Shower tray installation & bathroom tiling questions

[Jim_cam]

Hi all -- I've been away from this forum for a while but I found it really useful a couple of years back, not just for questions but for reading through various threads picking up tips from other people's expertise. So I'm really hoping someone can help me here!

I've installed bathrooms before in the course of general home renovation, so I've always been able to get the floors up, fix and reinforce joists where necessary, and install appropriate subflooring before tiling and putting in shower trays. Now I've been brought in to do some work on an extension, and the owners have already done the floor and stud walls for the proposed bathroom, which is in the corner of a larger room. The way they've done it has left me scratching my head a bit ...

The floor is a fairly thin OSB, with the sheets not staggered. Consequently there's a fair amount of deflection. Additionally, the studs have been installed so that the bottom plate fits snug against three sides of the shower tray (which they'd already 'installed' by sitting it on the OSB and siliconing round the top!). I'm at a loss as to what to do here -- in the past I've always boarded down to the floor and then put the shower tray up against boards and upstand seal, then tiled up from the tray, whereas here it seems like I'm expected to have the sides of the shower tray flush with exposed studs and have the bottom of the boards boards on the surface of the shower tray. I'm worried it means that any movement in the building or from people standing on the tray is going to gradually pull open a gap for water. If that's the case, the only alternatives I can see are to take the studs out and move the bottom plates back, or get a smaller shower tray.

Could anyone give me any guidance about how to proceed? I can install a decent bathroom left to my own devices, but I'm not sure enough of myself to do it on top of someone else's half arsed job. Would it be adequate to glue and screw some more appropriate flooring on top of the current OSB and bed the shower tray on tile adhesive / sand & cement on top of that? Is there a way of doing the walls that will keep a water tight seal against the top of the shower tray, or will it mean moving studs / changing shower tray?

I'm a bit desperate for help here -- thanks in advance!

 

[JaimieH]

If you are being asked to do the job then it will be you who gets asked to come back if any problems develop in the future. Explain your concerns and say that it needs prepping properly for it to last. Any wooden floor will need overboarding before tiling but the very least you need is for the subfloor to be solid and deflection free prior to this. I'd recommend removing the OSB and replacing with 22mm P5 tongue and grooved chipboard flooring, screwed not nailed. Then overboard with 6mm No More Ply using their Mega Adhesive. Judging by the rest of the room the chances are the tray will only have been siliconed down so it should be easy enough to take out and fit correctly. I'd also continue with NMP under the tray. Don't know what you plan to do with the walls around the shower but if it's plasterboard, plaster or any of the cement based tile backing boards they will need tanking. Also bear in mind the size and weight of tiles that are to be fitted as skimmed plaster walls have a max weight limit of 20kg sqm. If you dont feel comfortable with any of the current prep then just explain why and sort it because you'll be the one expected to sort out any problems afterwards.

 

[Jim_cam]

This is really helpful, thank you. 22mm t/g chipboard is what I've used in the past. The tray is indeed only siliconed down so removing and reinstalling will be fine. I've been asked to tile inside the shower enclosure, which is fine, but I'm worried about the space -- is it ever good practice to have the cement based tile backing boards coming down to the surface of the shower tray, rather than fitting down around it? Would a line of ct1 at that joint be sufficient?

Photo attached. Given the size of the room -- I'm not sure where the joists are -- getting the OSB up might be tricky. Would overboarding it with NMP or chipboard then NMP offer sufficient rigidity?

I completely take your point about good communication with the owners! Just want to have this one done to a high standard quickly so I can get back home.

 

[Jim_cam]

-- or can tanking the walls include the joint with the shower tray, is that what you're suggesting?

 

[JaimieH]

First thing to sort out is the floor. Going over the OSB with chipboard would stiffen things up but really the chipboard needs to be screwed into the joists. The location of these should be easy enough to find by looking for the screws or nails in the OSB but if you are not going to lift the OSB you run the very real risk of screwing into a hidden pipe or cable, something you definitely need to avoid. You'll also have a huge height build up by the time you've overboarded and tiled. As for the walls around the tray, you could cement board straight onto the tray if there is enough room on the tray lip but I would check the technical spec of the enclosure or screen just to make sure the minimum tolerances will allow you to do this. The tray needs removing because thin OSB is not an adequate base, and neither is a bed of silicone for the majority of trays. You would be better boarding down to the floor, overboard floor, fit tray on flexible tile adhesive, bond sides of tray against wall with silicone and then tank. If you try to make the best of a bad job I think you'll struggle to achieve the high standard you're wanting.

 

[Jim_cam]

This is so helpful, thanks. I've got a meeting with them in the morning and I'll talk them through the options!