OSB boards as base for Wetroom

[lennyd]

Hi all,

When installing a wet room (with toilet and sink) would it be ok to use 18mm OSB boards as a floor base on top of the joists?

The fake wall will be placed on top of the boards to conceal a toilet cistern for a wall-hung tolier, and put away the water pipes and shower valves. The aim is to create a wetroom like in the design attached.

Any advice welcome!

 

[One Day]

No, no and thrice no! Even osb3 would not be good enough. In the US they do, but their timber is different/better. Use 18mm marine grade or wbp ply and ensure it's deflection free before waterproofing.

 

[lennyd]

No, no and thrice no! Even osb3 would not be good enough. In the US they do, but their timber is different/better. Use 18mm marine grade or wbp ply and ensure it's deflection free before waterproofing.

Thanks impish! What sort of thing could go wrong if I use OSB3? Is it because of the strength of the board or the water proofing?

 

[Tile Shop]

OSB is one of the most temperamental timbers you could have. if moisture gets into that you will end up living another kind of hell. The movement, swelling and expansion of it just makes it a whole lot worse. British Standards is 18mm WPB Ply screwed at every 300mm with extra noggins if the joists will not allow this.

Go for a pre-formed waterproof shower tray, like the Schluter Kerdi Shower. That will need to have full structural support underneath so can't be laid directly onto joists alone, so that will also need boarding. The 18mm Plywood (sealed on the back and sides with an SBR primer) will be fine, but as impish said, it will need waterproofing. Weapon of choice for this would be Ditra Matting and the Kerdi Tape for the joints and edges.

You are going to need to play around with the levels depending on the tray you go for, but you may need to consider a further over-boarding of the rest of the floor with a further layer of Ply, Hardibacker or SLC to get it right. They do a couple of types of tray, around 22mm thick. so if thats the case, an additional layer of 18mm ply with the Ditra on top, would be pretty much spot on.

 

[lennyd]

OSB is one of the most temperamental timbers you could have. if moisture gets into that you will end up living another kind of hell. The movement, swelling and expansion of it just makes it a whole lot worse. British Standards is 18mm WPB Ply screwed at every 300mm with extra noggins if the joists will not allow this.

Go for a pre-formed waterproof shower tray, like the Schluter Kerdi Shower. That will need to have full structural support underneath so can't be laid directly onto joists alone, so that will also need boarding. The 18mm Plywood (sealed on the back and sides with an SBR primer) will be fine, but as impish said, it will need waterproofing. Weapon of choice for this would be Ditra Matting and the Kerdi Tape for the joints and edges.

You are going to need to play around with the levels depending on the tray you go for, but you may need to consider a further over-boarding of the rest of the floor with a further layer of Ply, Hardibacker or SLC to get it right. They do a couple of types of tray, around 22mm thick. so if thats the case, an additional layer of 18mm ply with the Ditra on top, would be pretty much spot on.

Thanks Paul! So OSB is no go, even if it's OSB3?

 

[Tile Shop]

Correct. Bare in mind, the guarantees for all the products you will use for this project, will have a clause saying that they must be used in conjunction BS5385, and BS5385 says it must be Ply. So if anything was to go wrong, you'd have no comeback.

You'll probably get people on here saying "well I've tiled on it before and never had a problem". And yes I can see its still tempting to use it baring in mind the layers you'll need to cover it. OSB3 is a far less density board than ply and nowhere near as structurally sound. So with humidity and heat, it will expand, in cold conditions, it will contract, with walking on it, you'll get deflection, if it gets damp with humidity, it will swell and go mouldy. And whatever is put on top of it, will inevitably follow.

Fare enough, you covering it all with all these waterproofing materials, but if anything can go wrong, it probably will.

 

[lennyd]

OSB is one of the most temperamental timbers you could have. if moisture gets into that you will end up living another kind of hell. The movement, swelling and expansion of it just makes it a whole lot worse. British Standards is 18mm WPB Ply screwed at every 300mm with extra noggins if the joists will not allow this.

Go for a pre-formed waterproof shower tray, like the Schluter Kerdi Shower. That will need to have full structural support underneath so can't be laid directly onto joists alone, so that will also need boarding. The 18mm Plywood (sealed on the back and sides with an SBR primer) will be fine, but as impish said, it will need waterproofing. Weapon of choice for this would be Ditra Matting and the Kerdi Tape for the joints and edges.

You are going to need to play around with the levels depending on the tray you go for, but you may need to consider a further over-boarding of the rest of the floor with a further layer of Ply, Hardibacker or SLC to get it right. They do a couple of types of tray, around 22mm thick. so if thats the case, an additional layer of 18mm ply with the Ditra on top, would be pretty much spot on.

Hi Paul, is there somewhere I might be able to lookup the British Standards for this? Sorry if the answer is obvious.

 

[J Sid]

Hi Paul, is there somewhere I might be able to lookup the British Standards for this? Sorry if the answer is obvious.

I believe many uk libraries have them or if you find you are a uk lottery winner you may be able to purchase a copy

 

[Kenneth]

Hi lads just reading up on this too. I have an osb3 floor. I placed the 22mm wettoom tray in adhesive on that. Acrylic primed first. Then i beded cement board on the rest of the floor and also screwed it. That left me with about 7mm to the top of the tray. So i joint filled the joints and corners and siliconed the joint. Etween the tray and cement board. Placed in my underfloor heating and filled it up with leveling compound to b flush with the tray. How does that sound thanks