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

[akist]

For fixing 18mm plywood sheets over the joists, I am thinking of using 4.5 x 60mm or 4.5 x 70mm ?

Like these

Screw-Tite PZ Countersunk Screws 4.5 x 60mm 200 Pack

Order online at Screwfix.com. Tri-Lock TS threadform, PZ recess, high performance, multipurpose screw. FREE next day delivery available, free collection in 5 minutes.

www.screwfix.com

or

Timco Velocity PZ Countersunk Multi-Use Screws 4.5 x 70mm 200 Pack

Order online at Screwfix.com. Premium countersunk screws, with twisted shank and low friction coating for rapid drive-in. Suitable for use in thin sheet metal, PVCu, MDF, chipboard, softwood and hardwood. FREE next day delivery available, free collection in 5 minutes.

www.screwfix.com

Or do I need 5x90mm like these

Timco C2 Strong-Fix PZ Countersunk Multipurpose Premium Screws 5.0 x 90mm 325 Pack

Order online at Screwfix.com. Multi-material screw with twin-cut technology. Ideal for applications when optimum holding force is required, such as hinges, brackets and high load environments. Features deep recess to prevent cam-out, giving precise and positive drive control. Under-head ribs...

www.screwfix.com

I do not know what is more appropriate,

 

[big-all]

2'5x material thickness so 45mm minimum but no more than 70mm as you will encroach into the 50mm safe zone if naked joists

 

[Deleted member 174758]

Generally for 18mm flooring we use something like 4 x 50mm collated screws, or sometimes 4.5 x 50mms. They are more than adequate when driven flush. You can use 60 or 70mm screws, but it will cost you more, take longer, you'll be recharging your cirdless more often and most importantly it won't deliver any advantages over using 50mm screws.

Make sure that you sink the screw heads flush or just below the surface and ensure that the board doesn't "jack up" off the joist below - I find that putting my full body weight on the impact driver and running it at the fastest possible speed (2500 to 3500rpm) does the trick (my corded collated gun runs at 4500rpm). If it does jack up, back the screw out, then drive it in again

 

[akist]

OK thanks for the advice, will put some orders in now.

How many screws? Every foot or so for example? Or every two feet?

 

[Deleted member 174758]

300mm maximum centres.

 

[akist]

OK thanks!

How do go about the short edges that straddle the joists - every 400mm or 350mm in places. Do we need to add noggins below? Do we just ignore because the gaps are only 350-400mm? Do we glue the edges together but with nothing underneath?

My personal preference would be that a noggin is placed where we have an edge of the plywood board above, but the carpenter may think it is superfluous and waste of time?

 

[Deleted member 174758]

How do go about the short edges that straddle the joists - every 400mm or 350mm in places. Do we need to add noggins below? Do we just ignore because the gaps are only 350-400mm? Do we glue the edges together but with nothing underneath?

Two ways to do this - trim the board to length so it ends on a joist centre (requires a circular saw and a home-made straight.edge guide, or a plunge saw and a rail), or support the board ends by adding 3 x 2in or 4 x 2in noggins at something like 400mm centres between the two joists where board ends end up.unsupported by a joist. I favour the former as it makes for a more stable floor (BTW I'm a joiner, and I wouldn't let anyone working for me do it any other way)

Depending on glueing unsupported ends is downright foolish - the joint won't be that good to start with and will fail in short order. Don't do it!

My personal preference would be that a noggin is placed where we have an edge of the plywood board above, but the carpenter may think it is superfluous and waste of time?

If a carpentet does that, then you need another carpenter!

 

[akist]

Thank you, hopefully he will agree to adding noggins where needed.

 

[Deleted member 174758]

As I said, you can also trim the plywood. Uses a bit more plywood, but takes less time to do and is a stronger, more stable job

 

[akist]

As I said, you can also trim the plywood.

Two ways to do this - trim the board to length so it ends on a joist centre (

You cannot trim the ply across the joists, you can only trim it in the direction of the joists so that it lands on a joist. But in the other direction you will be across the joists and the only solution there is to insert noggins in between the joists right where the ply ends.

Am I not understanding this right?

 

[Deleted member 174758]

Am I not understanding this right?

No, you are not. The plywood gets laid with its' longer side perpendicular to the joists. You'd trim across the ends of the boards, the cut being in the same direction as the joists,in other words across the width of the sheet (a 1220mm cut). If you end up with up to about a 25mm overhang it is possible to side fix a piece of 2 x 2in or 3 x 2in softwood to the side of a joist to carry the end of the next board board. Saves making a amall cut

 

[akist]

Ah I see. And what do I do about the longer side which is across the joists+gaps? Just leave it as it is? I made a sketch and drew arrows where some of the gaps are.

 

[Deleted member 174758]

Just leave it as it is. It will be fine.

On heavy duty jobs like restaurants I've been supplied with boards which were edge grooved (and sometimes even grooved then myself on site) to take a loose tongue (1/4in or 6mm ply), ⁰which would be glued in, but then a commercial floor needs to take a lot more loading than a domestic one, and for a house it isn't really necessary

 

[akist]

OK, thank you very much.

 

[JohnD]

If, like mine, it is 18mm ply and not T&G, I recommend putting a strut ("noggin") under all unsupported edges.

Or there will be a dip when you walk on it.

I can feel them with my feet.

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