Specifying our roof; cement board and OSB3

[ToughButterCup]

Slowly the specification for our roof is emerging from the mist that seems to swirl around at that level.

 

It has been suggested ( in the words of the architect, ' ... or similar...')  that we use 11 mm OSB3  and 6 mm cement board. That's easy I thought, look up what it costs on Tinternet and buy some.

 

Well Stanley, here's Another Nice Mess (steep learning curve) I've Gotten Myself Into.  Partly caused by the usefully ambiguous architect's phrase 'or similar'

 

 2 hours in to digging around on YT, Google and other murky bits of the Internet I realise;

• OSB3 is water resistant - but what does that mean (it needs covering during boarding if it rains)
• OSB3 ' ...or similar...' could mean what?
• cement board can also be waterproof or water resistant : have you seen how many types of cement board there are! Which should I get?

I have learned the hard way that architect's specify top-performance stuff. OK, fair enough.

 

Could you help me specify something sensible in terms of 11mm OSB3 or something with the same performance, and suggest a sensible 6mm cement board, please?

 

Yours, 

Semi - Irritated of Lancaster.

 

[Crofter]

If 11mm OSB3 is acceptable, I would just go with that. I guess WBP ply would be a substitute but likely to cost more and IMHO more likely to delaminate.

[joe90]

I lined my roof with 11mm OSB and glued all joints to make it airtight, it works a treat ( yet to do a pressure test)

[Vijay]

could you roller on watered down PVA onto the OSB to make it more rainproof?

[Onoff]

I, possibly mistakenly, re-roofed the kid's old Wendy house with 18mm OSB3 and then painted both sides with No Nonsense wood treatment. Pretty sure the stuff damages/softens the "glue" that holds the OSB together as the edges have "grown" and they've never been wet.

[Russell griffiths]

Can you explain what the cement board is for, what is your final roof covering 

there is a company called medite smart ply that make lots of waterproof boarding. 

[ToughButterCup]

11 hours ago, Russell griffiths said:

Can you explain what the cement board is for, what is your final roof covering 

[...]

 

Fire-resistance.

From the OSB3 up

• 6mm cement board
• counter battens
• some form of breathable membrane
• battens
• shakes

The cement board is for fire-proofing. 

I'm at the bottom of a very steep learning curve on what cement board can and cannot do.

 

There are many questions; for example can cement board be fitted ' as is ' or do I have to waterproof  the fixing holes ?

Does cement board need special fixings?

Do I have to seal the gaps between the fitted cement boards? 

If so, with what?

Which cement board should I fit: water resistant, or waterproof or ' normal ' ?

Which brands of cement board have we (BH members) commonly used? 

 

[PeterW]

TBH I would ditch the OSB and go 12mm cement board which is easily capable of taking your weight at 600 centers. 

 

One less layer to fix 

[ToughButterCup]

Thanks Pete,

Which type of cement board? We are using 400mm centers.

Sorry to be so thick....

[Alexphd1]

I have never seen cement board used in a roof, everyday is a school day! 

[PeterW]

I would suggest “Any”..!

 

Knauf is sold by the sheds such as Wickes but is pricey. HardieBacker is another expensive known brand. 

 

We have used STS construction board which is also known as NoMorePly and its amazing stuff. Rock hard but has been great to line the fireplace etc. it comes in 1200x600 boards and there is a lot of data on their website. 

 

Top tip for cutting this stuff is go on eBay / Gumtree and buy a cheap circular saw and Toolstations cheapest saw blades and only use that saw to cut the boards (good mask too) as the cement dust will destroy the bearings in anything else. Then chuck the saw when you are done ..!

[Pete]

Hi Ian

I have used moisture resistant render board from Berry's at Leyland. 2.4m x 1.2m which is 12mm thick. Ask to speak to Jonathan Aspin and if you open an account you get free delivery. Be warned they are quite heavy!!

[Russell griffiths]

Ok couple of points

do you have to supply fire proofing if there is no room above?

have you not got a plasterboard ceiling? Are the joists exposed 

 

that company i stated Medite smart ply, they do a fire resisting board used for racking and sarking. 

 

 

[Russell griffiths]

https://mdfosb.com/en

 

see if this works I’m a complete Luddite when it comes to computers 

[HerbJ]

I'm with @Russell griffiths on this - what is the reason for fireproofing the roof? Is it something to do with the wooden roof shakes?  Is it necessary, if you have a plasterboard interior  lining?

[ToughButterCup]

@HerbJ, @Russell griffiths, @Alexphd1, @Pete; it's a matter of fireproofing. 

 

We are having shakes for the final roof covering:  (not the shakes ! ) and they will be treated with Non-Com  Exterior fireproofing. From the email;

' ....

Just to confirm that [the BCO ] has looked through the Non-Com Exterior fire certification and has confirmed that the use of it on the shakes/shingles along with 6mm fibre cement board would be satisfactory for for fire resistance.

...'

 

Ours is an open, vaulted roof. The space between the rafters  ( they are 9 by 2s ) is fully filled with (yet to be decided) and underlaid by a further 82.5mm of insulation, and then skimmed to finish.

 

The thing is: given that we need to put cement board just under the counter battens, aren't we going to have to seal the gaps between the cement board ?

 

[Mr Punter]

3 minutes ago, recoveringacademic said:

they will be treated with Non-Com  Exterior fireproofing

 

Have you had a quote for this?  Might be cheaper to have natural slate.

[Ian]

1 hour ago, recoveringacademic said:

aren't we going to have to seal the gaps between the cement board ?

 

If you want the cement board layer to act as a fire barrier then, yes, any gaps will need sealing with a fire rated material.

[ToughButterCup]

6 hours ago, Ian said:

If you want the cement board layer to act as a fire barrier then, yes, any gaps will need sealing with a fire rated material.

 

Oh foxtrot. 

[Russell griffiths]

Still confused. 

If you have plasterboard and skim then isn’t that your fire proof layer. 

If it is because you are using timber shakes, then what are they saying the fire will start from outside and it is to stop the spread indoors. 

 

This is is hard to believe, you don’t live in Australia or California. 

 

Have you priced this cement board up?

it generally comes in very small sheets so to cover a roof area could be very expensive. 

 

I think i would definitely look at some of the fire resistant osb. 

[Ian]

@recoveringacademic

just a few thoughts about your roof spec:

- unless your house is very close to the boundary with one of your neighbours there’s no requirement in the Building Regs for you to have a fire resistant roof (just think of all those flat roofs finished in bitumen felt!)

- if you’ve decided for personal reasons that you want the roof to be fire resistant then a more practical solution would be to ditch the OSB and the cement board and replace them both with a single layer of metal roofing sheet. The cheapest wriggly steel roof sheet you can find will do as you won’t be able to see it. It will need battens under and counterbattening over etc to provide an air gap to prevent condensation forming on the u/s. The cedar shakes can go on top to hide the metal.