Bathroom floor, From Joists upwards.

[dilz000]

Guys Im redoing a bathroom floor as the tiles that were down were coming up on there own, turned out it had been tiled straight onto the floorboards by the previous tiler. I told the client that the floorboards needed to come up as most of them were water damaged and twisted, so now I have bare joists.

My plan is to brace the joists by adding plenty of noggins made from 3x2 timber.
I then plan to lay 18mm WBP ply, and then 6mm cement board, and then tile.

However I have a few questions...

does the above sound ok?
how far apart do i place the nogins
do i need to seal the underside and edges of the ply even if its not being tiled straight onto?

Cheers

Chris

[faithhealer]

that sounds ok Chris, Noggin wise, don't think there is a rule for that, more the merrier obviously. and yes prime underside of ply. If you could get thicker ply that would be even better

[aph257]

One thing John, regarding noggins, are they suppossed to be in line with each other?

[Alberta Stone]

The crossbracing should be at least 2 X 6 and you can put them staggered slightly-1-1/2inch (-_-_-_-_)as that makes it easy to get a screw into each end.
You need to assess the situation when putting them in, but more are always better. I space them at 4 ft typically or even 2 ft if the situation needs it.
Why would you prime the underside of the plywood, I have never seen this done....ever.
The top is primed.
Use exterior grade 3/4 inch plywood (using subfloor adhesive) and sheath again with 3/8 inch ext grade ply (using more subfloor adhesive) to get your 1-1/8 inch minimum (prime that surface) and then use cement board or ditra and tile on that.

[tfs]

I would preffer to use noggins the same thickness as the existing joists but, have seen peeps use thiner with no problems.

Some peeps preffer to prime back and sides of ply to prevent moisture or temp changes etc affecting the ply. BAL reccomend this practice (or maybe ditra. Cant remember) some guys do it, others dont.

The more preventative measures taken the better. In terms of quantity of noggins I would aim to have them at 1000mm - 1200mm centres. Staggering would be my prefference and most common. I would advise you to space noggins to suit your ply if possible so that the each end of the length of ply falls onto the centre of the noggin with a noggin in the centre also. This should give you a solid floor.

I would use the thickest ply possible, 18mm would be the absoluate minimum, if it was my decision I would go for 24mm ply unless you intend to sheet with backer board.

best of luck

[Scott]

I would use noggins the same size as the exisitng joist and stitch them in a line. What size joists are they and what spacing are they?

I would use 18mm BUT i would have to satisfied that the floor was solid enough.

[Alberta Stone]

The Tile Association of North America recommends a minimum of 1 and 1/8th inch plywood in 2 layers upon which you can commence to build your floor, meaning then adding concrete backer or ditra or just tile.
That would be either 3/4 + 3/8 or 5/8 + 1/2 and this is the minimum recommended thickness of wood ply over wood floor joists either 2 X 12 or silent truss system.
You may use crossbracing the same thickness as the joist, and this is the best to keep the joist from twisting over time, but 2 X 6 is sufficient if installed properly with screws (4 inch, #10 deck or wood screw) and PL premium (forget the nails). One has to consider what is running under the floor either ducts or gas pipes or electric cables, water lines waste pipes, etc, as well as then holes will have to be drilled through the crossbracing. You can put a piece on the top and another on the bottom if there is a lot of piping or such already in there.

As I said before, I have never seen or even heard of anyone ever priming the back side of a piece of subfloor in 25 years.
If there is moisture below, then one has to install vapor barrier (10 mil poly) over the joists and then screw down the primary decking (using acoustic seal on the joists to keep moisture from infiltrating where the screws puncture the plastic and technical tape on the poly seams), but I have only run into a job like that once in 25 years.
All others have been suitable designed as to not have moisture issues under the subfloor.

I do know that Schluter has indicated that one can go over the 3/4 inch primary deck (single layer of board on joists) with Ditra, but I would never recommend it or warranty it.