Screws needed to screw 25mm wbp plywood into joists

[aled]

Hi,
I am removing floorboards in my bathroom and replacing them with 25mm wbp plywood which i will be tiling on. I know I need to screw the plywood down every 150mm with countersunk screws. What screws would you advise I use? As I will be screwing into the joists I am not sure what length screws I need. I have had a look at the Screwfix website and there are so many different types of screws that to me look exactly the same!! Any advice would be great.

Thanks in advance

[Alan.P]

Any galvanised screws will do really, I like the double threaded gold variety from screwfix, you'll need something of 40mm + I would be tempted to go 45mm, don't forget to prime the underside and edges of the ply with an sbr primer.

[aph257]

as above

[Wood's better]

The old guideline is 2 and a half the size of the thickness of material. Therefore, a 25mm plywood floor should have a 60mm (approx) screw.

I use 4.5 x 60mm spax flooring screws. They are not full thread as fully threaded screws often lead to a tiny separation between the joist and floor leading to squeeks. These are available from Screwfix part no 56431.

Although it says no pilot hole required I would always pilot with a 3mm drill to the full depth of the screw and a slightly shallower countersink than required (especially in hardwood ply)

Hope it helps.

Richard.

[cornish_crofter]

As Richard says, 60mm is good for the job in question, though 50mm would do the job.

Although the Spax are good, any decent screw that is not full threaded will do the job. If you can only get the full threaded ones, then just make the pilot hole a bit bigger so the thread is not binding on it.

[aled]

Thanks everyone. I think i will go for the screws as mentioned by Ruchard above.

[aled]

Hi,
I went to ScrewFix today to purchase 4.5 x 60mm spax flooring screws (part number 56431) and was told that they were no longer in stock and that they could not be ordered in. Could you please advise on other screws found at ScrewFix that could be used to do the job?

Thanks

[devonmark]

these are a good screw, self cutting counter sink, I have used them many times


Toolstation > Screws & Fixings > Torx Fast

[aled]

I was told this morning that I could use the following screws to carry out the job in question. Spax Decking Screws 4.5 x 60mm (96947) As they are outdoor decking screws would they be suitable for screwing 25mm wbp plywood into joists?

Thanks

[aled]

I have also seen these at ScrewFix Spax Flooring Screw Zinc & Yellow Passivated 4.5 X 60mm Pack of 300 (88716). WHat do you think about these?

[Scott]

Any of those will be fine, they dont need to be zinc coated or deck screws. Its a bathroom floor thats covered with tiles and wont be getting constantly wet. If they get wet enough to corrode that much then you will have more serious issues to worry about than zinc or galvanised screws

[Wood's better]

Disagree, it is good practice to use zinc plated screws. Most of them are anyway! The powdered adhesive, being cement based can cause corrosion.

[Scott]

Corrosion to what end? A screw head that may rust but isnt exposed to moisture or air but is encapsulated in adhesive or slc. It would take forever, literally, for it to fail. I have removed steel cistern screws that have rusted from being in damp conditions for 20+ years but they are still holding the cistern to the wall

[doug boardley]

whichever you use, be careful of services, pipes, cables etc!

[deanotile]

The old guideline is 2 and a half the size of the thickness of material. Therefore, a 25mm plywood floor should have a 60mm (approx) screw.
I use 4.5 x 60mm spax flooring screws. They are not full thread as fully threaded screws often lead to a tiny separation between the joist and floor leading to squeeks. These are available from Screwfix part no 56431.
Although it says no pilot hole required I would always pilot with a 3mm drill to the full depth of the screw and a slightly shallower countersink than required (especially in hardwood ply)
Hope it helps.
Richard.

as long as we are talking guidelines
bs
BS5385
The use of sheets and boards that are subject to movement from changes in moisture content, e.g. wood-based materials such as plywood, chipboard, wood particle boards, etc. should be avoided if at all possible. If such boards have to be used they should be restricted to small areas and tiles should not bridge joints between boards. It is good practice to seal all exposed edges and the backs, but not the faces, of such boards with a suitable sealer to prevent distortion by atmospheric humidity changes. Care should be taken to ensure such boards are not installed in a condition where their moisture content is higher than the ambient equilibrium moisture content once the tiled installation is in use. Failure to observe this can lead to subsequent warping and distortion of the boards with consequent cracking and delamination of the tiling.
so you may want to consider the use of a tile backer board or an uncoupling membrane.

[TF Ed]

you need 60mm screws on 25mm ply. they need to have some decent bite into the joists.
the screws should be corrosion resistant too..

galvanised screws are not seen as a good option anymore due to the fact that the zinc can break down..the whole building industry changed its format on galvanised materials from about 1990.. angle beads, strap hangers, fixing, screws, cavity wall ties all were recomended to be stainless steel or corrosion resistant in exterior or moisture sensitive areas.

[Wood's better]

Here what your saying, then with your explaination, why not use ordinary screws to fit Hardi and not very expensive ceramic coated screws?

[Scott]

I use drywall for hardi 12.5mm and have never used hardibacker on floors.

[Wood's better]

Dean,

All very good quoting this and is all true. However, the origianal post says that the origianl boards are being lifted and replaced with 25mm ply. Surely this is very good practice?

The ideal here would be to then overboard with 6mm hardi do you not agree?

R