In the process of updating our bathroom and planning to replace our existing relatively deep square step-into shower cubicle with a rectangular walk-in shower, using a low profile tray.  Have been looking at the Mira Flight 1400x900 40mm tray which comes with a Vortex waste.

Bathroom is on a wooden floor and with the old shower we’ve never had any problems with the fall, with the pipes being set just into the top of the joists. With the new tray the pipes would now have to go through the joists and the waste has to be in either the left hand corner or centre of the 900 width, which would need a 2.5 meter run of pipe under the floorboards.  (If we placed the tray with the waste at the walk in end we would need only a 1 meter run but I’d rather the waste was at the wall end under the shower head.)

The Mira spec says that a fall of 50mm per meter run is required to prevent risk of flooding the low profile tray, and we don’t have that.

(1) Do all low profile trays require that amount of fall per meter run?
(2) Could we safely get away with less than 50mm per meter if we went    for the Mira tray, and
(3) are there limits on how much we can cut into the joists?

Any help or advice would be greatly appreciated.

Notching joists for this size of pipe is not normally practical and certainly not good practice. The best bet, and may give you your fall is to drill holes through the joists as near the centre of the joist as possible and then feed the pipe through the holes which can be a bit of a pain but normally possible. Plus I wouldn't bother about fitting the waste at the back of the tray these traps fit flush with the tray and are designed to be walked on.

Do you specifically need/want the tray sat on the floor?  I'm reasonably sure mira do riser kits for these trays

As I said what you can do depends on the size of the joists plus unsupported span length

I think the decision has been made to create a raised floor all along the wall where the shower will go.  We can't go out of that wall because there is a roof in the way from a little bay window below and apart from working blind, where the pipe would emerge he would hit the lead tray, (apparently!)

Unfortunately the joists run the wrong way for the wall we can go out through.  He'd have to drill through about 7 joists and from the muttering I gather he'd exceed the limits if he did that.  Also, we can't place the shower along that wall because it has the window in it, so raised floor it is.

Thanks to all for your comments and advice.

Not easily - cooker hood in the way.

The honeycomb give it the strength..

You should have instructions with it. But we used to mix a weak dry ish sand and cement then spread it over the floor and work the tray into it.

About an 1" plus depth, if it's going on wood seal the wood first.

The tray only needs supporting on the high points.

The PVA stops the wood drawing moisture out of the cement, and provides a key.

You just lay a good layer of stiff ish muck, run your trowel through it to make it wavey, lightly lower the tray on it and settle it in.  You don't need to ensure all the voids are filled.  Don't over settle it.