Do I cement Hardy backer board to the floor?

[ColinM50]

In the process of ripping out the bath and replacing with a shower. The shower tray will be on a plinth to get enough drop for the waste so my plan is to build up a 5 or 6 inch frame, for the shower tray and put the hardy backer board on that frame then the tray on top

Now the Q is, do I need to put say 12mm ply or OSB on top of the frame before putting the HBB down and do I put cement down on top of the HBB before placing the shower tray?

Advice I've had from tile supplier and tray supplier is conflicting. One says yes t'other says you can but probably not necessary.

What would/did you do?

TIA

[CoolHands]

I don’t know anything but I thought hardy backer board was just for the walls. Not for a shower tray to sit on? I think that’s a waste.

[OutInTheShed]

In my view the important thing is to secure the floor, walls and tray together to reduce movement which stresses the sealant.
Whatever the tray is sat on needs not to flex significantly relative to the tiling or whatever.

I'm not clear exactly what the OP has in mind, but cement doesn't sound helpful.

[ColinM50]

Thanks all, when I said cement, I really meant waterproof glue. Think I'm now going to build a 5x2 structure on the bathroom floor to give me the height for access to the drain, then glue/screw 12mm hardy Backer board to that and then glue the shower tray to that having painted the HBB with a waterproof sealant. Does this sound OK to you experts?

Have ordered it all from Topps Tiles, just can't remember the actual name of the glue or sealant.

TIA

[Aluminati]

I cannot see the need for the Hardie on your base frame. Your tray will be waterproof you would think .

[CoolHands]

I’m pretty sure they used 18mm ply under my shower tray, I think 12mm is too skinny and could have movement (flex)

[Baldchap]

Contrary to what I thought the manufacturer claims they're suitable for flooring under tiles.

[Vtekkers]

I did when doing my shower

had to build up for the waste so a 4in frame topped with a backer board and the shower tray laid onto the backerboard with adhesive no flex in the tray

[bennno]

ColinM50 said:

Thanks all, when I said cement, I really meant waterproof glue. Think I'm now going to build a 5x2 structure on the bathroom floor to give me the height for access to the drain, then glue/screw 12mm hardy Backer board to that and then glue the shower tray to that having painted the HBB with a waterproof sealant. Does this sound OK to you experts?

Have ordered it all from Topps Tiles, just can't remember the actual name of the glue or sealant.

TIA

I'd not do it like that, firstly id strip it all out first and then see which way your joists and drains run, with this information you might be able to select a tray with the drain in the right position to enable it to go directly on to the floor. We've managed this in the last 5-6 bathrooms we refurbished in various properties - always looks much nicer.

The Shower tray will have its own fitting instructions, the only one we've ever had an issue with movement and cracking was siliconed down. Most modern slimline trays are very low profile (3-4cm) and to comply with the manufacturers warranty need a full sand / cement bed to support them. Once thats in you can bond them to the walls to create a waterproof seal. Weve used hardy backer board or an equivalent for the vertical surfaces in shower areas as plasterboard can be problematic over the longer term. Weve also used the thinner version under floor tiles.

Hardy backer board is in 12mm thickness only or walls, it's in 6mm thickness for oversheeting the existing wooden floor in a bathroom under tiles, in which case it gets tile adhesived down, plus specialist screws. So you shouldnt use it under a tray by itself / unsupported as its not designed for that.

[dhutch]

Aluminati said:

I cannot see the need for the Hardie on your base frame.

Agree, just bed the tray down direct onto the frame/ply/osb.

[Paule2359]

When I did mine, I had to lift the tray up for the waste pipe, I used 4 2x4" lengths of wood running front to back then 12mm marine ply doubled on top, the ply was primed then the tray
was fixed on top using I believe tile adhesive.
The surrounding walls were covered in aqua panel.
The tray size was 750 x1000mm