the leak only shows up when the rain is coming from west/southwest -> east/north-east - i.e., when it's *not* howling against the gable end. I've been over that side more than once as well; for a while, water would soak right through the blocks when it was coming ifrom the east, confusing matters. That's stopped now, at least.


Current plan, on the assumption that the top of the chimney pot is how the water is actually getting in, is to remove the wood stove, stop up the flue, and put a cap on the pot. If that works, I can then slap a layer of insulation against the (previously plastered, now exposed) block wall on the inside, plasterboard across the gap in the timber frame to get a nice smooth wall, and then forget about it forever.


If that doesn't work, I'll honestly be at a bit of a loss. As a "joke", I suggested demolishing the whole stack a couple of years ago...


Will get some pics up later today, they're on the other laptop.

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stopping up the flue will create the next ledge for water to reach then spread. perhaps needs  a vent.


Having said that, i have a chimney that i have stuffed with rockwool in a plastic bag.  it stops most of the heat loss, but I'm sure still allows some air past.

About once  a year it gets wet from heavy vertical rain and dribbles onto the hearth for a few days.

Low tech but working.


previously there was damp which simply required all flashings to be changed whether hey looked sound or not.

That worked so what it tells me is that my inspections of lead and flaunchings were not spotting the one or 2 points of entr.


Good luck.


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Thanks; yeah, entirely possible that we've missed something. If so, it'll become obvious when the rain continues to come in, and I can get on with those more drastic measures...


So, pics. Exterior:




Probably the best pic I've got of the top of the chimney stack, partway through a great deal of work. It's less awful than this now, honestly, but still... what a mess.


It currently looks more like:




(We didn't *just* cover up the awful with a layer of waterproofing, we also remedied the fabric).


And interior:




The only thing that's changed there is that we've stripped off the rest of the plaster. It was ruined anyway.


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Well, we're up now and here's the first bit of progress. Fire out, mantelpiece off (I was worried those two would be hard, but they were easy \o/), mouody plaster removed.


Will have a go at capping the chimney tomorrow.


Christ on a bike that wall is bad. Doesn't matter though, the gap between the two wooden uprights is 1700mm so I can just slap plasterboard in front of the whole thing - with insulation behind.


The inside of the flue pipe is dry as a bone - I had soot coming down it while whaling on the wall. There were streak marks all over the outside of the flue pipe though, and there was plenty of rust too.


Turns out Nome of the guttering or soil stack was replaced when the scaffolding came down though. So that's an unexpected extra job. Still, the top and bottom of each pipe is established so - famous last words - how hard can it be?


Tomorrow, onwards.


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Just ordered some https://www.jewson.co.uk/p/gyproc-thermaline-pir-2400-x-1200-x-38mm-BGB28492 in anticipation of being able to make the leak go away. It's not a lot of insulation, but I'll feel better with an air gap between it and the stack - and *any* insulation on this bit of wall is an improvement.


Butyl tape to go all around, and i guess for the seams between the boards. I'll probably lay them lengthwise, so the joins run horizontally. I guess I need studs going up the centre of the wall and along where each seam will be, for screwing into?

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Got the chimney capped, but I'm pretty sure that won't solve the problem. The corksol hadn't stuck well to some parts of the capstone, so I went up with a wire brush to clear it off ready for a different water-tight coating, as I'm pretty sure it's porous, to find...




The south side - the one where rain causes the strongest leaks - is to the right in this picture and the crack extends about halfway down the chimney stack. It wasn't like that two years ago; it's new movement/separation since then.




Just. What.


I've got until Sunday to do something to this. New plan is to knock off the chimney pot, to get a nice cuboid shape to work with, then drape it with heavy-duty plastic sheeting, ratchet straps near the bottom, pop some concrete blocks on top to hold it in place. Then come back another time and demolish the stack down to well below the crack. I could pop a new cap on the new level, or get it below the existing roof and have the roof cover it, I suppose either would work.


Happy to take suggestions for other ways to do it ^^.



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Aye, definitely the plan when I've got a bit more time, I don't want to put myself under pressure with it when we're definitely leaving on the boat on Sunday :D . I can probably make it back a bit later in the year, but wanted to give it every chance of staying dry until then. So I've applied copious butyl tape to the cracks, knocked off the chimney pot, fitted a chimney cap to the hole, and wrapped the whole thing in tarp.IMG_20230706_104926.thumb.jpg.2a147c42b0ea9a2dfab8c31d00c1d766.jpgIMG_20230706_122224.thumb.jpg.3deda64a70d8f5408f04f139dbaef271.jpg


It'll do for now.

Edited by Nick Thomas
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The rain came down and the wall stayed dry, so I went for it:




First time plasterboarding. Still need to tape&fill but reasonably proud of it. Lifting that middle piece into place by myself - several time, to adjust the fit - was an absolute killer. Time for a beer I think.

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  • 9 months later...

It's been a while but I'm back up around this weekend - there's a break in the weather and I'm keen to do something permanent to this chimney. I'm reliably informed that the tarp has been ripped to shreds over winter; hopefully the butyl tape I popped over the various cracks has kept the water out, though.


I found a few more pics from when the builders rebuilt part of the chimney stack, chasing leaks and rotten block - back in 2021:







Judging from the courses of blocks, I'm looking to take down ~1.2Mx0.6Mx1M of assorted concrete fun?, which is apparently about 1½ tonnes. Bit by bit, for sure. I've got an angle grinder and a hammer drill to go at it with. In my head, I've allocated ~2 days to the demolition and ~3 to making good, but it's strictly a guess. It's just me and a ladder, so I'm going to need to be veeeery careful.


I'm still on the fence between just taking it down to below the cracked portion and casting something like a chimney crown - only without a flue - over the top, vs. going below the roofline, then making good the roof over the top of it. Commentators above (and thanks for the input) were keen on the latter, but I'm a little stressed about the possibility of demolishing to that level, then failing to build a decra roof "patch" over the ex-chimney, either correctly, or at all.


That said, the crown approach seems pretty complicated - I found https://www.concreteconstruction.net/_view-object?id=00000154-22df-db06-a1fe-73dff6220000 which talks about the process some. Obviously, hard to say without seeing what's *around* the chimney, under the roof tiles, but am I just psyching myself out here for no good reason?




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OK, I'm up and making some progress.




I wasn't really expecting another chimney crown a third of the way down, but it made a decent place to finish for today. Same again tomorrow; I should be able to get to the roofline if I stick at it.


I'm told it only takes 2-3 weeks for decra panels to arrive once ordered, so I'm definitely not rebuilding the roof- I was hoping they held small quantities in stock. Ah well. I'll pour yet another chimney crown instead.

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OK, the stack is gone.




I ended up going down one course further than I wanted though, as the blocks on the inside face were completely rotten - they crumbled away as I removed the blocks on top of them - so I'm kind of in no-man's-land, height wise. Too low to easily cast a crown, too high to rebuild the roof, even if I had the parts. Fun fun.




Yup, that's expanding foam - making up a good chunk of one of the block that crumbled away, and the one below, bless it. The top of the roof deck is just visible; it's above the surface near the ridge line, but then dips down below.


If I try to cast something, I'm struggling a bit with the form - it'll have to come up flush against the lead, and then have an overhang past it to keep the water out, and needs to be at least 50mm thick everywhere. Tricky, but... doable?


I suppose I could lay some block to get some height, then cast over that, but presumably I'd need to leave several days between the two stages?


I could dig out yet more block and try to extend the wooden roof, but I really don't have the parts 😬.


Big sheet of lead and a hammer? :D

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This thread is like an episode of time team. 


Gradually digging deeper into the history of the chimney! 


At least there's no annoying artist impressions of what life was like when the chimney was first built 😂

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I rather liked the artist impressions - especially back when they were watercolours!


I spent the day fiddling with mock-up formwork and not really getting anywhere; the height requirement is just so impractical, I think the concrete requirement was up from 80 ->240kg or so. Fortunately, a neighbour with the same roof system happened by and just happened to have a stock of spares, so I've got a new plan A:


1. Get some timbers and battens put together, fastened to the blocks making up the outside of the stack - those are new and not crumbly at all.

2. Nail the decra into place

3. ???

4. Celebrate


Still working on 3, as I don't have the ridgepole-shaped bits or the edge trim bits, but even if they end up being lead, there'll be a lot less of it than my original plan C!


The island life is great sometimes, it really is.

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1 done, working on 2 ^^.




For 3, I've grabbed a "timloc dry ridge kit". I'm not very interested in most of it, but it comes with a nice membrane with sticky stuff either side that can pretend to be the ridge *and* edge trim - hopefully. If not, I've got lots of offcuts of panel, I suppose I could try to fabricate the required bits.


Turns out lead is absolutely not wise to use on these metal roofs, due to the risk of bimetallic corrosion. I could really see it when I took the chimney flashing off, too - rust everywhere. We're pretty close to the sea, of course.

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OK, I'm watertight - and it is very conveniently raining, so I'm just sat waiting to see if I get a leak or not :D.


The dry ridge membrane is less sturdy than I imagined, but in fairness it's designed to go under a real ridge, rather than being the ridge itself. I'm happy enough leaving it like that until I can get back in the summer with the real metal pieces, though.


The bargeboard is really just a placeholder, below that very skinny wood will go some 8x1, and if I'm lucky that might hide all the missing render - if not, there'll be a small patch to reinstate. Since i need to cut away some of the other render to install it, I won't be getting to it on this trip either, but I'm happy to call it a success overall.


Take that, chimney!



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