I'm about to rip out our shower cubicle and start from scratch. It's currently a fully tiled in recess, with a 760x760 shower tray. The tiled recess is 750mm wide and 740mm deep (from front to back, with a single door onto the front.

I'm removing the wall boards (normal plasterboard) and replacing with aquapanel, as i'm putting up heavy quartz tiles. The stud framework from either side and front to back of the recess is 810mm wide. Would you think it is better to have an 800mm x 800mm shower tray in there, with the wall boards overhanging it, rather than a 760mm x 760mm shower tray where the wallboards won't overhang the tray before tiling, but will slightly overhang afterwards?

With the 800mm shower tray, there is 68mm all around from the edge of the tray (Mira stone shower tray) to the start of the scoop that you stand in. The walls would overhang the tray by about 26mm when fully tiled (12.5mm for the board, 10mm for the tile, 3 or 4mm adhesive), giving a 42mm gap between the tiled edge and the scoop out.

I think this could be a lot better for protecting against water ingress if the gap where the wall boards meet the shower tray are well siliconed before tiling, and the tile bottom to tray is also well siliconed, as opposed to using a 760mm tray where the tray just overlaps the outer edge tiled wall by 5mm all around. The 800mm tray also gives slightly more standing space in the shower.

Am I right in thinking the 800mm option is better, or is there a technical reason not to do it? I'm asking as a competant DIY'er, rather than a pro.

I'd go for the 800mm tray if I've read your post correctly

As it is, the current tray and shower door are set back 60mm from the outer corner of the recess (if you imagine a "U" shaped recess, where the shower doesn't quite fill the recess, it's 60mm short), if I use the 800mm tray, the door and shower tray will come out flush with the wall corner and look better too.

Just making sure there wasn't a technical reason not to have the boards over the tray, couldn't think of any reason why not, but best to check. Thanks again Doug.