Three, all before the floor exists.

The minimum floor build-up height.

The depth of the water seal.

And where the waste pipe must run.

All buried once the screed is down.

Setting the body to the finished floor.

Level along its whole length.

And at exactly the right height.

A millimetre proud and water stands.

A millimetre low and the tile edge shows.

Because floor depth is scarce.

A full-depth seal will not fit.

So the seal is reduced to suit.

It works, but with less margin.

Which matters if the shower sits unused.

Drain odours reach the room.

Evaporation empties a shallow seal faster.

In a guest or holiday bathroom especially.

Running the shower refills it.

Or specify a trap seal valve alongside.

So the trap can be cleaned at all.

It lifts out through the grating.

Without disturbing anything buried.

A sealed body cannot be serviced.

Which becomes a problem, not if but when.

Yes, and it is often skimped.

A long body bridging soft screed settles.

After everything above is finished.

Bed it properly along its length.

And re-check the level before tiling.

At the same time as the body.

It fixes where the waste can run.

Horizontal outlets suit shallow floors.

Vertical ones suit a slab with depth below.

Match it to what is actually under the floor.