Floor help needed

Good grief. So, some friends have just repurposed an old school cafeteria, and turned it into the beginnings of a black box theater. The learning curve has been steep. I've been asked to design and build their first true set (they will open with a small, 3 actor show and a few flats and lights, nothing more.) My set will be something more ambitious; a two-story thing, Victorian era. The set itself is always a challenge, of course, but to be honest I've never built one on a hard, unforgiving, tiled surface.
Further, the director has asked me for a floor that's NOT the recently painted tile, but something more like a stage. Originally she wanted a 30 x 20 foot stage build 20 inches high with the set on top of that. I told her that I didn't feel comfortable doing that; the set itself is challenge enough, and to build both stage and set is probably dangerous, not to mention expensive and time-consuming. And, of course, time is of the essence. I need ideas. I considered simply laying down plywood, perhaps with an acoustic underlayment or paper or something, but then how do I join the plywood? I can't glue it. I would almost have to join it with some sort of thin, slotted steel bar, maybe even recessed into the plywood? She's looking for a way to distinguish her play space, while at the same time giving her actors a more forgiving surface than painted tile. Plus there will be a lot of furniture moving, and scuffing, and building.... you get the idea. I'm floundering at the moment. Open to any and all suggestions.

tongue and groove plywood but like you said if you don’t want to build a sub floor and legs at 20” then maybe go with Marley. However if the tile has grout lines the Marley might do some dipping.

Thank you. I learned a few things this hour.

One think we do when we're performing in the round is to make a very thin floor by laying down protective sheeting then building a thin lattice/grid on that with thin wood then attaching the "stage" floor to that. The grid supports the sheets of ply and keeps them off the main floor, so it has a little give and feels right.

Thank you. Another option with considering. Have you built any heavy platforms on that deck, were there any problems you had to overcome?

UPDATE: (just riffing for the record.) Good ideas are being offered, both here and from other sources, but I can't get over the issue of stability for tall (8-9 foot platforms, post and beam style). Building upon the flat of the floor, or on a surface flat on the floor, is the most stable and defensible from a safety standpoint. On the other hand, building the stage and the platforms as SINGLE UNIT has its structural benefits, too, including the natural strength that comes from joining all of the fixtures into a single piece.
I've considered taking the platform posts THROUGH the stage to the floor, but this means that I might be wasting valuable plywood by virtue of cutting out slots for the posts. No decision made, although I'm now exploring building a frame out of standard 2 x 4 lumber, the pieces turned on edge (as opposed to faces flat on the floor), and that requires no legs, then reinforcing the downward pressure points with additional lumber. Of course, the same could be done by building the deck frame by laying the lumber on its face.
So far, though, tongue in groove layered on top of a thin .08 sheet of underlayment looks the most promising to date: sturdier, less expensive, less time consuming.

Chairman Moe said:

Thank you. Another option with considering. Have you built any heavy platforms on that deck, were there any problems you had to overcome?

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No, anything like that would be built first on the floor directly, standing on suitable surface protection, then the lighter weight floor would be built around it, but most things are low height set and props such as a kitchen for example.

Many years ago a large platform was built up into the air from steeldeck and scaffold, then the rest of the ground based set was built around it.

If I remember correctly, my stage is two layers of 3/4 plywood, and a layer of Masonite on top for a really nice smooth surface. Alternate the direction on layers and screw it all together.

Faced with a brand new blackbox space with a concrete floor, we laid down sheets of 3/4" or maybe it was 1" blue Styrofoam insulation then 3/4" plywood and used powder-activated fasteners to attach to the concrete.
Topped it with good quality 1/4" hardboard screwed down. Been in use for over 40 years with zero issues.
Probably has at least 60 coats of paint on it by now. Used exclusively for plays and as a classroom.
Doesn't sound like it would last but it obviously has.

microstar said:

Faced with a brand new blackbox space with a concrete floor, we laid down sheets of 3/4" or maybe it was 1" blue Styrofoam insulation then 3/4" plywood and used powder-activated fasteners to attach to the concrete.
Topped it with good quality 1/4" hardboard screwed down. Been in use for over 40 years with zero issues.
Probably has at least 60 coats of paint on it by now. Used exclusively for plays and as a classroom.
Doesn't sound like it would last but it obviously has.

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That sounds like a fantastic solution.