Flue passes too close to rafters

17 Oct 2006
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United Kingdom
We are having a new woodburning stove and the HETAS fitters have belatedly discovered that the existing twin-wall flue passes only 10mm away from the gable end rafters. Having installed the stove they are now refusing to issue a certificate since Approved Document J and the Selkirk twin-wall instructions both specify a 50mm air gap.

They have proposed fitting a smaller diameter flue (6in ID in place of the 9in flue for the original open hearth) but this including storm collar, rain cap and flashing will be a lot of expense.

Is there a fire resistant board which will provide adequate fire protection in a 10mm thickness? As it will only touch the curved outer surface of the flue in one place common sense suggests this is the way to go, but I cannot fiind anything with formal approval for this application.

All suggestions gratefully received!
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What fire resistance is required, 30mins? I don't know about a board in a 10mm thickness, but one layer of 15mm Fireline board will get you 30mins FR.
Unfortunately there is at most only a 10mm gap.

9mm Supalux would fit and is rated for 60mins fire protection when used for stud walling, but is not certified for the current application. Ideally I would like to find some reputable authority for using it (or some other barrier) between a twin-walled flue and structural timber.
Can you not simply kick the problem back at the HETAS engineers ?

This is obvously easier if you haven't paid them, but presumably they quoted you a certain sum to fit a compliant system and if they haven't done so, then legal action is a last resort , but also a useful negotiating tool before it goes that far.

EDIT: I don't know about the UK, but it is no longer permitted in France to fit a reducer between fire and flue and 9" down to 6" is a huge reduction: it is in fact more than 50% of the volume!

This smacks of desperation to me and I wouldn't countenance it without both checking with the fire manufacturer if it was acceptable and asking the HETAS engineer to certify in writing that it was legally permissible, would not affect the efficient operation of the fire and was approved by the manufacturer.
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Obviously I am pursuing that line of thought too, not least because they did not inspect the flue in the loft at the time of the initial site visit but then decided to once the installation was under way. The additional work they are proposing will increase the installation cost to nearly 4 times the figure they originally quoted!

But I am looking here for possible technical solutions that I can propose to them as an alternative to simply increasing the air gap to 50mm.
What about splicing a new rafter onto the existing, spacing it over 50mm away and then chop out the offending rafter section? Its only a short section presumably so hardly gonna be a major structural issue provided its done correctly.
Sadly the rafter is only 1/4 in from the gable end wall. We have thought of reinforcing it with (non-combustible) angle iron or channel along the bottom edge and then cutting a section away. But this is messy as ideally we would fix the reinforcing to the face as well but that would make it impossible to get the saw in!

Temporary props from the ceiling joists, putting trimmer rafters to the adjacent ones round the gap (as you would do for floor joists) or bolting the rafter to the blockwork gable end are all possibilities but none of them is very appealing.

Re mointainwalker's edit, the fitter said they would also line the rest of the flue with 6in twin-wall liner down to the the stove (which would be OK as it has a 6 in outlet) but when I reminded the office of this their quote for the additional work went up from £600 odd to £900 so they are clearly making it up as they go along.

Latest idea is to provide additional protection by fitting a linked smoke alarm in the roof void. There clearly isn't a real problem, as the sarking membrane in contact with the flue has not been affected by heat in 18 years (they have now cut it away).
Seems then that you have dscussed the limitations.

Therafter come the possibilities and who is liable for the cost.

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