6mm backer boards on floor

[23vc]

I’ve laid a 18mm wbp ply bathroom floor with all board edges meeting on joists. Before tiling I’m going to lay 6mm cement board glued and screwed down to the ply.
Am I right in thinking,
1) the backer boards edges don’t need to meet on a joist, and the screw length only needs to be enough to hit the ply
2) the backer boards don’t need taping/joints filling after fixing them?
Thanks.

 

[noseall]

1) the backer boards edges don’t need to meet on a joist, and the screw length only needs to be enough to hit the ply

Correct.

2) the backer boards don’t need taping/joints filling after fixing them?
Thanks.

Not unless you intend skimming them.....

 

[23vc]

Nice one, ta.

 

[ree]

OP,
You lay the ply in a broken bond with 5mm expansion gaps at the abutment edges & , say, 2/3 mm gaps between ply sheets.
The backer board is also laid in broken bond while avoiding the joins in the ply.
Its common knowledge in the building trade that backer boards joins are fibre meshed taped - and a tight skim of thin set adhesive used over the tape.
If your trowelling is not much good then as the thin-set starts to dry go over it with a damp sponge to feather it out.

 

[^woody^]

5mm expansion gaps in flooring boards?

 

[ree]

Not for T&G sheet materials.

 

[^woody^]

Hmmm. Wondering if those same boards used as a flat roof deck, or timber frame sheathing (ie a floor turned vertical) have 5mm gaps too?

 

[ree]

Poster #7,
Your question is irrelevant to the thread - but I'll answer it.
The thread is talking about a bathroom floor where my spacing dimensions would be typically adequate.

Roof sheathing, timber frame sheathing & flat roof deck would have 3mm or 4mm between sheets, and edge spacing according to the overall size potential for expansion.

 

[^woody^]

Bombshell news.

 

[Deluks]

The sign of a weak person Old man, is that he cannot bear to be challenged - can you?

You've turned into Darth Vader

 

[mrrusty]

Here's a fitting guide from a flooring plywood supplier, including a nice video. Spot where they show big gaps between the boards. - clue - it isn't there...https://sp101.co.uk/fitting-guide/

and here's the pdf version https://sp101.co.uk/wp-content/uploads/2018/09/SP101-Application-Guidelines.pdf

 

[Nige F]

Poster #9,
You asked what looked like an obvious trolling question but, out of politeness,

 

[Nige F]

You've turned into Darth Vader

maga man more like

 

[^woody^]

 

[^woody^]

Poster #10

Au contraire. Your mention of 5mm gaps for a floor deck and 3mm and 4mm gaps for a roof deck and frame sheathing is quite a bombshell after all these years of me following what I'd assumed to be standard industry practice and butting these boards together.

This major revelation needs further investigation and clarification, would you like to "expand" on this bombshell requirement?