OSB or Ply for flat roof

padstar · 1 Sep 2021

Hi i have been told 2 different things by contractors for the structure of the roof.

Should I use 18mm plywood or 18mm OSB (OSB3)?

£180 difference over the entire roof so i dont mind either but would rather save the money if there is no real gain.

Swwils · 1 Sep 2021

It depends.

Certified ply will be better than OSB, but all OSB is graded. OSB 4 better than a generic ply which varies from batch to batch.

Without super expense like a WBP certified ply, nothing will survive leaking, so make sure it doesn't leak.

If you going to walk on it or see it from below most will prefer ply.

dilly · 1 Sep 2021

OSB 3 it’s been tested and comes with BBA approval very stable but will need to be kept dry,unless the ply has BBA approved stamp on it the quality is very hit and miss unless it’s metsa spruce very good but much more expensive

padstar · 1 Sep 2021

By certified ply do you mean Structural grade? Is this required or can standard ply be used as there is quite a difference in price (£10+ sheet).

Regardless of what route i go down the roof structure is likely to be exposed to the elements for about 2 weeks. I can put a tarp over it to keep most the water off i guess if needed.

padstar · 1 Sep 2021

Went with ply in the end.Got offered a good price for it so made my mind up for me.

Should boards be glued and screwed or just screwed? If glued any preference in product?

Makie · 1 Sep 2021

padstar said:
Went with ply in the end.Got offered a good price for it so made my mind up for me.

Should boards be glued and screwed or just screwed? If glued any preference in product?

What kind of finish is the roof getting?

padstar · 1 Sep 2021

oldbutnotdead · 1 Sep 2021

padstar said:
GRP.

In that case should have gone with OSB3, the resin bonds much better to it than to ply plus T & G boards mean you don't have to support and tape all the joints (which you will have to do with ply)
EDIT Re the getting wet, whatever boards you use have to be 100% dry before laminating otherwise it will fail. If you've plyed it already, go for EPDM instead

Makie · 1 Sep 2021

padstar said:
GRP.

As above do not GRP it. It will be a problem further down the line..

padstar · 2 Sep 2021

What sort of issues are you talking about. I raised this with the roofer and architect and both were surprised that i thought there could be problems?

Captain Nemesis · 2 Sep 2021

padstar said:
GRP.

Search this site for noisy GRP / noisy fiberglass / GRP noise / etc

padstar · 2 Sep 2021

I have read things about noise but assumed it was poor install?

oldbutnotdead · 2 Sep 2021

Noise can be a problem (creaking etc), usually caused when the deck isn't solidly fixed to the joists. Common point seems to be when structure is joists then insulation then deck screwed through insulation.
GRP on ply- none of the resin suppliers I use would entertain any claim for GRP failure on ply, they all specify OSB3 for the deck.
EDIT Which problems were architect and roofer surprised about? It is very easy to throw GRP at a poor deck, it'll look fine and stay waterproof for a year or 2 but then it'll become a maintenance nightmare. GRP will not bond to damp boards. Bubbling, delamination, eventual leaks. If you use square edge boards for the deck, every joint has to be supported (either by joists/noggins or plated). If your boards have gone on top of insulation this won't have happened, you'll get movement between boards, cracks in laminate, leaks.

padstar · 2 Sep 2021

So workmanship on the substructure causing the noises.

What is teh issue with ply Vs OSB. I can do the top layer in OSB if it is an issue (i am sure i will use the ply elsewhere). Most GRP manufacturers mention OSB and ply in their literature?

oldbutnotdead · 2 Sep 2021

padstar said:
So workmanship on the substructure causing the noises.

What is teh issue with ply Vs OSB. I can do the top layer in OSB if it is an issue (i am sure i will use the ply elsewhere). Most GRP manufacturers mention OSB and ply in their literature?

Depends how you're insulating the roof. If the deck is fixed direct to the timbers with chunky nails/screws there seem to be fewer creaking problems than when a floating deck is used (where insulation is between joists and deck).
I could link to a few which say 'don"t use ply'. Here's one https://www.roofingkitsdirect.co.uk/fibreglass-installation-guides

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OSB or Ply for flat roof

Captain Nemesis

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