Shower tray direct onto floating floor: 22mm chipboard /polythene / 100mm PIR insulation / concrete

[Hastings]

The joiner who built the 3 timber frame walls around where the shower tray will go says it will be fine but I am nervous that in time the tray will drop a little and stress the tanking/join between tray and walls.

 

The stud walls are supported on timber floor plates directly fixed to the concrete subfloor.

 

I plan Classiseal tape for the tray (fitted to outside of tray before tray is put into position), overlapping and joining the Classi membrane tanking on the MR plasterboard walls.

 

Should I cut out all the 22 chipboard flooring, polythene slip layer and the insulation and bed the tray on sand and cement directly onto the concrete subfloor? My worry with that is there is no route for the mortar dry out except for down into the concrete where it will be trapped.

Edited August 11, 2022 by Hastings

[Nickfromwales]

I’ve done a few like this in garage conversions, no complaints to date.

I cut that section of insulation in last, the area under the tray plus a few inches more, and set that back down onto wet expanding foam. 
Whilst the foam cures you will need to put a piece of timber deck down and lay some very heavy weight on it, evenly distributed, until cured ( 2 hours should be plenty ). 
Bond the membrane to the insulation with CT1 or Sikaflex and roller it out with wallpaper joint roller, and leave to dry for 24 hrs. 
Fit the timber deck down, bonding it to the area of membrane that you previously bonded, and leave again to cure. 
Set the tray down into a bed of Sikaflex ( NOT a sand and cement mix ) and be generous enough with it to use that to level the tray. Bond the tray to MR plasterboard the 2 or 3 sides around, and remember to screw into the plasterboard when fixing it to the studs, at the same level where the tray will be bonded to the plasterboard, so it’s nice and solid.

FYI, I always do these types of conversions using 2x layers of deck board ( 18mm or 22mm P5 Egger etc ) with the joints overlapped for 2 belts and 3 braces, gluing snd screwing the hell out of them so they become one bit of deck. 
I have put washing machines on these types of floors ( garage to utility etc ) and even spin cycles etc never see a mm of movement. This is quite a solid way to lay a floor as long as the subfloor is near perfect in level and uniformity. If not, self levelling compound should go down first to get it flat and smooth, as there is no other practical way of levelling once the insulation has gone down.

 

Done properly, this will be fine. 

Edited August 11, 2022 by Nickfromwales

[Hastings]

Thanks Nick.

I can see the merit in bonding/foaming the insulation down onto the concrete at the stage of laying the floor but I am too late for that now and if I do it now to just the tray area of floor by cutting out a section I loose all the support strength from the surrounding deck while depending even more on the compressive strength and long term dimensional stability of the insulation under the tray.

 

One thing I didn't understand please:

44 minutes ago, Nickfromwales said:

Bond the tray to MR plasterboard the 2 or 3 sides around, and remember to screw into the plasterboard when fixing it to the studs, at the same level where the tray will be bonded to the plasterboard, so it’s nice and solid.

"when fixing it to the studs" When fixing what to the studs?

[Nickfromwales]

On 11/08/2022 at 14:08, Hastings said:

Thanks Nick.

I can see the merit in bonding/foaming the insulation down onto the concrete at the stage of laying the floor but I am too late for that now and if I do it now to just the tray area of floor by cutting out a section I loose all the support strength from the surrounding deck while depending even more on the compressive strength and long term dimensional stability of the insulation under the tray.

 

One thing I didn't understand please:

"when fixing it to the studs" When fixing what to the studs?

Ah, OK.

 

When screwing the plasterboards to the studwork, you should make sure there is timber, and that you screw into it, at the position that the tray will be bonded back to the plasterboard. That make sense?