Plywood over T&G "Slip Layer"???

[glock339]

Hi I'm about to over board an old battered T&G floor that I have re-patched which is still a bit bashed & dinted etc. It's a complete house renovation so can use pretty much any thickness ply as no need to match any other heights so was thinking maybe 12mm or so ply?

As well as fixing the ply to the T&G I'd usually screw through to the joists for extra strength, however I've seen on a few U.S sites they recommend you ONLY screw to the T&G to create some kind of slip layer??? I've never heard of this before so wondering can anyone shed any light on the subject?

Heres a few quotes I've seen on the U.S sites:



"You will fasten the planks with screws to the joists, then fasten the plywood to the planks not into the joists. I know it sounds goofy, but in reality the plywood will "slip", and you are allowing it to do so by not putting it firmly into the joists. Your planks become part of the joists, and the plywood becomes part of the planks."

"With two layers of subflooring, they have to be given the opportunity to "slip" against each other, although, granted, this slipping is minimal. Fixing both layers to the joists prevents any possible movement in the top layer."

 

[Deleted member 174758]

That's at odds with what structural engineers sometimes require you to do here when laying a diaphragm floor in old buildings (basically two or so layers of thinner, 8 to 12mm, plywood laid on top of an existing planked floor) where it is important that you nail through into the joists on at least the first layer. The idea is that the diaphragm stiffens and strengthens the existing structure. We are often instructed to use 75mm ring nails on the first layer (going through softwood sub flooring between 25 and 50mm thick) then use 50mm rings on the second layer. All on 50 to 75mm centres (3 or 4 rows over each joist). The sheets are offset by half a sheet between rows to stagger the joints (stronger, better draught proofing and better for minimising noise transmission). I can say from experience that done correctly this really does produce a much stiffer floor than a single layer of 18 to 22mm ply would, although it is more labour intensive. It's also worth mentioning that there are good reasons why nails are used for this sort of task - nails will allow a certain amount of flexing in the floor structure without failure, whereas screws, being hardened and therefore much more brittle, won't allow for as much natural movement and are far more prone to failure under shock/collapse loadings

I don't think it is always a good idea to source construction solutions from the USA. Their building codes are markedly different from our regulations in many areas (e.g. stair construction, etc), so techniques which "work" on an American timber framed house (as many of them are) won't necessarily be suited to a British masonry built house with a suspended timber floor

 

[foxhole]

US properties often collapse with a stiff wind.

 

[glock339]

Cheers for the replies!

I wasn't really trying to source solutions from US sites but Google seems to bring up loads of US sites even after selecting "UK only" in the search the options so I always end up seeing a few weird ideas & was just curious if anyone was familiar with this one as it had me baffled!