Hi folks. We're having difficulty with a tub install and could use some input.

We purchased an MTI baths Andrea 11 airtub and intend on mounting it into a frame with deck material above the flange of the tub. The deck has been built and the tub is perfectly level when sitting on the frame.

However, the tub sits on 2-3 inches of foam under which is a thin sheet (maybe 3/8 to 1/2 inch) of plywood/particle board. The whole system is called a "pre-leveled frame". The undersurface of the plywood rests about 1/4 inch off the concrete slab, so there is a gap under the entire pre-leveled frame. Basically, the tub is only supported by the flange sitting on the wood frame built by the carpenter.

Our contractor wants to use expandable foam to fill the gap between the bottom of the plywood and the concrete slab. The manufacturer has stated that this might be ok, although there is a risk that it will cause the tub to rise unevenly and no longer be level. We are pushing for mortar. However, I am confused because of the rectangular piece of plywood that sits under the 2-3 inches of foam underneath the acrylic tub. Would we set the plywood right on top of several dollops of strategically placed mortar or would we have to have a uniform layer of mortar on which to place the tub.

Any thoughts? We're kind of desperate here, as our contractor seems to be more interested in just getting the job done, rather than getting it done correctly.

Any advice you can give would be most appreciated.

Thanks!

CM