Warm flat roof - 2 Decks?

[Martin Spindlow]

Hi all... hope you can help, I'm finding conflicting information online!

I'm about to put a GRP warm flat roof on an outbuilding, I'm using 100mm PIR foil backed insulation and was wondering do I need a deck between the joists and insulation or only one above the insulation (the joists are at 400mm centres)... some people seem to state that I need both... others state the insulation will be rigid enough once I have a deck installed over it to spread any loading.

It will not have foot traffic and it literally just a roof to keep water out

Hope you guys can help

Martin

 

[^woody^]

It's feasible, but not practical in some situations.

There is the potential for the insualtion to compress slightly under wind, or snow load, and then create ponding. The larger the roof, the greater the risk. If its just a couple/three meters wide with those close joist spacings, then the risk is greatly reduced.

 

[Martin Spindlow]

Thanks for the help woody... seen a lot of your replies on these forums

It's 3m wide by 4m long so quite small

 

[^woody^]

The problem you will have is where two of the sheets meet on a single joist - not enough support on the edges.

So make sure to lay the insulation sheets longways across the joists and where two sheets meet, fix a batten either side of the joist to support the sheet ends.

Bond the insulation sheets, and then bond the timber decking boards too, so that the joints are staggered and not in line with each other or the sheets below

 

[Elastomeric]

And when you've done all that, put a proper membrane system on it.