Advice to bed down SMC shower tray: options

[Camran Ahmed]

Hi guys

 

I'm a DIYer having learned mostly from others online teaching browsing YouTube and other forums. I'm in the process of refurbishment of the bathroom in my house and I'm after second opinions on my approach to bedding down a shower tray. 

 

The shower tray is to replace a bathtub. It's a 1200mm rectangular SMC tray (this exact one https://www.bathroommountain.co.uk/berlin-rectangular-matt-white-slate-effect-smc-composite-shower-tray-1200x800mm-c48184)

 

The only thing is that the instructions provided are sparse with zero wording. All I have a is a brief pictogram showing that it's recommended to bed down using some sort os silicone.

 

What it's doing down on is a floorboard subfloor, with supply and waste pipes on the surface. Given that I have spare studs and spare 18mm plywood on hand from other refurbishments happening in the house, I was thinking to create a level base using the two in order to avoided routing pipework below the floor. I don't mind a small step up to the tray and it suits me as I'm no the best plumber. Plus I intend to tank the plywood with a suitable product (don't know what yet, maybe SBR) and use jackoboard as the tile backer for the parts of the wall that didn't have tiles (due to there being a bathtub there before).

 

Would anyone in my situation still proceed with bedding down with silicone only, even if they constructed a plywood base? As I've seen some people use silicone with some sort of foam, others use sand and cement, some use flexible tile adhesive. What do you guys think?

 

I will be tiling the floors and some walls so could just use flexible tile adhesive if I have spare, right? Or are there certain adhesives not suitable for bedding SMC trays?

[Temp]

I've only done a few trays but they all recommended mortar for bedding the tray. In one case I didn't have much height to play with and used a flexible tile adhesive which seemed to work well.

 

I always use top access traps. These have a part that goes below the tray and a top part that goes through the hole in the tray and into the part below. So you can do all the waste plumbing before bedding the tray. Typically you cut a clearance hole in the 18mm WBP that you are going to bed the tray on. It helps if the pipework allows the lower part to move, in this clearance hole, particularly vertically so that it can be pulled up through the hole against the bottom of the tray when you fit the top part. A few dry runs are needed to ensure everything is in the right place.

 

You clean the trap from above by removing the chrome mushroom cap.

Edited August 4 by Temp

[Camran Ahmed]

thanks for the replies so far guys its much appreciated as its my first time doing this! i was leaning more on the tile adhesive route but wasn't sure. 

 

how would you guys even go about back-buttering this thing for good coverage? the ridges on the underside will surely make it so awkward right? im thinking of combing out the spread on the plywood base for sure, but would you slap on thick a layer on the underside of the tray too?

 

Cam

[JamesP]

I use flexible tile adhesive usually on 6mm or 12mm cement board base.

Your existing floor and joists need to be solid.

 

 

[Temp]

On 04/08/2025 at 16:17, Camran Ahmed said:

how would you guys even go about back-buttering this thing for good coverage?

 

What do the instructions say? May not need it.

 

I suppose you could make a slightly runny mix of flexible adhesive and use a paint roller to apply it to the bottom of the tray. Regular mix on the boards.

[Nickfromwales]

This type of tray is a-la Impey, so you need a flat, supported piece of marine ply that this gets screwed down to the ply with stainless steel screws. You drill back through a load of those holes on the underside, and then fix back through from the top, sometimes you'll need to countersink the holes eg if you use 5.0 screws. You picked a PITA tray to fit....sorry.

 

If the shower needs to be flat to the floor then you replace the floorboards with the ply, and then fix the tray to that. You need to go nuts with timber noggins etc to make sure this can't move a mm. The only place you need sealant (clear CT1) is around the perimeter and along the edges that meet the walls, then along the tray and to the wall.

[Nickfromwales]

Here’s an Impey one I did a while ago. 
 

 

 

 

Turns out I laid a load of D4 floorboard laying pu (expanding) glue down (yellow) and then CT1’d (white) afterwards.