what substrate on suspended bathroom floor ?

[sth]

hi - i have taken up old damaged floorboards (25mm thick) and want to put a ceramic tiled floor down - preferably to the same 25mm height as the old.
the joists are in good condition : 700mm wide and spaced 330mm apart.
is plywood the best option and what thickness would i need ? what are marmox boards - do they take the same weight ?
i have not bought tiles so don't yet know what thickness these will be...
i have time, so want to do a decent job - just don't have any expirience.
my fear is that water could get under the tiles and start rotting the wood beneath...

[doug boardley]

to TF sth, you can replace the floor with a minimum 18mm wbp plywood. I'd put extra noggins between the joists to minimise any deflection. The underside and edges of your ply will need sealing with an acrylic primer to negate water ingress, also leave a 2mm joint between plywood joints. You can tank the floor prior to tiling with a waterproofing kit. I'll post a link...
Concrete & wood wet room trays for showers

[peckers]

are you trying to achieve 25mm including the tiles or 25mm plus the tiles?

[doug boardley]

good point Peckers, I read it as FFL as close to 25mm as possible

[Dave]

Moved from guest forum to tiling forum.

[sth]

thanks Doug n Peckers for prompt reply -25mm including tiles if possible - Doug do i add noggin's parallel or at 90 derees to joists? re. tanking - it's kids getting out of the bath and dripping - would a good bath mat do the job instead ?
This job is a whole refit...i'v exposed the masonary on the exterior wall and need to work out what thickness of backerboard i need with regard to tile weight load and insulation...plan to attatch these to battons since masonary uneven + need space for 15mm piping ............ i could go on! so many questions...

[peckers]

thanks Doug n Peckers for prompt reply -25mm including tiles if possible - Doug do i add noggin's parallel or at 90 derees to joists? re. tanking - it's kids getting out of the bath and dripping - would a good bath mat do the job instead ?
This job is a whole refit...i'v exposed the masonary on the exterior wall and need to work out what thickness of backerboard i need with regard to tile weight load and insulation...plan to attatch these to battons since masonary uneven + need space for 15mm piping ............ i could go on! so many questions...

As Doug has said you will need to install noggings at 300mm centres to the floor these go between the joists from joist to joist if you dont already have them? This will stiffen up the floor and help to reduce deflection/bounce
Then prime the back and edges and install your 18mm wbp or marine grade ply on the joists and screw down to joists and noggings at max 300mm centres/Then I would apply a tanking to the ply, I would personally use the membrane type something like schutler kerdi or homelux floor membrane or similiar if you want to tank the floor?
you will be slightly over your 25mm ffl once you have installed your tiles but using a membrane instead of a backer board is the closest way of achiveing this height

Your wall depends on the weight of your tiles and how much insulation you want to achieve on your exterior wall and how far you can come away from the wall? and is the wall single skin or got a cavity etc?

You could use plaster board with insulation that is already attached to the back of it. Or you could put insulation between your studs and use plasterboard that way.

Or you can use 12.5mm wedi or marmox boards that have insulation in them and you can screw this back to your timber. you can check out there thermal insulation on there web sites. you can use thicker boards to get more insulation if you need it but it depends on how much room you got to play with?

Tile backer boards can carry more weight then plasterboard so it depends on the weight issues.

hope this helps

[sth]

Peckers - what a gent ! thanks again for great info and such a speedy reply -
as regards what space i have to play with - it's a small bathroom , single skin external wall (Edwardian house) :

- external wall (masonary) to opposite wall (partition,with lath and plaster removed) = 1880mm

length of bath between these walls = 1800mm

space for battons+boards = 800mm

the bath length will be against another partition wall (lath and plaster removed) thinking - bath against timber uprights, then board and tile down to rim of bath -good idea or should bath be against board

[brinkley]

board all the way down to the floor fit the bath on a baton , seal bath with sillicone then tile down to the bath

[peckers]

Imo i would install the boards first and then fit the bath against the wall and then silicone between the wall and bath and then tile.

imo if you have 800mm to play with on the masonry single skin wall what i would do is to erect a stud wall useing cls timber pack it off the wall to leave an air gap between the new and the old then in fill between the studwork with insulation and then board it out with a suitable board that wil carry the weight of the tiles. (it may be advisable to get a local builder to check your masonry walls for damp as there only single skin and he could advize on the air gap between the old and the new wall if this is needed?) I am not a builder! And i cant see what you are looking at so this is only advise and not gospel.
Hope this helps.

[brinkley]

thats what i said

[mikethetile]

take care against cold bridging at this point

you have room to allow a free circulation of air behind your stud. use a vapour barrier behind your boards

[peckers]

thats what i said

You got in just before me

[peckers]

take care against cold bridging at this point

you have room to allow a free circulation of air behind your stud. use a vapour barrier behind your boards

Thats what i was thinking

[sth]

thanks for the advice everyone. will sort floor first ...