Flue/roof flashings etc

[Crofter]

Looking at different options for making good the hole that I am going to have to cut in my roof for the stainless flue from the woodburner.

Basic plan is to have a single wall black stove pipe exiting the stove and going up to the vaulted ceiling (this will throw more heat out into the room, look neater, and save a fair bit of money). Just before reaching the ceiling I will switch to insulated twin wall stainless pipe, which will continue up to above ridge height.

 

I will need to seal around the flue at three points: the airtightness layer, the roof membrane, and the corrugated steel roof covering.

 

There appear to be several options for the final flashing. I'm leaning towards the 'Dektite' silicone flashing as a cheap and easy to use design.

 

 

Perhaps I could just use more of the same for the other two flashings?

There isn't much space to play with of course, especially for the seal to the membrane and to the corrugated roofing, which are only about three inches apart.

[PeterW]

Dektite may be your only option - they make one for corrugated roofing. 

 

Airtight layers are always a challenge as you can't put combustible materials next to a twin wall flue. 

 

Its a known issue and there is stuff here about it 

 

http://www.stovesonline.co.uk/airtight-roof-penetration-kit.html

 

and this is the product they refer to 

 

http://www.deks.co.uk/storage/deks/pdf/Air Seal Leaflet.pdf

 

[Crofter]

Revisiting this, I was chatting to the local supplier/installer for this sort of thing and he reckoned there was no need for a flashing at the membrane, or at least not anything that seals tight to the flue. He suggested simply diverting any water away from the hole with some sort of upstand, e.g. lead or even just something put together with battens. I'm not sure whether this is a bodge or actually quite clever, as it reduces the number of flashings, and avoids having them stacked right on top of one another.

 

One worry I have had is how to make sure that I have no flue joints in the region of the flashings. With a 42 degree roof, it is hard to fit three flashings onto a length of pipe without one of them clashing with a join. It is recommended that all joins are accessible and anyway I expect it would compromise the seal on the flashing.

 

I don't suppose anybody here has fitted a stainless flue to a metal roof? Although the issue with multiple flashings surely comes up regardless of roof construction?

[Nickfromwales]

1 hour ago, Crofter said:

 

I don't suppose anybody here has fitted a stainless flue to a metal roof

I've fitted gas burners in commercial buildings, and they've been stainless twin wall and on corrugated metal roofs. Don't see this being too difficult to sort . 

Fit one of these to the airtight junction, with a ton of CT1 / intumescent mastic, and then another at the roof junction. Simples. 

[Crofter]

9 hours ago, Nickfromwales said:

I've fitted gas burners in commercial buildings, and they've been stainless twin wall and on corrugated metal roofs. Don't see this being too difficult to sort . 

Fit one of these to the airtight junction, with a ton of CT1 / intumescent mastic, and then another at the roof junction. Simples. 

 

Ok so one at the VCB (which is my airtightness layer), by roof junction so you mean the membrane or the metal?

 

How do people seal the membrane penetration on a more conventional roof?

[Nickfromwales]

4 minutes ago, Crofter said:

How do people seal the membrane penetration on a more conventional roof?

They usually don't. The most a 'conventional' roofer would do is cut a cross in the membrane and poke the flue through it. 

The breathable membrane only needs to not let any accumulated water / dew etc through to the fabric of the dwelling, so making some form of upstand at the highest point would satisfy that criteria. 

As long as the outermost junction is rain / watertight then your protected against water coming down the flue wall, and if your sealed at the airtight layer your ok for airtighthess. The breathable membrane is the least of your concern or liability imo. 

 

[Crofter]

Bumping this as I'm about to install the flue and flashing.

The Dektite flashing didn't come with a fixings kit- I wonder if I can use some of the 'stitch' screws that came with my roofing, to fix it to the metal? They are self cutting and have sealing washers. Not sure how much of a bite they will get into a single thickness of the roofing sheet though.

[Nickfromwales]

Don't over-tighten them. Also a bit of clear CT1 on the underside of the fixing will do wonders. Don't worry about any excess squirting out, just clean it off with baby wipes and finish with MultiSolve spray.