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Advice on Fixing a Warm Roof to Joists and Creating a Fall

Postby SilvaBC » Wed Aug 16, 2017 9:30 pm

Hello everyone I'm new to the forum.

I'm currently project managing a single storey extension to my house which is going to have a flat roof. I'm very much a novice to DIY and building techniques in general and have many many questions but I'm only going ask a couple in this post.

I'm going for a warm roof design comprising of the following layers - going from outside to in

1) GRP weather proof layer
2) OSB3 18mm Tongue & Groove top deck ( Required so I can walk on roof for outside maintenance of house)
3) 120 mm of Kingspan TR26 rigid insulation
4) Vapour Control Layer - not sure which to go for yet e.g. loose laid or self-adhesive
5) OSB3 18mm Tongue & Groove bottom deck

Roof outline is L-shaped and approximately 60sqm with 4 skylights. The fall for the roof is pretty simple as it runs from right to left (as you look from the garden towards the house) all the way through. The fall is at right angles to the joists. The fall height will be approximately 130mm.

My questions are as follows:

1) My builder tells me that to achieve the fall he will have lay tapered firrings at right angles to the joists. My concern with this method is that the warm roof will not fully rest on the joist along its length only on the tapered firrings so in effect there will be gaps between the bottom deck of the roof and the joists. Therefore the fixing (nails / screws see question 2 below) would be partially exposed running a potential condensation risk and have to be longer as well? My idea for the fall would be to use different height battens fixed along the length of joist to create the fall profile. In this way the whole bottom desk is in contact with the joist. What are peoples thoughts/experiences on the two approaches above - am I needlessly worrying over nothing?

2) Could somebody please provide some info/tips onto the best method of fixing the warm roof deck to the joist. My plan is lay the bottom deck first and secure with standard roofing nails and then to lay the vapour control layer and then secure the insulation to the VCL by gluing to temporarily hold in place and the place the top deck over the insulation and permanently securing by driving 195mm Helifix nails from the top board and through the insulation into the joists. My BIG concern about this is how can you accurately hit a joist through such a depth with such a long nail? Again tips or guidance/alternative approaches would be appreciated.

Many thanks for taking the time to look at this

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Postby diydoctor » Thu Aug 17, 2017 10:20 am

Get the roofer to fix your different sized battens bewteen his firring pieces. You then have the best of both worlds. Total support for your deck between joists and full insulation.

Measure carefully. Probe the insulation with a stiff wire coat hanger to find joist. A few "missing" holes will not compromise the insualtion
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Postby SilvaBC » Sat Aug 19, 2017 4:50 pm

Thanks for taking time to reply.

That's good idea about the battens between the firrings. Finding the joist I guess will come down to careful measurement as using a stiff wire is not an option as the insulation is the rigid PIR type
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