OSB3 Vs structural ply Vs WBP ply

[MisterBoy]

For roofing sheets (on top of insulation) we'd original specced 22mm OSB3 but our local merchants only have it in 18mm. Also worried a bit this time of year it might get drenched before it is shingled.

A timber yard guy recommended me plywood over OSB for greater strength but I lost the thread a bit over the types. I had thought about marine ply but turns out I probably wanted WBP as proper marine ply costs a fortune. When we got onto structural ply Vs WBP is where I got confused.

For something that is rigid, not going to get ruined this time of year, and not going to cost a fortune, what are my options and what's the pros and cons? Guess OSB3 is the standard but I'd like to spec a bit higher.

 

[MisterBoy]

Should add, any particular tips with this since rain is always a possibility? It'll be shingled so it will be able to breath and dry out but my concern is frost when wet if it rained halfway through then turned cold

 

[Notch7]

OSB3 -it has a rather water repellent coating which does dry out pretty quickly -its the edges that soak up the moisture and swell is left wet for a long time

Ive used OSB3 as the deck for orangeries for years -and it can get soaked, but its been fine.

i cant see any advantage to 22mm really -damn heavy for getting onto a roof

 

[MisterBoy]

Any point taping joints if it'll be exposed during bad weather to minimise edges getting wet?
Think 22mm was either some guy's recommendation or was to make it nice and rigid. Seems to me if we need that, ply does the same at 18mm for a few quid more. WBP sounded a great product but I never heard of it before today.

 

[Notch7]

Any point taping joints if it'll be exposed during bad weather to minimise edges getting wet?
Think 22mm was either some guy's recommendation or was to make it nice and rigid. Seems to me if we need that, ply does the same at 18mm for a few quid more. WBP sounded a great product but I never heard of it before today.

WBP (weather boil proof) doesnt exist anymore -and only related to the adhesive.

 

[MisterBoy]

WBP (weather boil proof) doesnt exist anymore -and only related to the adhesive.

A timber merchant mentioned it to me today, well he mentioned BS... but then definitely said WBP! This was when I asked about marine ply and he advised I probably didn't need that, but this product. Any guesses?

 

[Lower]

What is the modern equivalent of WBP ply?

 

[MisterBoy]

Guessing maybe builders call something WBP that isn't? If a Timber merchant knows what I mean great but worried I'd ask for the wrong stuff

 

[Alexnh]

I have been struggling with a similar ply question although for a shed floor rather than a roof.

I think the standards that now apply are en636 which covers the wood for the Plies used and en317 which covers the glue. For each of those there are 3 levels, 1 being interior, 2 for humid conditions and 3 for wet. I think both en317-2 and 3 are sometimes described as being equivalent to or sold as WBP.

I am not sure what standard covers structural Vs non structural.

Any cut edge you will have to seal and it is possibly sensible to seal the surfaces and factory edges if you have the time too, especially the ones that have the greatest risk of getting wet.

 

[d000hg]

I hadn't considered that factory edges are pre-sealed. Is this typically the case or do they simply cut the boards from a huge sheet?

 

[Notch7]

I hadn't considered that factory edges are pre-sealed. Is this typically the case or do they simply cut the boards from a huge sheet?

OSB is made as a continuous sheet because the softwood strands go through rollers, compressing it down.

At the end of the process the endless sheet is chopped into 2440 lengths and trimmed to width.

 

[Alexnh]

I suppose I have kind of implied that the factory edges may be sealed but really I am not sure if they are or not. I'm certain that the edges you cut yourself won't be though.

Seal all the edges if you have the time. Never heard of anyone taping the joints unless they're building a boat But again, if you can find a suitable tape it won't hurt.

This is possibly DIYer lily-gilding and most builders would just put it up reasoning that the shingles etc will keep the roof boards dry enough to last many years without time consuming steps like this.

 

[d000hg]

I am thinking of sealing the edges exposed to the air - they'll either be behind a fascia or overhung by the tiles but still. With PVA or perhaps epoxy resin. Never thought about the cut edges - our roof is 1.5 sheets 'tall' so one cut edge per sheet width right in the middle which isn't so bad. Perhaps we'll slosh some PVA/epoxy in the cracks while we're up there before the shingles go on though I would guess it probably is overkill.

 

[Alexnh]

Epoxy, especially thinned, is a fine penetrating sealer but relatively expensive and will degrade if exposed to UV. PVA very cheap not much good as a sealer. Exterior wood sealers are another option or you could use exterior paint or varnish on the edges if you have some spare.

 

[muggles]

On our extension I used OSB, but did it as 2 layers of 12mm screwed and glued together with PU adhesive, making sure to get plenty where the boards butted together. The advantage of this is that you can stagger the edges between first and second layers, so there's no direct path between two boards for water to get through if the roof covering fails. Possibly overkill, but I was happy with it