Fixing 18mm plywood into floor joists - what size/type screws?

[Deleted member 174758]

I previously used the T&G but it was a much rougher grade of ply.

T&G plywood is generally rotary-cut spruce (softwood) plywood and is not meant to be a "visible" grade. In one respect it is a bit like the T&G chipboard he so dislike - it is designed to be glued at the tongues and glued to the joists. Without that it isn't completely stable. Did you glue yours?

Hardwood plywood is a lot harder and stiffer and if a flooring grade doesn't normally need any support down the long edges, after all, you don't do that with square edged softwood planking, do you?

 

[JohnD]

T&G plywood is generally rotary-cut spruce (softwood) plywood and is not meant to be a "visible" grade. In one respect it is a bit like the T&G chipboard he so dislike - it is designed to be glued at the tongues and glued to the joists. Without that it isn't completely stable. Did you glue yours?

No, I pulled it up and built a shed with it

 

[Deleted member 174758]

The manufacturers of spruce ply generally recommend gluing it down. It gets used a bit to build multiple skin floors in public buildings (as a means to reduce the carbon footprint), so 3 to 4 layers with the nailing pattern from hell is not unknown

 

[JohnD]

I don't do much woodwork so I may not have found the ideal board yet

 

[^woody^]

18mm ply will require joist spacing of about 300mm and lots of noggins, else it will flex.

 

[Deleted member 174758]

So 18mm chipboard onto joists at 400mm centres will be even worse, then? By your inference almost every house built since the 1960s was built wrong and they all have flexy floors...

 

[^woody^]

You may be on to something about every house built since the 60's being built wrong, but I would not make such a bold assertion.

However, chipboard is more rigid than ply and less prone to permanently cup, so much better as a floor deck, and the OP is not even using T&G sheets is he?

Ply is a poor choice for flooring. Has the OP stated why it's being used?

 

[Deleted member 174758]

However, chipboard is more rigid than ply and less prone to permanently cup, so much better as a floor deck, and the OP is not even using T&G sheets is he?

I'd say that depends on the plywood, although admittedly some of the Chinese stuff I've been supplied with in recent years has been horrendous. So maybe go back to the same technique that we use on chipboard flooring, i.e. glue to the top of the joists (I'll probably have to duck for saying that... OP, by "glue" I mean a low foaming D4). As to T&G sheet ply, I've only ever dealt with T&G spruce ply which isn't an off the shelf product from your average builders' merchant - I can't recall ever seeing T&G hardwood ply (spruce is a softwood); it's always been a case of get the workshop to supply it (and narrow rips of 6mm ply as loose tongues) or machine it on site with a 1/4in groover in a router, then make your own rips from 1/4in ply. These loose tongues get glued in (and I did touch on this earlier)

Ply is a poor choice for flooring. Has the OP stated why it's being used?

But is it? It has its' uses - you can't make diapragm floors from chipboard, plywood is a far better proposition for high load floors (e.g public buildings), plywood is far better if you are screwing tit (e.g stud walls), etc. You are painting it as all bad whereas that just isn't so, although granted my own preference genuinely is for P5 T&G chip in domestics (glued and screwed)

 

[^woody^]

But is it? It has its' uses - you can't make diapragm floors from chipboard, plywood is a far better proposition for high load floors (e.g public buildings), plywood is far better if you are screwing tit (e.g stud walls), etc. You are painting it as all bad whereas that just isn't so, although granted my own preference genuinely is for P5 T&G chip in domestics (glued and screwed)

If using ply, it's only good if it's a specialist designed floor, for a specific purpose or temporary. Ply is no good in the context of the OPs use or domestic floors in general.

 

[akist]

It is a standard loft floor, made of 225x50 joists spanning from 2.8m to 4.5m, in between walls and steel beams. We have placed the joists 400mm apart at the shorter lengths and 350mm apart at the longer lengths. The area is about 250m2.

We have used "structural hardwood" 18mm plywood.

We laid the first two areas today the carpenter used only screws on every section, no foam or other glue.

The partition walls have been set out first, so in theory, in the future we will be able to lift a board by just taking out the screws (excluding skirting boards etc).

Between the 3 available materials, chipboard, "structural OSB 3" and structural ply, the ply and then the OSB are the more robust. In cases of extreme conditions, eg laid on the ground, in rain and with lorries and a forklift driving over, the plywood is by far superior. There are about 60 structural OSB boards upstairs used as temporary flooring, for about a year, exposed to rain and sun for about 8 months, some look to be in good shape, some are coming apart. I would not use either ply or OSB that's been in the rain for months, but the ply is still together structurally whereas the OSB cracks and pieces are coming off.

Some people have warned me that some plywood manufacturers use low quality glues and they delaminate with time. My most recent batch came from China. I hope they used good quality glues to make them!