Ply subfloor questions

[RF Lighting]

Evening all.

I don't know if you remember, but a little while bick I was asking about the best subfloor for tiling onto for the toilets at my local scout hut.

I was advised to get 25mm WBP ply, so that's what I've gone and got.

Now, while reading the forum the other day, I saw something about priming the under side and edges of the ply before fitting it.

Mine will be fitted on to joists about 600mm above the concrete in a ventilated floor void.

Is this the sort of situation which needs primer, and if so, what should I use?

I saw a link to some bal primer. Is this the best stuff, or would normal wood primer do the job?

Also, one other thing. The ply is to be screwed to the joists every 100mm. (the joists are on 400mm centers) What size screws should I be using? I think I have seen to use 2" screws, but that would only leave 1" to fix into the joist, which doesn't seem much to me.

Will no 8 srews be ok, or should I use no 10?

Thanks very much.

 

[gcol]

You'll see it recommended to seal the backs/sides of ply on instructions for adhesive, but to be honest, it's not often done. It's basically a barrier to reduce the amount of moisture that can be readily absorbed into the ply. Fence stain will do at a push!
Screw down every 250mm along the joists with 8's or every 300mm with 10's - your choice. You'll really need a decent drill/driver to drive in 10's through 25mm ply though. Either way, use 2 1/2" long screws.
Just a tip.... stick a small amount of tape over each screw head so that if you ever need to remove the ply in future, the screw heads will not be filled with adhesive and will be easy to remove.

 

[Richard C]

Priming/damp proofing is recommended by adhesive manufacturers &, as gcol says, is to prevent any risk of moisture absorption curling the ply from the back & cracking the upper tile surface.

Agree it's not usually necessary if it’s a suspended floor in the 2nd story of a well heated & dry residential property but in the case of an infrequent use, Scout hut sub-floor over an externally ventilated & potentially damp concrete base/floor void I would recommend it.

 

[RF Lighting]

Right oh. I'll set to it with some primer undercoat

Thanks again chaps

 

[RF Lighting]

Evening chaps.

Just thought I'd check in with you and make sure I'm doing everything right.

I took up the old floor, and the joists didn't really look up to much to me.

I've beefed them up with some noggins, and also put in extra joists for where the sheets of ply will meet.

Put down the ply. I've painted the underneath and edges.

Then screwed it down. I might have got a bit carried away here, but I couldn't remember what spacings were recommended

I've screwed it down every 200mm.

Oh and I got mixed up buying screws in metric, so ended up using 5 x 90mm screws

 

[regsmyth]

Give yourself a pat on the back, nice tidy job.

You did put some preservative on the noggins and joists?

 

[jefoss]

Give yourself a pat on the back, nice tidy job.

You did put some preservative on the noggins and joists?

im not sure strawberry jam is going to help here.

 

[RF Lighting]

Preserve surely?

I've not done anything to the joists. They have been fine for 30 odd years, so I'll take my chances

 

[gcol]

Looks good so far.

 

[regsmyth]

Well maybe I'm overly cautious but I would have put some wood preservative on the new timber, just in case....
After all, that's the sort of environment that woodworm probably fantasize about - slightly damp, poorly ventilated, and untreated timber.

And you don't know that the existing timber wasn't treated originally, which is maybe why it's been fine for 30 years.

So if anyone else reads this, I would advise always treating timber in locations like this.

I mean, it wouldn't take long to whip out those few screws would it??