Options for raising shower tray?

Hi,

This is a ground floor bathroom with a concrete/screed floor. I need to raise the shower tray by 50mm to get the minimum fall. Tray manufacturer say min 8mm of sand/cement under tray so that is 42mm. If I use a 18mm OSB3 base that is 24mm to floor. Could I scree 24mm and then set the OSB on that? With long torxs to the concrete sub base. Or would I be best to dig out the existing scree so I can set timbers to get level "joists" to screw the OSB3.

Suggestions?

Or I suppose I could scree 42mm and do away with the osb but the edge I think would be suspect, likely to crumble. With OSB3 then the edge under the tray would be stronger. Or I suppose a concrete slab 42mm thick is another option.

What minimum fall do you need, is it for the waste? Are you not able to channel the waste?

Build a wooden plinth, much easier to get level and secure long term.

Sand and cement is such an old way to install shower trays these days IMO.

Yes I need to raise the tray level to get the waste fall and the trap height. I'm already joining below floor level, into to a soil manifold. That is concreted in so I'm not digging that out. With a knuckle bend my pipe centre line is 59mm below floor level. I have 2.1m run at 18mm/m so that is 38mm, now 21mm below floor level for pipe centre line. Trap needs a hole in floor about 45mm deep. Trap is 60mm from underside of tray to pipe centre but I think that might be with the thin seal for thicker trays so need to allow extra 5mm, say 65mm, hence min of 44mm above floor for shower tray base.

BTW maker says uses sand cement 5:1 so no choice in that if I want the lifetime guarantee. But that would be on top of a wood base, if I went that way.

I'm leaning towards casting a concrete slab. Bonded and with 10mm ballast should be ok this thin. If I allow 10mm for mortar and say I want 50mm then it would be a 40mm slab. Doing it in wood is awkward. 18mm OSB3 only leaves 22mm to make up and timbers 22mm think would be very bendy. I would think the best option would be 18mm OSB3 for base and then cut 18mm OSB3 bearer strips to glue and screw to underside of base, then set these on bedding strips of mortar for levelling (~4mm of mortar but could be more if tray is on 8mm bed and/or go up a few mm). I think the concrete slab sound more solid.

Who is the maker of the tray? I come across more tray's with movement problems using sand and cement than anything else and I've yet to find a manufacturer that would invalidate a warranty if it was placed upon a wooden plinth and supported over its whole footprint but ultimately that's up to you.

Your fall is actually 1mm on the wrong side of the min limit to ensure the waste is self cleaning but that's just semantics, you will need to make sure the tray waste will drain as fast as the water entering it.

2.1 x 18 = 37.8mm which I've rounded to 38mm but it will likely end up a bit higher just to be sure I've above the limit. Tray is made by premier. Hmm, I'm a cynic and I think if I didn't follow the instruction then they would wriggle out of a claim. Shower trap is rated at 42l/min and this calculator gives 45 l/min for 18mm/m and 40mm pipe (36mm internal). Can't see the shower exceeding that .