Housing - New Build: flexible board with racking strength

1 to 29 of 29

- CommentAuthordjh
- CommentTimeNov 29th 2013
I'm looking for a board for the underside of our roof structure. It needs to provide some racking strength, be fairly vapour permeable, and be able to bend with a radius of 6 m or so.

It needs to be vapour permeable because the upper surface of the roof is not especially open, so there'll be an 'intelligent' membrane underneath to allow drying to the interior when conditions are right.
- CommentAuthorTimber
- CommentTimeNov 29th 2013
9mm OSB? There are other possibilities like ply or hardboard - all of which will provide racking resistance which can be calculated to BS 5268 or EC5. I think all of them will bend to that kind of radius, although it will probably need something fairly strong to bend it into shape.
- CommentAuthordjh
- CommentTimeNov 29th 2013
Posted By: Timber9mm OSB? There are other possibilities like ply or hardboard - all of which will provide racking resistance which can be calculated to BS 5268 or EC5. I think all of them will bend to that kind of radius, although it will probably need something fairly strong to bend it into shape.

Do you have permeability numbers for OSB? I thought they were a bit hard to come by. It's been suggested that 9mm ply is not vapour-open enough. Hardboard is an interesting thought; maybe Kronotherm or similar? I don't know the numbers immediately.
- CommentAuthorEd Davies
- CommentTimeNov 29th 2013
How about ply in fairly narrow strips (200 or 300 mm, perhaps) with breather gaps of 2 or 3 mm between, like Scottish sarking boards? Can't really visualize your curvature requirements vs racking directions to think whether that makes sense.
- CommentAuthorSaint
- CommentTimeNov 29th 2013
Have you looked at thin MgO boards? The 4mm ones will bend to a much tighter radius than that
- CommentAuthorfostertom
- CommentTimeNov 30th 2013 edited
Posted By: djhDo you have permeability numbers for OSB?
Surely they all publish that? (unlike air-permeance which is equally important). I'd go and look up the nos for the different OSBs in the WUFI database but I'm temporarily locked out of that till I convince myself to plonk down E1950. Anyway OSB is adequately water vapour permeable.

Tho a recent paper tested OSB for air permeance
http://www.mendeley.com/download/public/35879/3686773611/9b3c4e723d2042867cf783960db3ebd04e66c472/dl.pdf
discussed in http://www.linkedin.com/groups/OSB-is-not-airtight-recent-2163729.S.60928033
found a huge variation not just between the 8 (sadly anonymous) brands tested, but also between samples within a brand. No reason why that shouldn't mean the same variability for water vapour permeance too.

Makes you wonder about such products (even e.g. Panelvent) - can we believe the performance figures even for a particular brand - are they continuously quality-controlled to maintain the published performance, once the testing agency has gone away?
- CommentAuthorTimber
- CommentTimeNov 30th 2013
As mentioned OSB figures are published - 9mm OSB has a mu of about 30 if memory serves, but the manfufacturers will have all this data available.

Hardboards have a number of pluses potentially - BS5268 considers that a 6mm hardboard is a type 1 racking board. 6mm hardboard is relatively cheap and flexible. I can't remember the specs off the top of my head, but can confirm the exact type of board you will need (there is a fairly complex series of codes in the fiberboard standards). The other advantage is that there is NO GLUE in a hardboard! It is just wood and nothing else (well perhaps a little oil in the oil tempered products).
- CommentAuthorfostertom
- CommentTimeNov 30th 2013
Smartply MDF also claims as USP 'no added formadehyde' i.e no added glue - but timber has plenty of formaldehyde/glue in it naturally and offgasses just like (but less than) the dreaded MDF. Prob the Irish softwood that Smartply uses has exceptionally high natural formaldehyde. Smartply is currently going thro Fraunhofer (WUFI) testing so I expect their published figures will get revised by better info shortly.
- CommentAuthorTimber
- CommentTimeNov 30th 2013
Posted By: fostertomSmartply MDF also claims as USP 'no added formadehyde' i.e no added glue

That is not true, they still use a separate adhesive in the manufacture of the board, it is just that the glue has no formaldehyde in it. It is something like Methal Diiso..blah blah! I have the name of the glue at work if anyone is interested. And that is only their OSB3 product. The OSB2 still has normal formaldehyde adhesive in it.
- CommentAuthorfostertom
- CommentTimeNov 30th 2013
Ah that's interesting. Any guess then how use of that glue instead of OSB-industry-standard stuff, wd affect water vapour permeance and/or air permeance? I wonder why they do that - for the extra strength reqd in OSB3 presumably, and the 'zero added formaldehyde' USP is just a bonus?
- CommentAuthorDarylP
- CommentTimeNov 30th 2013
'USP'...., and because OSB3 is designed for areas of greater humidity than OSB2?
- CommentAuthordjh
- CommentTimeNov 30th 2013
Posted By: fostertom
Posted By: djhDo you have permeability numbers for OSB?
Surely they all publish that? (unlike air-permeance which is equally important).

Sorry, yes, I'm getting myself confused.
- CommentAuthordjh
- CommentTimeNov 30th 2013
It's a barrel-vaulted roof. From the top it's currently:

- standing seam aluminium
- one of the metal underlays
- timber board of some kind (probably in multiple layers to allow curved surface)
- timber beams, joists etc structure filled with warmcel or other blow-in
- timber board of some kind (the subject of this thread)
- intelligent membrane
- 9mm plasterboard

It sounds as though either hardboard or USB might be good contenders, or indeed perhaps MgO. I'd be grateful for any more details about the hardboards or MgO boards. The difficulty is finding a buildup that somebody is happy to sign off on.
- CommentAuthorfostertom
- CommentTimeNov 30th 2013
Surely you'll need copious through ventilation under the metal? With or without internal VCL, massive condensation on underside (and top) of the overcooled metal facing clear night sky. Otherwise you're just describing caravan construction, and we know what happens to them within 10yrs! Expose the underside of a bit of caravan roof skin, watch, and marvel at what happens!

Having done that, you've got a simple breathable roof. In WUFI I've found that a big thickness of even something as vulnerable as wood fibre+Warmcel, is amazingly tolerant of such condensation, even leakage, beyond off-the-scale 4% of incident rain - provided there's no inboard VCL, not even an 'intelligent' one, to obstruct inward drying (as well as outward to the vented space). Absence of VCL is vital - with any VCL, instant disaster area!

you might think that a watertight, outward draining breather under the through-vent layer, to drain the condensation outward, (when it doesn't evaporate by airflow) wd be essential - but WUFI says not necessary.

Usual proviso - don't take my word for it (unless I'm being paid for consultancy) - satisfy yourself by whatever means. But WUFI! - constant turnarounds of conventional wisdom!
- CommentAuthorTimber
- CommentTimeNov 30th 2013
Posted By: fostertomAh that's interesting. Any guess then how use of that glue instead of OSB-industry-standard stuff, wd affect water vapour permeance and/or air permeance? I wonder why they do that - for the extra strength reqd in OSB3 presumably, and the 'zero added formaldehyde' USP is just a bonus?

Not sure really - I guess that this data can be found on the smartply website so it should be easy to compare. Not sure on air tightness of the OSB2 and 3 products I am afraid.

Posted By: djhIt's a barrel-vaulted roof. From the top it's currently:

- standing seam aluminium
- one of the metal underlays
- timber board of some kind (probably in multiple layers to allow curved surface)
- timber beams, joists etc structure filled with warmcel or other blow-in
- timber board of some kind (the subject of this thread)
- intelligent membrane
- 9mm plasterboard

It sounds as though either hardboard or USB might be good contenders, or indeed perhaps MgO. I'd be grateful for any more details about the hardboards or MgO boards. The difficulty is finding a buildup that somebody is happy to sign off on.

As Tom mentions above, that is a very risky construction. Condensation will form on the underside of the metal roofing. Now one of the hairy underlays may be sufficient to drain that away without it getting back into the structure, but I certainly wouldn't build that. I would be adding a ventilation space in there above the insulation and below the metal roofing. As mentioned I have seen a number of un-vented cold flat roofs fail spectacularly within a few years so get it wrong and you will be taking it back off again in the not too distant future regardless of whatever intelligent do-dars you might fit.

Go for conventionally accepted design. Might seem boring and 'safe' but these things have been proven over time. Survival of the fittest kinda thing.
- CommentAuthordjh
- CommentTimeNov 30th 2013
The unventilated top is as per the manufacturer's BBA and their repeated answers when questioned.
- CommentAuthorTimber
- CommentTimeDec 1st 2013
djh - what is the BBA reference you refer to (certificate number or manf name)? I think I have come across this before and the BBA is all down to interpretation - If memory serves the BBA says that the metal roofing and hairy membrane must be laid over a solid board matieral for support. Fine. What it doesn't say is that that board can be the top of an un-vented cold roof structure.

I might be wrong (often am) but lets see what the BBA says.
- CommentAuthordjh
- CommentTimeDec 3rd 2013
Timber asked: "djh - what is the BBA reference you refer to (certificate number or manf name)? I think I have come across this before and the BBA is all down to interpretation - If memory serves the BBA says that the metal roofing and hairy membrane must be laid over a solid board matieral for support. Fine. What it doesn't say is that that board can be the top of an un-vented cold roof structure.

I might be wrong (often am) but lets see what the BBA says."

Hi, it's http://www.bbacerts.co.uk/CertificateFiles/41/4151PS4i1.pdf

I think section 9.4 is the relevant bit. Clearly the 'as appropriate' decision is down to 'others', who in this case have come up with the proposal we're discussing, except that their proposed internal board was too rigid.
- CommentAuthorTimber
- CommentTimeDec 5th 2013
hmm, I read 9.4 differently - the last bit says '..and/or ventilate..'
- CommentAuthordavidfreeborough
- CommentTimeDec 5th 2013 edited
From: http://www.bbacerts.co.uk/CertificateFiles/41/4151PS4i1.pdf9.4 When used as a cold roof covering, the installed breather membrane protects substrates from condensate which may form on the back of the panel. Measures should be taken to minimise water vapour reaching the substrate by incorporating a vapour control layer (VCL) in the roof construction, providing an adequate seal around ceiling/wall joints and/or ventilating the void under the substrate as appropriate for the construction.

I would read this to mean that ventilation is required in all cases & that a VCL may be required in some cases.

David
- CommentAuthorTimber
- CommentTimeDec 5th 2013
^ indeed, that was what I was trying to say :)
- CommentAuthorfostertom
- CommentTimeDec 5th 2013
And I'm saying forget the VCL as much more trouble than solution, and go for copious ventilation.
- CommentAuthordjh
- CommentTimeDec 5th 2013 edited
Posted By: davidfreeborough
From: http://www.bbacerts.co.uk/CertificateFiles/41/4151PS4i1.pdf9.4 When used as a cold roof covering, the installed breather membrane protects substrates from condensate which may form on the back of the panel. Measures should be taken to minimise water vapour reaching the substrate by incorporating a vapour control layer (VCL) in the roof construction, providing an adequate seal around ceiling/wall joints and/or ventilating the void under the substrate as appropriate for the construction.

I would read this to mean that ventilation is required in all cases & that a VCL may be required in some cases.

David

Your reading of and/or differs from that I would expect and as stated for example at:

http://www.thefreedictionary.com/and%2For
http://en.wikipedia.org/wiki/And/or

Indeed, I can't even see how to parse it in the way you want to! But as the wikipedia article suggests, it would be better not to use it.
- CommentAuthordavidfreeborough
- CommentTimeDec 5th 2013
At best its ambiguous. It depends critically how you interpret the "and/or" operator.

My initial reading was that you should have a [VCL and ventilate] or [ventilate]. In other words, that the "and/or" was a short-cut allowing you to avoid the following:

"...incorporating a vapour control layer (VCL) in the roof construction, providing an adequate seal around ceiling/wall joints and ventilating the void under the substrate as appropriate for the construction or ventilating the void under the substrate as appropriate for the construction".

I can see how it can be read as just a list of potential things you might do, but that doesn't seem very definitive.

David
- CommentAuthordjh
- CommentTimeDec 5th 2013
Posted By: davidfreeboroughAt best its ambiguous. It depends critically how you interpret the "and/or" operator.

My initial reading was that you should have a [VCL and ventilate] or [ventilate]. In other words, that the "and/or" was a short-cut allowing you to avoid the following:

"...incorporating a vapour control layer (VCL) in the roof construction, providing an adequate seal around ceiling/wall joints and ventilating the void under the substrate as appropriate for the construction or ventilating the void under the substrate as appropriate for the construction".

I can see how it can be read as just a list of potential things you might do, but that doesn't seem very definitive.

David

Well, OK, I see how you parse it now except that your way it reduces to meaning exactly the same as

"...ventilating the void under the substrate as appropriate for the construction".

That is, your interpretation makes most of the sentence pointless. If anything, I'd say the way it was ambiguous is that it can be interpreted as meaning:

"(incorporating a vapour control layer (VCL) in the roof construction,
OR providing an adequate seal around ceiling/wall joints
OR ventilating the void under the substrate)
as appropriate for the construction

with the usual 0111 meaning of OR

And that interpretation would definitely lead to some dodgy but apparently fairly common constructions.

You can get to London using a car, bus and/or train.
- CommentAuthordjh
- CommentTimeDec 5th 2013
Oh, and I like the last entry here

http://www.oed.com/view/Entry/7283?redirectedFrom=and%2For#eid4002106

"1998 N. Lawson How to Eat (1999) 68 Grate in a cooking apple and or a quince."

Very topical
- CommentAuthorTimber
- CommentTimeDec 5th 2013
Well I read that bit of the BBA as...

Either you
incorporate a vapour control layer (VCL) in the roof construction and provide an adequate seal around ceiling/wall
joints and ventilate the space below the zinc

OR you
Ventilate the space below the zinc (perhaps a bit more 'cause you have not done the above)

It is very poorly worded though, and the picture showing zinc on the SIP certainly doesn't help matters.
- CommentAuthordavidfreeborough
- CommentTimeDec 5th 2013 edited
Posted By: djhIf anything, I'd say the way it was ambiguous is that it can be interpreted as meaning:

"(incorporating a vapour control layer (VCL) in the roof construction,
OR providing an adequate seal around ceiling/wall joints
OR ventilating the void under the substrate)
as appropriate for the construction
Agreed. If we can agree that "providing an adequate seal around ceiling/wall joints" is not sufficient on its own then we have to ask whether either of the other two options is sufficient on their own.

The web dictionary references I have found only address the case of A and/or B. Here we have A, B and/or C, which adds an extra level of confusion (in my mind at least).

David
- CommentAuthordjh
- CommentTimeDec 6th 2013
Posted By: TimberWell I read that bit of the BBA as...

Either you
incorporate a vapour control layer (VCL) in the roof construction and provide an adequate seal around ceiling/wall
joints and ventilate the space below the zinc

OR you
Ventilate the space below the zinc (perhaps a bit more 'cause you have not done the above)

It is very poorly worded though, and the picture showing zinc on the SIP certainly doesn't help matters.

I think the [lack of] need for ventilation below the zinc is dealt with by "the installed breather membrane protects substrates from condensate which may form on the back of the panel". That's part of the system that they do take responsibility for. But for obvious reasons, they don't take responsibility for the build-up below the deck over which they have no control.

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