The bathroom refit continues…

I’ve cut and laid 25mm marine ply above new joists (grain across the joists) I’ve not screwed down yet but plan to use D4 and screws every 100mm or so onto the joists, then use noggins underneath the ply between the joins and around the edges of the joists to reinforce. There’s a 30cm gap or so down one side but I’ll fill this in with the old 25mm floorboards I have left over – most where shot so ply was bought.

Do the joins on the top face of the ply need filling with something?

Was then thinking tile backer board and porcelain tiles on the floor – there seems a few different options for backer board adhesive – any recommendations?

I also need to get all four walls of the room sorted with dot and dab + Moisture resistant plasterboard, should this be done before or after the backerboard and floor tiles are down?

it’ll be a walk in shower with a low profile tray rather than a wet room so hoping there’s no need for tanking etc.

I’ve not done this before so I’m feeling my way a bit, just don’t want to do anything major in the wrong order!

I drylined my bathroom with green board and it’s a messy job for an amateur so I would put the tiles down last.
Also rest the boards on an off cut off greenboard + the depth of tile/ adhesive and remove when dotdab adhesive has set. This will help avoid any water being sucked up into the board and ruining it.

If you’ve got marine ply down, I’d use ditramat or Dural rather than backer board, as you’ve got the stiffness you just want a decouple layer. I’d wallboard before flooring as you’ll get it filthy when boarding. I’d still tank to the wet areas, or better still use aqua panel to board the inside of the shower area.

As above.  You need something to decouple the tile from the board

This is why I asked, I’d heard of decoupling but need to know what it’s actually referring to in this scenario!

Basically your subfloor needs to be solid and sound, which marine ply certainly is at 100mm centres. Duralbase or Ditramat is then stuck to the ply with thinset, it has a fleece layer on the underside which sticks like poo to the proverbial, you then bond the tiles to the top face with flexible thinset (I prefer Setaflex semi rapid). The decouple membrane basically allows the subfloor to move a bit whilst not affecting the tiled surface.

Thanks, the joists below are 16″ centers but I replaced as was hence the 25mm ply

As long as the floor is flex free then you’ll be fine with decoupling membrane. If there’s still flex then 6mm or even 12mm no more ply glued and screwed will make it stiff enough to tile.

why backfill that 30cm with floorboards? I’d aim to have the same subflooring down on the entire surface
a inch of ply on 16″ centers is well inside Schluter’s spec for ditra (assuming you’re not on a ridiculous span or using 2×2 tile)
Adding another ply layer, schluter don’t call for gluing it down, there’s a specific offset and screw patterns and 10mm minimum thickness.
Schluter also recommend unmodified (or lightly modified) thinset to set tile to the ditra. Ditra’s uncoupling is doing the same job (but better) than flexible thinset and the modified stuff will take much longer to dry with large format tile as the modifiers need air to dry, which tile over ditra doesn’t allow for much of.

Stolen from tilerforum but it’s info I was told by a Rep also…

The words modified and unmodified mortars relates to products used in the U.S.A.

Schlüter®-DITRA 25 can be adhered to a suitable substrate using a cement based C2/C2F classification adhesive applied using a 4 x 4 mm notched trowel.

You can also use the same cement based adhesive above Schlüter®-DITRA 25 with a maximum bed of 10 mm.

Please see our installation guide linked below:

Schlüter®-DITRA 25

Should you require any further assistance, please do not hesitate to contact us.

Yours sincerely

Sarah Meller
Technical Services
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Schlüter-Systems Ltd | Units 3-5 Bardon 22 Industrial Estate
Beveridge Lane | Coalville | Leicestershire | LE67 1TE

Thanks, reason for the floorboards was there’s about a 30cm by 3m section that the 3 ply sheets came short of, to fill that I’d need less than half a sheet and there £70 each! Agree same throughout would be ideal but I’ve got no use for the left over £50 of ply

I’ll take a look at the Schluter product, thanks for tips it’s helping me find my way, I’d get someone in but enjoying the learning and taking my time (till it goes pear shaped!)

Weird that the north american arm is more conservative with thinset recommendations. Science is the same everywhere 🙂
I would note that the above response from schuter refers to c2/c2f but it’s the S1 rating that would have me second guessing.
The product technical sheet states “polymer modified”, which is where the north american term comes from.
My work is based on TCNA/ANSI so I do appreciate it is indeed irrelevant to UK stuff.
As you were. Sorry.