flat roof upgrade

21 Jul 2010
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Weymouth, Dorset
United Kingdom
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One dormer flat roof and one bay felt roof. I'd like to upgrade the insulation on both and have GRP fitted instead of felt. All I read about is warm roof being best, but adding 100mm+ Celotex to the bay will look strange I feel. Not so concerned about the dormer roof.
On the bay, can I have 50mm Celotex above the joists and extra 50mm in-between the joists? Can I do the same to the dormer window or best to do it proper.
On the dormer roof it would mean adjusting the tiles at the rear edge either way.
I know Part L demands 100mm+ above the joists, but I also read there can be exceptions.
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I wouldn't install 100mm on that bay. I'd be more inclined to go with a cold roof & insulate between joists.. For the visual considerations.

The regs guidelines are to be achieved where possible, but exceptions are allowed where installing to the guidelines is not possible.

Stick with a warm roof on the dormer. Better overall & you won't have to mess around with vents.
That's what I was thinking. Good to know the regs are for guidance mainly. Weymouth council are sending me a letter or something about this.
With the dormer roof, I understand I will need to provide a vapour barrier at ceiling level, is that correct? Maybe foil backed plasterboard.
I think you can use foil backed plasterboard if you use tape with it.. Unsure about that though to be honest.

We would use a sub deck (11mm is sufficient) with a bitumen felt layer as vapour barrier. 100mm insulation over that & an 18mm OSB top deck screwed into roof joists with large screws. *GRP has to be 18mm OSB.. No other deck is suitable for fibreglass.
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thanks for your spec, looks good.
For the bay cold roof, if the joists are 100mm, 50mm Celotex or more?
so ventilation is not so crucial I guess, for a small area like that. It's not a high humidity room anyway.
Ventilation becomes quite technical when you get stuck into it. Quite a few factors can affect what measures are required.

Thinking out loud here, but...

I guess you could incorporate some kind of ventilation around(or underneath) the fascia boards. Maybe extend/double the drip battens & create an airflow with some cut outs(eg put pieces of batten around the drip edge leaving gaps. Then fit another drip batten over that. You'd then have air gaps. You'd have to counter batten the joists, which would raise the roof up the thickness of a batten.

That's something like what building control would likely suggest.

I've seen hundreds of bays that don't incorporate ventilation. Not saying that's right.. Just saying.

I've never seen a bay roof with a vent in it.. Put it that way.
exactly for such a small area I wouldn't bother. Full fill the void nice and tight, foam any gaps. Maybe consider taking down the bay ceiling and putting a VCL in but I wouldn't say it's essential.

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