OSB3 top-deck for warm EPDM roof

[MisterBoy]

3 quick questions...

1. Am I right in thinking nothing other than 18mm OSB3 should be considered for this?
2. Has everyone switched to the half-size T&G boards these days despite the higher cost?
3. We would be screwing through 150mm to the sub-structure, should I be doing anything to the screwheads or using specific types/technique to avoid them showing through the EPDM?

Thankyou.

 

[DiyNutJob]

I am looking at building a tool shed and interested in a mini EPDM roof. My searches suggest OSB can't handle moisture (moist air?) well. My other option is to use plywood. I want a light weight roof and will go for 9mm board. My thinking is that so long as there is support frame under the board, thinner boards can work. To get a flat roof, I would countersunk the screw.

Sorry, I can't help you. I find bouncing ideas around often produces a gain.

 

[MisterBoy]

Osb3 is reasonably good with moisture, definitely better than plywood which delaminates easily (all the layers unstick)... This is the problem we have and why I'm having to replace it.
You can get moisture resistant marine plywood but it costs a fortune. Every video I've seen they use OSB and it seems to be what EPDM companies recommend too

 

[Notch7]

3 quick questions...

1. Am I right in thinking nothing other than 18mm OSB3 should be considered for this?
2. Has everyone switched to the half-size T&G boards these days despite the higher cost?
3. We would be screwing through 150mm to the sub-structure, should I be doing anything to the screwheads or using specific types/technique to avoid them showing through the EPDM?

Thankyou.

Ive always used 18mm square edge OSB and 180mm normal screws.

you can see the screw heads sometimes as the screw transmits enough heat to dry the EPDM out in a circle.

you can buy those helical ones but they are quite dear.

the trend these days is to use glue down insulation with a ply coating ready for the EPDM

 

[DiyNutJob]

Osb3 is reasonably good with moisture, definitely better than plywood which delaminates easily (all the layers unstick)... This is the problem we have and why I'm having to replace it.

The old shed I demolished had a chipboard for roof. That got wet and failed. I suppose they will all fail when water gets to them. I can stop water getting to them. The only unknown to me is the effect of moist air.

 

[MisterBoy]

Ive always used 18mm square edge OSB and 180mm normal screws.

you can see the screw heads sometimes as the screw transmits enough heat to dry the EPDM out in a circle.

you can buy those helical ones but they are quite dear.

the trend these days is to use glue down insulation with a ply coating ready for the EPDM

The comment I'd seen on screws was the heads can dimple it a bit - but maybe this is being a bit anal, I'm not sure. If it's cosmetic only, perhaps doesn't really matter. I wonder if one can simply tape them but then you don't tape the OSB joints as far as I've seen so may be OTT.

This is a re-deck as the plywood has delaminated so the insulation is already there - regular foil-faced PIR on top of a regular black plastic sheet VCL so not really set up for gluing.

 

[MisterBoy]

The old shed I demolished had a chipboard for roof. That got wet and failed. I suppose they will all fail when water gets to them. I can stop water getting to them. The only unknown to me is the effect of moist air.

Chipboard is probably even worse than ply for water unless you get a special type. You often see buildings faced in OSB left for quite some time in the weather, it seems to be the go-to option these days.

 

[DiyNutJob]

Chipboard is probably even worse than ply for water unless you get a special type. You often see buildings faced in OSB left for quite some time in the weather, it seems to be the go-to option these days.

I reckon they are all roughly the same. The shape, size, and type of wood chips are different, but the glue is the same. Water and UV will destroy the glue over time. The higher grade glues might last longer in the more expensive boards. I will make my mini roof replaceable, so that a failed board will be less of a trauma. Given that OSB is cheaper to buy, I will go for that.

 

[cdbe]

I've never understood 18mm OSB for this (I think it's just a "done" thing), when it's going on PIR on a sub deck it's not really spanning anything, it's just something to fix the EPDM to - if you consider that you can just stick the EPDM to tissue faced insulation. My garage is EPDM on 9mm OSB on the joists. Takes my 17 stone on the odd occasion I have to walk on it, but I don't go dancing up there!

 

[phatboy]

I used 9mm ply as the top deck, regret it. Delaminated over night last summer, just from condensation forming on it. Took a bit of rain in one area, and it's a bit wrecked. Should have used OSB3