I think the British standard for overboarding prior to tiling is 15mm wbp.
I’d always go with 18mm wbp ply and then staggered wedi board over the top. Screw the ply at 200mm centres.
Don’t use shuttering/spruce ply. Get quality hardwood wbp/marine.
If the floor boards are dodgy, or you want the depth, take them up and upsize the ply to inch/25mm. Leave a 3mm gap along edges and joints for expansion. Also seal all edges.
Use a thin bed of tile adhesive on the wedi board to fill any gaps and also screw it down.
Is it only that edge where the gap is? Looks like water has been getting out of the shower seal and on to the floor (as evident by scale, could also be from body when exiting shower). If the silicone went and then water got under the silicone, it could have them soaked in to the 6mm and cause it to deteriorate/rot, potentially causing the gap.
Looks like a redo. If it is tiles on to 6mm ply on floorboards, it’s bad workmanship. Even my dad wouldn’t do that.
I’d always go with 18mm wbp ply and then staggered wedi board over the top. Screw the ply at 200mm centres.
Don’t use shuttering/spruce ply. Get quality hardwood wbp/marine.
If the floor boards are dodgy, or you want the depth, take them up and upsize the ply to inch/25mm. Leave a 3mm gap along edges and joints for expansion. Also seal all edges.
Use a thin bed of tile adhesive on the wedi board to fill any gaps and also screw it down.
Is it only that edge where the gap is? Looks like water has been getting out of the shower seal and on to the floor (as evident by scale, could also be from body when exiting shower). If the silicone went and then water got under the silicone, it could have them soaked in to the 6mm and cause it to deteriorate/rot, potentially causing the gap.
Looks like a redo. If it is tiles on to 6mm ply on floorboards, it’s bad workmanship. Even my dad wouldn’t do that.