Shower tray help please

[0Floz0]

Hi

I’ve got a tile tray and I’m planning to lay it on the floor pictured. I’ve inset the ply between the joists as I’m trying to minimise any height increase (dormer with low roof). The more I look at it the more I think it’ll move too much. The ply is 18mm and is attached with some battens underneath that go into the joist so my expectation is it’ll mostly move together if it does. I think I can spare another 6-12mm. I’ve got some 12mm ply that I could stick over the top. I’m just wondering if cement board would be worth the £20 expense to go and get some today.

Some parts of the ply sit slightly proud of the joist. About 3mm in the worst parts. I was thinking I could balance that out with the tile adhesive.

If it’s ply the plan is to Sbr it and then set the tray on a bed of slow set tile flexible tile adhesive.

If I do go cement board it’d be the same but don’t think I need to Sbr it.

Would appreciate some input as I’m second guessing myself.

The tiles are sat on 6mm cement board on top of 18mm ply. Not that it’s relevant.

Cheers all

 

[Madrab]

Only thing I would have added would have been in joist supports along the right hand side (dwangs) but if it's a stone resin then I'm sure it'll be fine. Always have the infills level though, saves relying on the setting in material to level it. Do you have a power planer to level the boards to the joists maybe?

 

[0Floz0]

Only thing I would have added would have been in joist supports along the right hand side (dwangs) but if it's a stone resin then I'm sure it'll be fine. Always have the infills level though, saves relying on the setting in material to level it. Do you have a power planer to level the boards to the joists maybe?

I think I’m going to go the cement board route. For the sake of £20 I’d rather be belt and braces.

I think it’s just a plastic one. It wasn’t expensive but it’s a low profile one which is what I needed for the height issue. It’s not heavy.

I’ve got a power planer but it’s the ply sticking over the joists in places. With the screws it’s going to be a pain holding it down. I’m hoping the cement board solves the issue with the level. I’ll probably build it out with a bit of tile adhesive where there are gaps. Or is it best just to leave the gaps?

 

[Madrab]

I think it’s just a plastic one

Ah ok - foam filled acrylic tray. Then definitely get a solid flat base and use the min of adhesive so it wont deform over time - I'd even use silicone adhesive to set that down on and silicone sealant along the 2 wall touch points. Shame you've already tiled the walls as I would have been tempted to tank that whole space too. Would recommend tanking the section that's there though and seal the corners/edges with tape.

If you're going to use cement boards then use a thinset to lay that down onto the boards with and get it nice and firm and flat before the tray goes down, screw it down if you can using spacers to level up. Then get the cement boards squeaky clean before the tray goes down.

 

[0Floz0]

Ah ok - foam filled acrylic tray. Then definitely get a solid flat base and use the min of adhesive so it wont deform over time - I'd even use silicone adhesive to set that down on and silicone sealant along the 2 wall touch points. Shame you've already tiled the walls as I would have been tempted to tank that whole space too. Would recommend tanking the section that's there though and seal the corners/edges with tape.

If you're going to use cement boards then use a thinset to lay that down onto the boards with and get it nice and firm and flat before the tray goes down, screw it down if you can using spacers to level up. Then get the cement boards squeaky clean before the tray goes down.

Cheers. Appreciate the advice. On the walls I’ve used the nomoreply foam core cement boards and used their sealing adhesive on all the joins. From what I read, and correct me if I’m wrong, but that should be as good as tanking it anyway.

I was going to put a decent bead of ob1 behind the tray between the wall so I can stop anything getting behind. Then another bead of silicone between the tray and the tile.

For the cement board when you say thin set do you mean flexible tile adhesive? I’ve got some screws to really nail it down as well but figured it may be tricky as the screws could pull the boards out of level.

So if I can get cement board really level would you then use some ob1 to stick the tray down?

cheers!

 

[Madrab]

should be as good as tanking it anyway.

Should be yes - as long as the joints seal properly - the issue can be where panels aren't say tongue and groove then there's always the chance that a joint hasn't sealed properly.

OB1 is good stuff but it's very unforgiving. As a polymer adhesive it's great to say bed the tray down with but I'd use a good quality sanitary silicone on the wall contact areas as that tends to be a little more flexible so there's more give, especially on a wooden subfloor. So I would OD1 underneath the tray and then use a good line of sealant along both walls and the push the tray in and then down squeezing out the silicone at the walls so it's filled it all then run a finger along to smooth it all out.

when you say thin set do you mean flexible tile adhesive

Yes, a thinset is a cementitious based adhesive typically used for cement surfaces so grips well to the boards but may need some latex additive to add flexibility and strength. Only thought though is it's going down onto wood so an epoxy adhesive may be better, check which one is best for both. Again though if it can be screwed down then that'll help. Only tighten the screws till it's level, then leave it to set. Don't tighten them so much that it's pulled it out of level.

 

[0Floz0]

Should be yes - as long as the joints seal properly - the issue can be where panels aren't say tongue and groove then there's always the chance that a joint hasn't sealed properly.

OB1 is good stuff but it's very unforgiving. As a polymer adhesive it's great to say bed the tray down with but I'd use a good quality sanitary silicone on the wall contact areas as that tends to be a little more flexible so there's more give, especially on a wooden subfloor. So I would OD1 underneath the tray and then use a good line of sealant along both walls and the push the tray in and then down squeezing out the silicone at the walls so it's filled it all then run a finger along to smooth it all out.


Yes, a thinset is a cementitious based adhesive typically used for cement surfaces so grips well to the boards but may need some latex additive to add flexibility and strength. Only thought though is it's going down onto wood so an epoxy adhesive may be better, check which one is best for both. Again though if it can be screwed down then that'll help. Only tighten the screws till it's level, then leave it to set. Don't tighten them so much that it's pulled it out of level.

Hoping I’ll be ok with the joins. The adhesive they recommended is an expanding one so fingers crossed it properly filled the joins.

I take your point on the ob1 I was a bit worried that it may crack the tray by the walls because of how strong it is. I researched the silicone as well and picked up some Dow 785+.

So would you advise against using flexible tile adhesive under the cement board to join it to the wood if I’m screwing it down as well?

 

[Madrab]

So would you advise against using flexible tile adhesive under the cement board to join it to the wood if I’m screwing it down as well?

No - if the floor will be out because the ply isn't level with the joists then the adhesive will sit underneath the cement board and 'fill those gaps' the screws will just hold it all level and stop it moving once everything has set. Give the adhesive plenty of time to set properly pack everything to the highest point and screw at the packers so the board won't be pulled down below those high points

 

[0Floz0]

No - if the floor will be out because the ply isn't level with the joists then the adhesive will sit underneath the cement board and 'fill those gaps' the screws will just hold it all level and stop it moving once everything has set. Give the adhesive plenty of time to set properly pack everything to the highest point and screw at the packers so the board won't be pulled down below those high points

Cheers Madrab. Much appreciated.

So I’m fine to use flexible tile adhesive underneath the cement board? I’ve not got experience with latex additives so will probably steer clear of those and hope the flex in the tile adhesive is enough. Should I Sbr the ply and joists before the adhesive and give it 30 mins to get tacky before laying?

Assume plastic packers are fine for this use?

 

[Madrab]

SBR could be added to the adhesive as SBR is the latex additive I mentioned to give the adhesive more flexibility and grip, nothing wrong with coating the wooden substrate first too though.

Yes plastic packer would be fine, I tend to use 2mm hardboard too and thin ply as well depending on how much needs taken up. Cut it the same size as the joist and use them to screw them too, then fill in the voids with the adhesive. Lay the board down onto and tap it flat, lift and check for voids that haven't been touched, add a little more until it's all being compressed by the board, then screw it down. Give it plenty of time to set properly

 

[0Floz0]

SBR could be added to the adhesive as SBR is the latex additive I mentioned to give the adhesive more flexibility and grip, nothing wrong with coating the wooden substrate first too though.

Yes plastic packer would be fine, I tend to use 2mm hardboard too and thin ply as well depending on how much needs taken up. Cut it the same size as the joist and use them to screw them too, then fill in the voids with the adhesive. Lay the board down onto and tap it flat, lift and check for voids that haven't been touched, add a little more until it's all being compressed by the board, then screw it down. Give it plenty of time to set properly

Thanks for your help! It went better than expected. I’m just waiting for it to set now. It’s slow set so think I’ll ob1 the tray down tomorrow. It was hard not to let the screws pull it down too much so a lot are sitting proud of the cement board. Is it best to take these out or drive them in once it’s set? Wary of cracking the board with the latter.

 

[Madrab]

Thanks for your help! It went better than expected. I’m just waiting for it to set now. It’s slow set so think I’ll ob1 the tray down tomorrow. It was hard not to let the screws pull it down too much so a lot are sitting proud of the cement board. Is it best to take these out or drive them in once it’s set? Wary of cracking the board with the latter.

Take them out and use a countersink, that'll allow them to sit flat. You will wear it out pretty quickly on the cement boards but you don't have that large an amount to do so should be fine.

 

[0Floz0]

Take them out and use a countersink, that'll allow them to sit flat. You will wear it out pretty quickly on the cement boards but you don't have that large an amount to do so should be fine.

If it was wood that would have been my first thought. Hadn’t considered that for cement board. Got a relatively new countersink bit so hopefully it’s still usable after! I’ll wait until tomorrow to do it so it’s full set and I don’t disrupt the bonding.

 

[Madrab]

With cement board you also get specific screws that countersink themselves. TBH in practice they aren't the best at countersinking but they do to a point.

CB Screws

 

[0Floz0]

With cement board you also get specific screws that countersink themselves. TBH in practice they aren't the best at countersinking but they do to a point.

CB Screws

Yeah the ones I’ve got have been doing it. But it’s pulling the board down further so couldn’t use the full force. I’ll play it safe and use the bit so it doesn’t crack the board.