Shower tray install – advice needed following cowboy job

[Beverly27]

As some of you may know from my previous posts, I'm currently in the process of ripping out and reinstalling our main bathroom because it had leaked. Another cowboy install has become evident, with no measures taken to prevent water ingress (plasterboard used instead of backerboard, no tape between board and tray), tiles and shower tray dot and dabbed etc), and the whole installation being generally shocking.

I now have to replace the shower tray. As you can see from the photo of the ply (non-marine) that was sat over the pipework etc shown in the photos, the previous one was installed very poorly, being dot and dabbed onto the ply with cement. My plan is to use 22mm marine ply base over the pipework etc, filling the same gap that you see in the photos, with a 5/1 sand/cement bed on top on which to mount the shower tray. I plan to seal the marine ply, including the edges, with a 50/50 PVA/water mix.
I will have to build the marine ply out to the newly finished wall - we will render the wall with a sand-cement mix and then mount the Abacus Elements backerboards to this using adhesive and cement screws, the latter screwing into the brick. This will still leave a gap at the edge of the wall between the final joist, which I have circled, in the photo and the wall. This all leads me to reaching out here for help from those more knowledgable than me with the following questions:

1. Do I need to seal the gaps where the edges of the marine ply will meet the outer edge of the adjacent subfloor, or do I just leave them and apply the sand/cement mix?
2. Should I attach some noggins to the wall, or to the joist (highlighted) closet to the wall, with these noggins protruding from the wall, or extending to it from the joist, so that the edge of the marine ply by the wall has a stable base and isn’t floating over the gap? I don't know whether this is necessary or not, and I don't want to expend unnecessary time and effort, but I do want to do things properly.
3. I want to use a Mira shower tray, which has a central waste. Do I just cut the waste pipe back and solvent weld the new waste? How do we establish the correct position to do this, so that the new waste fits in the correct position to connect to the shower tray? Does the waste need to sit on something solid, like a noggin, or is it ok floating above the insulation, as it is currently, which the photos illustrate?
4. The previous install had used foam around the edges of the ply, which then reached the wall on one side. I don't know whether this was more sloppy work, or needed. The surrounding floor is wet-heated - you can see WarmBoard in the fourth photo. I also don't know if I can screw through this to affix the 22mm marine ply on top of it.

I'd appreciate your help here, folks, thanks!

 

[Madrab]

a 5/1 sand/cement bed on top on which to mount the shower tray

Never use it these days unless the tray is being bed onto a concrete floor and even then it's a flexible tile adhesive. Using a mortar mix on a floating wooden sub floor never quite seems to last due to the inevitable movement. Yes, I would fill the gap to the wall with timber so it supports the Ply and tray edge

Do I just cut the waste pipe back and solvent weld the new waste? How do we establish the correct position to do this, so that the new waste fits in the correct position to connect to the shower tray?

If the waste needs moved then yes. Just need to ensure there is still enough fall on the waste run to the outflow in its final position. Place the ply on the floor then the tray onto the ply and mark out the wastes position. Then cut the ply access remembering to slightly over cut for wiggle room and cut a channel to take trap's outflow and compression coupler. The trap can be floating but it needs to be high enough so when the tray is finally laid you can grab the trap and pull it up tight to the tray and fit and tighten up the locking ring.

I also don't know if I can screw through this to affix the 22mm marine ply on top of it.

As long as you aren't screwing through any pipe work then no reason not to. Only thing may be to ensure the ply all sits perfectly flat.

 

[Beverly27]

Never use it these days unless the tray is being bed onto a concrete floor and even then it's a flexible tile adhesive. Using a mortar mix on a floating wooden sub floor never quite seems to last due to the inevitable movement. Yes, I would fill the gap to the wall with timber so it supports the Ply and tray edge

If the waste needs moved then yes. Just need to ensure there is still enough fall on the waste run to the outflow in its final position. Place the ply on the floor then the tray onto the ply and mark out the wastes position. Then cut the ply access remembering to slightly over cut for wiggle room and cut a channel to take trap's outflow and compression coupler. The trap can be floating but it needs to be high enough so when the tray is finally laid you can grab the trap and pull it up tight to the tray and fit and tighten up the locking ring.

As long as you aren't screwing through any pipe work then no reason not to. Only thing may be to ensure the ply all sits perfectly flat.

Thanks for your advice, Madrab! Would you seal around the edge of the marine ply, for example, using silicone to fill the gap?

 

[Madrab]

Would you seal around the edge of the marine ply, for example, using silicone to fill the gap

I'd tank everything - get yourself a kit and that should come with a roll of edging tape. Seal the wall and floor sections and use the tape to seal it all up at the transition points before the tray goes in.

 

[Beverly27]

I'd tank everything - get yourself a kit and that should come with a roll of edging tape. Seal the wall and floor sections and use the tape to seal it all up at the transition points before the tray goes in.

Thanks. I have such a kit, so I will do that. Much appreciated.

 

[Beverly27]

We've moved the waste (the noggin it underneath will be cut to size and fixed in place on the shelf!), which now, as the photos show, has quite a steep drop between it and the waste pipe. I don't know if this might be an issue; hence, I am reaching out to ask you, good folks here, for your thoughts - what do you think?

 

[Madrab]

Absolutely won't be an issue, the shorter the distance and and the quicker you can get the water to accelerate out of the trap then the better for the tray's drainage and as long as the rest of the run is at least 25mm per M then the pipe run should be self cleaning.

 

[Beverly27]

Absolutely won't be an issue, the shorter the distance and and the quicker you can get the water to accelerate out of the trap then the better for the tray's drainage and as long as the rest of the run is at least 25mm per M then the pipe run should be self cleaning.

Thanks, Madrab.