GRP Flat Roof Construction - Temp Coverings

4 Jan 2006
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United Kingdom

I am building a workshop in my garden project thread here.
The timber frame is nearly complete and will soon be fixed in place. The next step will be to get the Roof Joists on.

As I am doing this project in stages in between work / life, I would like to get the roof and sides on and waterproofed to help protect the wood and the inside of the structure but not have to undo anything when I get to finish it off.

The roof will be GRP so once the Joists are on with a drop from rear to front, I guess I need to fit the furrings and then OSB (18mm or 11mm?). As I wont be able to GRP this for a while I would like to temp waterproof. Looking at flat roof constructions, wether I go for a cold or warm roof, none of the ones I have seen have a waterproof / breathable membrane on top of the OSB / Ply so whatever I put on here would be temp and have to be removed before completing the roof - am I correct?

After much deliberation I am going to OSB or Ply on the inside, put Rock Wool between the Studs, cover with waterproof / breathable membrane (any recommendations of products) batten and then clad.
I have not decided on or sourced the cladding yet so if I build this up to waterproof covering stage would it be able to leave for a while?

Thanks for any advice
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I have just GRP-ed my garage roof and was told to use 18mm OSB. I was surprised how easy it was, you can watch videos on Youtube.

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yep 18mm OSB no point scrimping. I would put the deck on and go straight onto the fibre glassing ASAP

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