Mortar bed under stone shower tray.

[andreas]

I am a bit confused by the instructions on the resin stone shower tray we have bought. 

It says to lay a bed of mortar 15mm thick over the entire base, lay the shower tray on top and "manipulate tray a minimum of 5mm into the bed to ensure the underside of the tray is FULLY supported". Emphasis on fully.

The tray is a non-slip one, long and has quite a steep angle, with ribs underneath. The ribs are 20mm thick at the "shallow end". If I only manipulate the tray 5mm into the bed, only the ribs are going to be supported, not the entire underside of the tray.

Am I overthinking this? Do they just mean that all the ribbed bits need to be bedded in 5mm into the mortar? 75% of the mortar is going to be doing sod all on that basis 😅

[BotusBuild]

Yes, you're overthinking it. As long as the ribs are supported, the bit you will be standing on will be plenty strong enough.

 

FYI - one of ours sits on a wooden frame suitably waterproofed

[andreas]

7 minutes ago, BotusBuild said:

Yes, you're overthinking it. As long as the ribs are supported, the bit you will be standing on will be plenty strong enough.

 

FYI - one of ours sits on a wooden frame suitably waterproofed

Thank you. I thought I probably was but have never fitted a modern stone shower tray before. I have done plenty of old fibreglass ones in the past but they were so cheap I didn't care if I got it wrong. This one, was not.

[Nickfromwales]

1 hour ago, andreas said:

I am a bit confused by the instructions on the resin stone shower tray we have bought. 

It says to lay a bed of mortar 15mm thick over the entire base, lay the shower tray on top and "manipulate tray a minimum of 5mm into the bed to ensure the underside of the tray is FULLY supported". Emphasis on fully.

The tray is a non-slip one, long and has quite a steep angle, with ribs underneath. The ribs are 20mm thick at the "shallow end". If I only manipulate the tray 5mm into the bed, only the ribs are going to be supported, not the entire underside of the tray.

Am I overthinking this? Do they just mean that all the ribbed bits need to be bedded in 5mm into the mortar? 75% of the mortar is going to be doing sod all on that basis 😅

I would never use mortar, and have only ever used flexible tile adhesive.

 

FYI I have been 'at this' for over 30 years, and have done more wet-rooms than I can remember. Not ONCE have I used mortar for this and I cannot fathom why they are still suggesting you do so in 2026! Mortar was used "during the war".

[andreas]

3 minutes ago, Nickfromwales said:

I would never use mortar, and have only ever used flexible tile adhesive.

 

FYI I have been 'at this' for over 30 years, and have done more wet-rooms than I can remember. Not ONCE have I used mortar for this and I cannot fathom why they are still suggesting you do so in 2026! Mortar was used "during the war".

Thank you - that interests me.

How thick a bed would you use if you are using flexible tile adhesive? Any recommendations for which one?  Do you use quick setting or normal, and how wet do you mix? The base is 18mm ply on raised joists. 

LOL sorry that is a lot of questions but I've not heard of that before.

[Nickfromwales]

5 minutes ago, andreas said:

Thank you - that interests me.

How thick a bed would you use if you are using flexible tile adhesive? Any recommendations for which one?  Do you use quick setting or normal, and how wet do you mix? The base is 18mm ply on raised joists. 

LOL sorry that is a lot of questions but I've not heard of that before.

The thickness of the bed is based on how well you laid the ply, in a nutshell.

 

The stone you refer to, is probably 'stone-resin', is that correct? Either way, the biggest issues with this type of install is failing to clean (decontaminate) the underside of the tray; most of these are either covered in mould release agents (or remnants of), or are dust-covered from the underside being ground flat prior to dispatch.

 

Both of those will stop anything sticking well to the underside, so needs addressing. I recommend using some tile adhesive on a cotton cloth and using it like Ajax to 'scour' the underside. This will clean the tray and leave it ready to stick the adhesive to excellently.

 

I would mix this to a consistency similar to clotted cream, and if you're a novice then I would use a "standard" or "extended" set adhesive vs rapid, as this will give you an additional hour or more to feck about with it. Rapid will start to cure more or less as it hits the ply. I use Ultra tile adhesive, but Bal or Mapei are all very good gear; much of a muchness I am sure.

 

You need to SBR (PVA on steroids) the plywood before installing the tray btw, so the tile adhesive sticks to that like shit to a blanket. You should do this around an hour before fitting the tray.

 

The 2 or 3 sides of the tray that are abutting the walls should be filled up with clear CT1, NOT SILICONE, and that will complete the install for a trouble-free life.