OSB or (structural) ply

[Tim Watts]

Hi,

It became apparant when the tiles where stripped that the 1970s monkey who made my dormers (large dormers, half a room each) was a complete moron.

In short, the roofers strongly recommended sheetsing the inside of the 3x3" timber frame with glued and screwed 18mm sheet to add stiffness and to brace the odd bits of timber that are not particularly well jointed.

So the question is:

Ply or OSB for structural sheeting work?

And if ply - what does "structural ply" actually have over "regular" ply, apart from the price? Is "shuttering" (the cheapest) "structural" by default or the opposite.

When I was a lad, ply was ply, except for marine ply - we didn't have much of a choice to my memory :)

Cheers!

Tim

[sm_jamieson]

I've certainly seen OSB being used in lots of Grand Designs style projects = and in US stick-built homes and I guess it is performing a structural role,= though the frame is important too. 18mm osb is pretty strong and OSB type =

3 is waterproof to an extent. I would have thought OSB 3 would be fine for = the job.

Off some website or other:

Sterling Board - OSB Type 3 OSB (Sterling Board) Type 3

Type 3 is a strong, versatile board suitable for structural use in humid co= nditions, ideal for applications as diverse as roofing, flooring, hoarding = and wall sheathing etc.

Simon.

[Tim Watts]

On Tuesday 15 January 2013 09:39 sm_jamieson wrote in uk.d-i-y:

Thank you Simon.

The frame is not crooked - so it should sheet up nicely. And it's not falling down - it's just not as good as it should be.

OSB visually is not a problem - this would be further covered by extra celotex then plasterboard.

If anyone can explain "structural ply" I'm still interested though - for other reasons :)

[Tim Lamb]

In message , Tim Watts writes

Bit here....

[Dom Ostrowski]

For your application the incremental cost of using a better quality product is likely to be small, so I would suggest buying exterior ply.

AIUI "structural" means "tested as conforming to a standard".

With timber joists that means each individual joist rolls through a deflection test machine and is automatically labelled (e.g. C16, C24). Don't know how they test sheet products.

[polygonum]

But just what is better about ply than OSB?

[Tim Watts]

On Tuesday 15 January 2013 11:13 Tim Lamb wrote in uk.d-i-y:

[Nightjar]

On 15/01/2013 10:38, Tim Watts wrote: ....

It complies to BS5268: Part 2 and, as such, is deemed to satisfy Building Regulation 7, which requires that any building work shall be carried out with proper materials.

Colin Bignell

[sm_jamieson]

Quote from sterling board product details:

Sterling Roofdek is designed specifically for flat roof decking and pitched roof applications. The board combines all the features and benefits of SterlingOSB T&G and measures 2440x1200mm; perfect for roofing The sanded surface is designed to offer excellent adhesion for glues, self-adhesive felts and bitumen. The board is completely free from voids and knots and provides the ideal solution for roofing. In other words, it's as hard working as you are. Sterling Roofdek is BBA approved, and permitted for structural applications under BS 5268:Part 2.1996 and Eurocode.

That also implies that the other Sterling OSB boards do not comply with BS 5268.

Simon.

[polygonum]

BS 5268 has disappeared so there would be no need to!

"Farewell to BS 5268. Welcome to Eurocodes.

31 March 2010 saw the withdrawal of all parts BS 5268. For timber and wood-based products, BS 5268-2 is superseded by Eurocode 5 (EC5) or to be more precise BS EN 1995 - Design of Timber Structures."

more info. here:

interesting, that does not actually answer my question! If anything, it makes it sound as if OSB is better than ply.

[js.b1]

I think OSB4 is the highest OSB, and marine ply, and true marine ply BS1088= are the highest ply. You can sometimes get BS1088 on Ebay as job lots rela= tively cheaply - however I suspect not as cheap as OSB. OSB is routinely us= ed as the web in engineered timber joists for both floors and roofs - and c= an be amazingly stiff.

Just avoid "ply" from sheds which can be really poor quality, full of voids= , disintegrates if it sees a cloud.

[Dom Ostrowski]

An equivalent thickness ply is quite a bit more rigid, and more difficult to pull through nails/screws IME.

[sm_jamieson]

True, I put down some 18mm OSB3 on the loft floor (I had some left over) and certainly noticed it was more bendy underfoot than 18mm ply would (wood ?) be. Simon.

[polygonum]

certainly noticed it was more bendy underfoot than 18mm ply would (wood ?) be.

Thanks.