uneven joists

[dotto]

Hi all.
I will be helping a friend doing his bathroom.
1) would you recommend to use two sheets of 12.5 mm. of exterior ply on the top of each other for the floor before tiling or a single sheet of 25mm ply?
2) What would you advice if the joists are not even and therefore the ply once screwed down every 200 mm. would be bumpy?
3) The bathroom has two studs walls - if one of the stud walls is not resting on a joists, and therefore i would not be able to screw the ply down, how could i by-pass that problem?
Apologies for the so many questions, but i am not a tiler and i am just trying to anticipate a few problems that i may encounter. Thank you for your time.

 

[Symptoms]

dotto - don't use the thin ply. Once the old floor deck has been removed survey the joists (use a long straight-edge) for any bowing. Anything high (bumps) can be planed off ... WARNING - only small slithers can be removed, maybe no more than 5mm, otherwise you risk weakening the structure. Anything low will have to be packed-out. Using thick ply will prevent your bumps. We usually put 18mm ply down (ample enough in most situations) but if you're not concerned about levels to adjoining floors, and the joists are widely spaced then use thicker ply.

Where the ply edge is un-supported next to one of your walls introduce some additional 75mm x 50mm (at least) structural timbers (noggins) screwed into the existing stuff.

 

[dotto]

dotto - don't use the thin ply. Once the old floor deck has been removed survey the joists (use a long straight-edge) for any bowing. Anything high (bumps) can be planed off ... WARNING - only small slithers can be removed, maybe no more than 5mm, otherwise you risk weakening the structure. Anything low will have to be packed-out. Using thick ply will prevent your bumps. We usually put 18mm ply down (ample enough in most situations) but if you're not concerned about levels to adjoining floors, and the joists are widely spaced then use thicker ply.

Where the ply edge is un-supported next to one of your walls introduce some additional 75mm x 50mm (at least) structural timbers (noggins) screwed into the existing stuff.

Thanks 4 your advice Symptoms, so you would opt fora one sheet of 25 mm.ply instead of two sheets of 12.5 mm. each!?

 

[Symptoms]

dotto - one sheet of 25 or 18. Forget the 12½.

 

[dotto]

dotto - one sheet of 25 or 18. Forget the 12½.

Thanks 4 your time