Conflicting advice over ply subfloor, anyone advise?

[Tam71]

We're in the middle of a complete bathroom refurb. Walls, ceiling and floor all ripped out. Floor is becoming a problem due to conflicting advice and we've ground to a halt till we get this settled.

House is around 30/40 years old, stud and plasterboard walls, 18mm chipboard flooring throughout, joists are 400mm centres.

The bathroom flooring has been ripped up but we don't know what to put down in it's place.

My 'better' half has been asking for advice on the net and has been told:

a) lay 18mm ply direct onto joists and tile on top

b) lay 25mm ply direct onto joists and tile on top

c) lay 18mm ply plus a 6mm tile backer board such as Hardibacker because you need the ply for strength but the Hardibacker for moisture protection and good base for tiling on.

d) don't bother....lay vinyl instead!

I would prefer to use a tile backer board, I think they make sense. I dont know what ply to use though. The 25mm would be stronger but would the extra weight be a problem for the joists, especially if we put 6mm Hardibacker over the top? Would the 18mm be strong enough? I've often thought our 18mm chipboard wasn't as rigid as I'd have liked but I don't know what 18mm ply would be like.

We're going round in circles and our work is being held up because of the uncertainty so we could use some advice?

 

[clf-gas]

18mm ply and use the tile backer if you wish, though you dont need too.

 

[gcol]

No brainer. Fit 25mm WBP (exterior ply). Absolutely no worries about extra weight on the floor - I can't even understand why this would even cross your mind.

 

[Tam71]

Thanks for replies ....but you see our dilemma?

 

[gcol]

Read the sticky - if you follow that info, you won't go wrong.

 

[jefoss]

25mm ply for floor, sorted

 

[dextrous]

Thanks for replies ....but you see our dilemma?

Without being disrespectful, clf-gas is a plumber, and the other contributors are tilers.

 

[Tam71]

25mm ply it is then. Thanks for the help.

Is it wise to tank the ply (belt and braces approach) or still use tile backer boards to deal with the possible moisture damage aspect?

The floor does slope slightly towards the centre, the joists at the outside wall are a little higher than the ones in the centre, would the adhesive used for the tile backer boards even this out more maybe?

Or do we just leave it at the ply?

We're also having under tile heating if it makes any difference.

 

[gcol]

Unless you're making a wet room, there's no need to tank the ply at all.

 

[amsie]

Both would have been acceptable, but I would have gone for the 25mm ply as suggested.