Plywood for lining internal walls in timber frame

[Hannah82w]

Hello, we've built a timber frame single storey house, watertight and screed poured, so now looking to start lining the walls - internal studwork, internal load bearing walls and the inside of the external walls. The technical spec just lists '9mm ply' or '18mm WBP in the bathrooms about pan, cistern and wash basins' (with vapour barrier under the ply on the external walls, and 12.5mm plasterboard over the ply on all walls). We want to use 12mm ply throughout for extra depth to aid hanging things on the walls, and don't plan to have wall fixed basins etc (will opt for free-standing).

 

I've just rung round the various builders merchants and timber yards and got differing advice on what to use!

 

Is 12mm softwood ply acceptable in all cases except the bathrooms where we could use hardwood instead (for its more waterproof properties)?

 

Thanks

[Bozza]

I think the requirement for the 18mm in bathrooms is for future handrails / grab rails  / disability assess accessories, not for hanging basins etc.  Certainly on my drawings (TF build ) it specifies accessible bathroom walls to be of substantial construction namely 18mm ply then 12.5mm plasterboard.  But I’m in Scotland so may be different down south.  
 

there was no mention of using WBP though.    You may need to check with regs, your architect and/or building control.
 

 

[Hannah82w]

13 minutes ago, Bozza said:

I think the requirement for the 18mm in bathrooms is for future handrails / grab rails  / disability assess accessories, not for hanging basins etc.  Certainly on my drawings (TF build ) it specifies accessible bathroom walls to be of substantial construction namely 18mm ply then 12.5mm plasterboard.  But I’m in Scotland so may be different down south.  
 

there was no mention of using WBP though.    You may need to check with regs, your architect and/or building control.
 

 

Ah ok, thank you. I will take a look at the regs. I'm told WBP is an old term, but essentially is the current hardwood that is suitable for exterior use, so some water resistance.

[ProDave]

If the plywood is getting wet behind the plasterboard and other wall finishings, there is something seriously wrong.

 

Check the boarding is not for aiding the racking strength of the structure.

[Bozza]

44 minutes ago, Hannah82w said:

Ah ok, thank you. I will take a look at the regs. I'm told WBP is an old term, but essentially is the current hardwood that is suitable for exterior use, so some water resistance.

Yeah I was surprised it needs exterior grade board, as per what @ProDave has said.  He’s also picked up a on valid point regarding structural strength.  I would not downgrade any sizing as it must have been specified for a reason, but note you were increasing from 9 to 12 you just need to watch out because some ply is structural and some is not.   I would just also think about clear benefits in kitchen of being able to attach wall units etc easily you may wish to consider 18mm on the relevant walls.  

[Mike]

3 hours ago, Hannah82w said:

Is 12mm softwood ply acceptable in all cases except the bathrooms where we could use hardwood instead (for its more waterproof properties)?

Simply, plywood comes in Classes I (dry use), II (humid / occasionally wet) and III (unprotected externally / frequently wet). The old WBP would now be Class III - it's been that way for years, so someone must have cut and pasted a very old spec.

 

Personally I'd choose 12mm Class I everywhere, since ventilation & tiling should stop the ply from becoming humid / wet in bathrooms, though you could upgrade to Class II.

 

Edited December 12, 2023 by Mike

[Hannah82w]

1 hour ago, ProDave said:

Check the boarding is not for aiding the racking strength of the structure.

Thanks, I'll check (and find out what that is!)

[Hannah82w]

48 minutes ago, Mike said:

Simply, plywood comes in Classes I (dry use), II (humid / occasionally wet) and III (unprotected externally / frequently wet). The old WBP would now be Class III - it's been that way for years, so someone must have cut and pasted a very old spec.

 

Personally I'd choose 12mm Class I everywhere, since ventilation & tiling should stop the ply from becoming humid / wet in bathrooms, though you could upgrade to Class II.

 

Thanks, so class I is softwood?

[SimonD]

Most plywood you get from merchants is going to be Class II with some of them Class 3 - e.g even B & Q ply is class III. Softwood can be class II or class III as it depends on the glue. I would suggest you go for minimum Class II which is for humid conditions but it doesn't matter if it's class III they stock. My local supplier doesn't even stock class I but has structural 18mm Elliotis Pine Class III at £27 per sheet inc VAT and £20 for the 12mm, with this minor price difference I'd go for 18mm as then you know it's got the screw fixing capacity for almost everything.

[Alan Ambrose]

I think the 18mm in the bathroom is in case someone decides to hang wall mounted loos etc at a later date. Personally, I would use 'class III' everywhere if the price difference isn't much and definitely in the bathrooms to avoid a small leak doing a bunch of damage. It might be worth actually seeing/testing the product before you buy - plywood quality has gone way down over the years and has a great variation in quality whatever what the sellers call it in their listings. I used some 'class III' in the spring for a temporary outside job and gave it a couple of coats of varnish to help it along - it is already bowing and delaminating. 

[Hannah82w]

Thanks all for your thoughts and advice, very helpful. I haven't actually seen class I, II or III on any descriptions of ply products!

[Temp]

22 hours ago, Hannah82w said:

Ah ok, thank you. I will take a look at the regs. I'm told WBP is an old term, but essentially is the current hardwood that is suitable for exterior use, so some water resistance.

 

WPB is available in various grades ranging from absolute cr&p to the best quality boat building marine plywood. Some of the big timber stores sell more than one grade at different prices for the same thickness. Might be worth asking around.

 

I bought some that claimed to be WPB from a big store beginning with W and it delaminated quickly.

 

I suspect there is probably a CE, EN or  UKCA standard that the best grades should meet but I haven't bought any for awhile. 

Plywood is sometimes specified to prevent racking so check with the designer before deleting any.