under-tile ply minimum thickness

[Jimbohotpants]

Hi all,

I'm doing a spot of DIY in the bathroom and need a bit of reassurance: Is 18mm ply strong enough for tiles over 2" thick joists spaced 360mm between centres? The tiles will be 300x600x10mm and there'll be a layer of decoupling membrane between them and the ply.

Cheers,

James

 

[Ian]

You say 2", I assume that's the width, how deep are the joists?

 

[Jimbohotpants]

Hi, yeah that's right, 2" wide x 4" deep. The room's 2.4m along the length of the joists and 2.7m across.

 

[Ian]

I'd be adding some extra noggins while you've got the floor back to the joists, it'll give you some extra strength and some more fixing points. Also, what tiles are you fitting? If you can afford the extra height, use 25mm ply. The tile type may change my mind on your plans, I'll await your reply.

 

[Jimbohotpants]

The tiles are porcelain, 600x300x10mm. I can't really deal with any extra height without causing problems with the carpet/tile transition.

 

[Ian]

Ok, with a porcelain tile I'd go with, extra noggins, 18mm ply, 6mm cement board glued and screwed to the ply, then tile. The decoupler is not necessary for your tiles IMO.

 

[charlie1]

James, are you sure the joists are only that deep?? You may kind the real joists underneath the ones you can see. Have another check mate, my daughters dolls house has thicker joists lol... (joking face) in all seriousness, do have a check mate.

 

[DHTiling]

4 inch depth of joist.. ? .. have you tested for joist deflection and accounted for the extra weight.. ?

 

[beanz]

4" does seem very shallow!?! Wouldn't be putting a bath on those lol!

 

[Jimbohotpants]

I'll double check but I'm pretty sure the joists are only that big. I put some spotlights in the kitchen below and they nearly didn't fit; they're only 100mm deep! The rest of the house is different but this bit is the typical Victorian T extension to a terrace house.
I haven't tested for deflection (how do I do that?) but there is already a steel/enamelled bath in there so I think the floor will take the weight of a plastic one and tiles (I bloomin hope so!).
assuming the joists are sound, will it be ok to just use 18mm ply with noggins etc? I don't really fancy the 6mm boards if I can help it. What do you think?

 

[DHTiling]

22mm would be better and nogged at 300mm intervals along the joist length for fixing supports.