Installing a Stone Resin Shower Tray on Wooden Floor Boards

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We are planning to fit a stone resin shower tray into an upstairs bathroom that has wooden floor boards. It needs to be as low as possible and not up on adjustable legs.

I plan to ensure that the existing boards are in sound condition and then lay a thick sheet of WBP plyboard (18mm or so) on top of this for strength. I plan to take time to get the board level and true by using either mortar or a floor tile adhesive between the floor boards and the ply. Once this is done, I plan to sit the tray on top of the ply, butted up to the wall which will be tiled afterwards. I shall make sure that the waste is all in place first and that I have cut access holes and so forth.

Does this sound like a sensible plan or should I be doing it differently?

James.
 
When I put mine down many years ago, the ply was just screwed down to the floorboards and the resin tray layed on a bed of mortar
 
Those resin trays all need even support. The lump of ply over the floorboards is good (stops differential movement) but you need the mortar between ply and tray- the idea is the mortar squidges up into all the little undulations and uniformly supports the thing.
If yours is the type that offers a leg kit (on the underside there'll be some clear load points) I suppose you could just use big mortar dots at those points but if you're going that far you might as well follow the Mfr instructions on the thing.
 
Lift the floor boards and check the joists - run supports(noggins) in between the joists (every 300mm) to provide lateral support to the floor boards to minimise flex in the boards. Double screw the boards down at every joists/support.

Ensure there is a rectangle/square of joists/noggins that's tracing the outline of the tray. Use packers to level out the ply, don't use an adhesive as the floorboards will move/expand/contract and can break up the adhesive over time. Screw the overlay down every 100mm.

Some use mortar some use adhesives to bed the tray. Personally I use a good quality silicone adhesive (not sealant) to bed the tray into as it hardens to a stiff rubber that allows it to absorb a little flex when the floor moves so minimises the pull on the wall/floor seals (silicone sealant).
 
Resurrecting this thread. Rather than using plywood underneath the shower tray, is there any good reason not to use cement board such as Hardie or no more ply?
 
Depends on the thickness I guess, 12mm would be the min I suggest. Just remember there is no sure way to really secure the cement board to the floorboards unless you use bespoke screws and countersink them.
 
Recyling an old thread, but similar question... Old house with wonky floors, now within 10mm level on a 1200x800 base! Plan to put 18mm marine ply on top of 22mm floorboards, screwed down then bed my resin stone tray onto mortar to get perfectly level. I also wondered about rapid set tile adhesive as I have plenty left from laying a sandstone floor. Anyway, unwrapped the tray and the instructions tell me I MUST use polyurethane adhesive and not tile cement. What's peoples thoughts on this? I can't see why I can't use a nice thick bed of tile adhesive so I can level it...
 
Ideally you would start a new thread and link back to this one, just avoids any confusion about what’s being answered.

If you want the warranty to be upheld then you need to follow the MI. Me as an installer, just use my own methods and I then warranty the install. PU adhesive is a bit of a strange one though, typically use for wood, chipboard t&g flooring etc but manufacturers keep changing their specs. I typically use a silicone adhesive (not sealant) or a flexible tile adhesive when it’s a badly laid concrete

I usually do all the levelling up when fixing/setting up the sub floor, so when it comes to the tray it will always be level. If the floorboards are badly out, they would normally be lifted and new flooring/boards would be fitted.
 
The last two I’ve done, I did with the Hardie backer cement board, 12 mm.
Prime floorboards with SBR, 6 mm notch trowel with Bal rapid set flexi, 25 mm turbo gold screws from s’fix. If you need the 18mm, then put a 6 mm Hardi on top of it, again with a thin spread of adhesive, and use longer screws.
I used the same adhesive on top of this to bed the tray onto.

You don’t need to use rapidset, standard ( ‘slow set’) will do.
 
Thanks for the quick reply. Yes, probably should have started a new thread. I spoke with the supplier, the reason they want you to use a polymer adhesive is actually to allow it to flex, its kind of opposite to what I thought. As the ply is screwed to the floor its rigid, if the resin stone tray is bedded on mortar or even a flexi tile adhesive they say its still to rigid and if there is any floor movement it may crack the stone tray. If the tray is set on a flexible PU adhesive the floor can flex and doesn't stress the tray. I suppose its logical. I think I'll get the ply base level and then glue as the manufacturer requests...no arguments then!
 
Mmm, that’s interesting what they say, and I understand their reasoning.
I would follow their advice.
( though I still think flexible tile adhesive was specifically designed for, and used in, tiling floors that flex and bend )
 

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