warm flat roof details Information, help and advice on maintaining, installing and repairing a flat roof. Post your questions and view answers here

[jg61]

Hi Im new to forum so any help greatly appreciated. Plan is to lay 100 pir over ex ground floor felt roof 12'x8' (cracked but not yet leaking), then new osb deck folowed by grp system.
Do i need to fix new osb through pir (using long screws into ex joists) as this would seem to stop any expansion taking place? how to prevent uplift etc without fixing down.
thanks in advance

[Perry525]

The best way is to waterproof glue t&g osb into one monothic fully floating piece, this to avoid any chance of leakage. Then cover with a geotex fabric followed by a one piece made to measure #firestone rubber covering. Then cover with pea shingle, the shingle will hold the roof down and will prevent any ultraviolet damage. This presumes the joists are up to it.
The problem with fibreglass is that you cannot be sure of the coverage and cannot see any pin holes, unless you go over it several times and build up to at least a quarter of an inch.
While fibreglass may sound good, afterall they make boats out of it, unless you do it yourself and take a lot of care you will probably end up with a rotten roof. (if a boat has a hole you can easily see - with a roof you cannot)

[jg61]

Hi thanks for info sounds like a better option than grp what depth of shingle would be needed ?
I would have pref a mechanical method but that will stop expansion? Will that matter with epdm? Seems to be turning into bit of an epic! TIA jg

[RubberRoofer]

Sorry - have to disagree with your first reply.

You will NEVER stop expansion - it happens. The firmer you screw boards down the more damage they do when they move!!!

Put your hands flat on a table, fingers touching and then push together. It's uncomfortable, but if you push hard enough they will buckle and lift up - that's what will happen to your boards.

Screw down the OSB3 with 2-3mm expansion gaps between the boards. Get a very thin sheet of hardboard, or thick cardboard to push between them to make it fast.

You don't need to leave these with the PIR insulation as it will squeeze sufficiently to absorb the slight movement.

You should be gluing the EPDM down - do NOT ballast it.

A) There is no need - if it's glued properly it won't come up.
B) The Ballast could puncture the membrane.

DO NOT try to stop expansion! You can't - work WITH it - if you try and stop it you will wreck your roof.

[RubberRoofer]

I should have added - I do agree to steer clear of GRP.

Firestone is a good product, or if you are having it done professionally go for a fleece reinforced membrane as it's even better.