minimum thickness of ply subfloor

11 Jan 2010
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United Kingdom
I'm trying to find out what the minimum sensible thickness of plywood i can use to lay over our joists. Its an old house and the joists centres vary from 380 - 400 (most less than 400). The joists are 45mm x 160mm, and there are noggins dotted around the place. Most joists are pretty stable, but there is the odd one that flexes a bit. We've added extra noggins to this.
Next step is to lay the subfloor directly onto the joists, but we have very limited floor to ceiling heights so I'm trying to find out what the minimum thickness plywood I can use for subfloor. The final floor will be a floating bamboo floor (10mm).
Is 18mm WPB plywood required, or can i get away with thinner sheets, e.g. 15 mm?

Many thanks
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The absolute minimum thickness you can use on 400mm/16in centres is 18mm. Never heard of flooring being done with anything thinner. Don't skimp on the sub-floor - it's the subfloor which carries the load, not whatever fancy stuff you stick on top of it.

Most joists are pretty stable, but there is the odd one that flexes a bit. We've added extra noggins to this.
Noggin? By that do you mean that you've added solid strutting between the joists, or do you mean that you've sistered the joist? If the joists are flexing, you need to consider sistering them, if possible with deeper material, because bowing that way tends to indicate that the joists are undersized for the span. TBH 160mm seems a bit on the small size to me for any sort of joisting and should only support a span of about 3.2 metres (10-1/2 feet) assuming regularised C16 timbers. So what is the span? Asking this because you may need to look at this a whole different way
The joists spans are around 3 - 3.2 m. We've added solid strutting between joists, not sistered the joists.

Almost everywhere I've read talks about 18 mm ply or chipboard for standard subfloor over joists. However, if 18 mm chipboard is regarded as sufficient, and hardwood ply is stronger than chipboard, then would 15 mm hardwood ply be strong enough to use as a subfloor?

I can't find any scientific or structural analysis that shows why 18 mm is the industry standard. Is it just a case of 'that's what we've always done'. I've seen documents from other countries that say 12 mm ply wood is ok to use over 400 mm centre joists. I'm not suggesting I would use 12 mm ply, but i do wonder if 15mm ply is sufficient.
is 3mm going to make any difference to your loss of height?

18mm is such a common thickness its often cheaper than 15mm
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Yes, that's a good point. But every mm counts, and I didn't want to just waste 3 mm if i can use 15mm rather than 18mm.
15mm ply will flex, have it in my loft and moving around can feel it give underfoot.
Thanks for the info and advice.
I've opted for the 18 mm hardwood structural ply from Travis Perkins. I guess I'll just have to duck that extra 3 mm!

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