I'd pour concrete. Simple, solid and no water problems to worry about. Otherwise, standard p.t. framing on 6 mil plastic, but just under the 2x8s.

"standard p.t. framing on 6 mil plastic"

I need a transalation for this into "weekend novice contractor" language.

Also, the sauna will have very little water in it, so I didn't think concrete was necessary. The drain is just precautionary in case excess water is thrown on the rocks for steam. The sauna is 6.5 x 6.5, and will need 8" of depth, so I'd prefer not to mix and haul concrete for this. The tiled shower, however, will need concrete.

Thank you.

(And I accidently posted this twice thinking the first didn't go through)

Sorry for the jargon.

P.T. = pressure treated

For further protection from the moisture that is always coming up through the floor, place the p.t. joists on strips of 6 mm thick polyethylene sheeting. It comes in black rolls about 10 in. wide.

Instead of the traditional mud base for your shower, you can use pre-formed foam bases and drains from Schluter.

The system is pricey but is easy to put down and no bags of heavy materials to haul in.

How far apart would you space the p.t. floor joists?

For best results, 2"x8" @ 12" on centre.

Our shower size is going to have to be 3x4' and Schluter only offers the pre-formed bases in 4x4'. Can you cut the base to fit the opening (as long as the slope still allows water to the drain)?

According to page 11 of the Schluter Shower System Installation Handbook, you can cut the base.

Even if you choose not to use the pre-formed base, you can used the Kerdi membrane over everything.