Leaking chimney, but where is the water getting in? (see pics)

[Calum]

Hello all

I've just had a loft conversion completed on a victorian semi. The rear of the house is one large dormer with a (nearly) flat roof, and we've left the chimney stack in place as we might use the dining room fireplace one day.

The problem we have is that rain is getting in somewhere and coming through the chimney breast plasterin the new loft room, but the roofer can't work out where this is coming from.

Here is a picture of the plaster with a roll of masking tape for scale. There are pencil lines around the damp circles as I was measuring to see if they got bigger.

are pictures from the outside: roofer has re-pointed and patched up the mortar slope to the (very old and unused) central heating cowel. The cowel was covered in a plastic bag, but that made no difference.

If it rains moderately for an hour, the patches appear/get worse and then take a week or two to dry out. To me that looks like the water is being funnelled down quite effectively.

Could it really be soaking in from the brick? What else could it be?

Thanks for your thoughts Calum

[stuart noble]

I'd replace all that mortar round the pots. The rain could either be coming straight through it, or there are hairline cracks at the edges. Either way, it's no big deal to replace it

[Jim K]

me too defo. do the other pot as well - if neighbours spose you should ask first....

Jim K

[Nitro®]

I personally don't like the look of the lead flashing, might be worth checking for water pooling in that area.

I built a shed once, it fell down.

[The Natural Philosopher]

You have a flat roof and f*ck all flashing on the stack.,

Go up and prise up the lead. Bet you its all wet underneath.

You need to take roofing felt an carry it up the sides of the stack, to where the flashing starts, and probably mastic it in, then put in new flashing and dress it several inches out along and over the roofing felt. So that driving rain under the flashing has to go a long way and then UP to the flashing top, before it meets the brick.

Yu might also, as others have suggested, at the same time repoint the stack and remortar the cowl top.

Personally I would scaffold up, or whatever, and demolish the stack top, and start again and build a new top, done properly.

I take it the flat roof is part of the new conversion? If so its been badly done.

It should be redone in conjunction with the chimney.

[Jim K]

care to expand - for the benefit of the OP?

Jim K

[tony sayer]

I'd go along with most of that, tho If it were me and it seems that the water doesn't take that long to appear, I'd prolly chop a bit out of the wall where damp and then try a hose at the bottom sides of the chimney and see if thats where its at and if not the top, might just might show up where its leaking...

But as its just been done why isn't the builder sorting this out?..

[Andrew Gabriel]

Difficult to tell.

I bought a couple of those elephant's foot cowls like the one you have, decided they wouldn't reduce the amount of rainfall going down the chimney, and took them back. Actually, they may make it worse by capturing water from a larger area than the original pot opening, and making it run down the inside of the pot - there's nothing to make the water running down the outside of the pot (unless you siliconed it on). You could repeat the plastic bag test on this flue (but you can't block the airflow permanently like this).

I then went a bought a couple of these instead:

I fixed them in a different way so the strap isn't visible ?pid=813158&l=46e9b39859&id=1619546457In theory, rain water going in the pot shouldn't cause any problems. In practice it might because the internal pointing has been washed away over the years of rain exposure without any heating to dry it. Also, if the flue isn't ventilated at bottom _and_ top, it will fill up with condensation, which will soak through the brickwork in time.

[The Natural Philosopher]

I thought I had..

I don't like the way the flat roof has been sealed to the chimney. It looks inadequate.

Water can run down the sides inside, and appear ..well wherever there is a route. Probably through porous mortar.

[Jim K]

your last critique appeared to me to be a general "crap flat roof" broadside so was hoping you could enlighten the OP with specifics taht he could use to bolster his possible arguments with the professhunals who did it - seems all you are picking at is the height of the stack flashing and what may or may not be underneath it...

Jim K

[The Natural Philosopher]

Well all flat roofs are carp, because simple overlapping of waterproof elements will not in general be enough, as water can and does work its way between the layers in any sort of wind.

The whole point of a roof slope is to make gravity work for you against wind pressure.

Typically what you might do for a stack is to create a flashing, soldered together that extends a LONG way from the chimney base

And run the felt up inside it..but then the corners are vulnerable as well.

So you probably end up with some sort of 'soaker' system. a lead layer under the felt, the felt and a lead layer over it, and all raised up at the stack, so that water has to run up the felt under the top lead, then down the lower lead and a long long way before it ends up dripping off the edge of the soaker through the ceiling..so put another layer of felt under hat as well that goes all the way to the flat roof edge etc. And seal it all with flexible mastic and pray.

It's a series of defenses in the end, all compromised by the fact that gravity on a flat roof is just as likely to make water move towards the places you don't want it, as the places you do.

[Nitro®]

There appears to be very little if any overlap.

Looks very patchy to me, in the larger picture it shows up even more just how naff it looks, especially the area around the stack.

[robgraham]

In line with the others I can't see quite how the flashing round the stack is working.

On the basis that the CH flue isn't used and your 'one day' could well be a long way away, I would take down the whole stack and seal the roof properly I'd leave a ventilator out of the flue to allow air flow and mark where it is. If 'one day' does arrive some time then put in a stainless steel liner and flue outlet, again properly sealed in. Rob

[Jim K]

zooming in on the pics it seems someone has "pointed" the stack and flaunching around pots with silicone?....

Was that done after the problem 1st manifested or is that a previous bodge?

Jim K

[Adam Aglionby]

TBH flashing looks pretty normal, read lead/zinc for copper here

normally think flashing failure giving damp signs higher up wall or on ceiling as water runs down outside of stalk internally.

Brick has lready suffered water damage, the damaged faces look as if damp has penetrated behind then frozen and blown the faces off.

Stack looks like its beginning to split as well, silicone sealant is great but structurally gluing split chimney stack back together from outside is probably pushing it.

Demolish and make good seems best option if don`t need vents and can get [planners to agree, take down and put back together again properely if they don`t.

Looks a bit far gone for this sort of treatment to work, water proofness on lot of bricks has gone.

ferking hate roofs, its all down to opinion and any of them, in any combination could be right.

Cheers Adam

[Calum]

Yes, it's part of the conversion. They've been back once already to look at it. They'll be back again soon...

[Calum]

They are coming back. At one point they were talking about dry-lining inside - definitely not doing that. They appear to have come to the end of their ideas tho, which is why I thought I'd post and see if anyone had any thoughts - which they do! All interesting comments...

[Calum]

I see what you're saying. They have been back once to try and resolve, but I'm not sure exactly what they did. I think I need to get up there and take a look myself as camera pics (camera stuck to broom handle poked out of windows!) aren't ideal.

[The Natural Philosopher]

Lift the flashing and take pictures of what's underneath.

If the roofing felt is carried up a bit that's a help, and there ought to be gobs of tar or mastic there as well. Nit bloody silicone either.

If the roofing felt finishes at the chimney, as does that crap excuse for flashing, its a total disaster. They MUST overlap.

[Phil L]

Who's to say the felt doesn't already go up behind the lead? - my bet is that it does, but this isn't the problem.

IME you can't seal a chimney with lead on a flat roof - where they've took the felt up the chimney sides and then flashed over it, they can't seal the corners and this is where it's getting in, that is to say, the felt is split at each corner and doesn't go 'around' the corner in one piece, and as you say, the lead is just covering brickwork at this point.

I wouldn't flash a chimney like this with lead onto a flat roof - it needs something completely different, torch-on would probably be best but not very permanent