How to create air gap by joist for flue?

bobbidill · 8 Jun 2019

I have to create a 75mm air gap between a joist and the twin walled flue which runs from the stove beneath it. The joist sits below a pitched roof which is made from cement fibre tiles in a single storey side extension. This air gap between the joist and flue is currently 25mm. The 1st photo is taken looking up at the ceiling from below and the second from standing in the room. They show the joist on the right which needs to be moved over to create the 75mm air gap. I've considered doubling up the adjacent joists and boxing the flue in. Would that be a reasonable solution and do you guys have any better ones please? Thanks.

KenGMac · 8 Jun 2019

bobbidill, good evening.

Any chance of "moving / diverting" the twin wall by 50.mm from [I assume] the appliance within the room where the appliance is situated?

Ken

bobbidill · 8 Jun 2019

KenGMac said:
bobbidill, good evening.

Any chance of "moving / diverting" the twin wall by 50.mm from [I assume] the appliance within the room where the appliance is situated?

Ken

Hi Ken, good evening to you too and thanks for responding. Unfortunately moving any of the walls is not an option now.

KenGMac · 8 Jun 2019

bobbidill, Hi again.

What I was suggesting was to slightly divert the flue? is that an option?

Ken

bobbidill · 9 Jun 2019

KenGMac said:
bobbidill, Hi again.

What I was suggesting was to slightly divert the flue? is that an option?

Ken

That's not an option either sadly Ken. What I'd really like to know is can I move the part of the joist adjacent to the flue over by 50mm. We can access the space beneath the roof from above fairly easily by removing as few tiles.

big-all · 9 Jun 2019

surely with a minimum say 50mm you can apply a fireproof material or indeed a heat sink ??
in my workshop the space was quite tight round the wood burner so cut up and old pressed steel radiator spaced it off the wall by 25mm and the heat at the wall whilst quite warm wont burn just store the heat in the metal
so what i am suggesting is heatproof material or metal at least 12mm thick as a heat sink

bobbidill · 9 Jun 2019

big-all said:
surely with a minimum say 50mm you can apply a fireproof material or indeed a heat sink ??
in my workshop the space was quite tight round the wood burner so cut up and old pressed steel radiator spaced it off the wall by 25mm and the heat at the wall whilst quite warm wont burn just store the heat in the metal
so what i am suggesting is heatproof material or metal at least 12mm thick as a heat sink

I did enquire about this and the flue manufacturer informed me that the air gap is a necessity, so the option of a heat sink isn’t possible either.

^woody^ · 9 Jun 2019

The 75mm is normally measured to the inner flue, not the outer casing.

Can't you use an insulated flue instead, which would require a reduced distance.

Normally, manufactures will relax the requirement when presented with a suitable design showing the rafters protected with a section of calcium silicate or magnesium oxide board (eg Supalux)

bobbidill · 9 Jun 2019

^woody^ said:
The 75mm is normally measured to the inner flue, not the outer casing.

Can't you use an insulated flue instead, which would require a reduced distance.

The flue is twin wall insulated. Normally, manufactures will relax the requirement when presented with a suitable design showing the rafters protected with a section of calcium silicate or magnesium oxide board (eg Supalux)

Thanks Woody. I'll give the manufacturers a call tomorrow and see firstly whether the 75mm referred to is to the outer or inner flue and I'll also ask them about your suggestion regarding protecting the beam.

^woody^ said:
The 75mm is normally measured to the inner flue, not the outer casing.

Can't you use an insulated flue instead, which would require a reduced distance.

Normally, manufactures will relax the requirement when presented with a suitable design showing the rafters protected with a section of calcium silicate or magnesium oxide board (eg Supalux)

Thanks Woody. The twin wall flue is insulated. Thanks for your suggestions, I'll contact the flue manufacturers and see what they say. Have you any idea if there's a way to create a cavity in the beam, perhaps by double boarding it in order to create the space if they decline your proposals?

cdbe · 9 Jun 2019

Just do this:

bobbidill · 9 Jun 2019

cdbe said:
Just do this:
View attachment 165717

That's super, thanks cdbe. What do you suggest that we use to join the 2 beams together, which is represented by the 3 lines on either side in your sketch?

cdbe · 9 Jun 2019

I'm not an engineer so I'd just over-engineer it - 600mm overlaps, 5 X M10 bolts either side with spiked washers in between and big washers on the outsides. They sell it all in Screwfix.

bobbidill · 9 Jun 2019

cdbe said:
I'm not an engineer so I'd just over-engineer it - 600mm overlaps, 5 X M10 bolts either side with spiked washers in between and big washers on the outsides. They sell it all in Screwfix.

Excellent thank you cdbe, you think that'll do the trick? I know nothing about what's needed in terms of the beams being strong enough to support the cement fibre tiles, hence asking for advice!

cdbe · 9 Jun 2019

For a single rafter, and with the join being so close to the end I wouldn't have any worries.

geraldthehamster · 10 Jun 2019

^woody^ said:
The 75mm is normally measured to the inner flue, not the outer casing.

I don't think that's right. Flue manufactureres give the minimum distance between the flue and the combustible material. They don't expect the user to extrapolate to the inner wall of a twin wall flue.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

How to create air gap by joist for flue?

Similar threads