ᐅ Bathtub renovation: which materials to use?

I need some advice again. We are going to install a Repabad acrylic bathtub. This tub will be placed in a niche in the bathroom, meaning one long side will be exposed, the short side near the foot end will be against a solid wall, and the other long side as well as the head end will have a ledge about 20cm (8 inches) wide. The tub will not have a support frame but will stand solely on feet on the raw subfloor (new build with a concrete slab).

The woman from the bathroom studio, who has many years of experience, recommended building a 40cm (16 inches) drywall frame around the 20cm (8 inches) ledge, then supporting the bathtub edge up to the rim with plasterboard or Wedi panels so the tub can rest on it. The tiles will be flush, so the tub will essentially be clamped on three sides.

I asked the plumber and the drywall specialist on site today. They didn’t understand this approach at all and said they have only seen versions with Styrodur (extruded polystyrene foam) for years, which were then tiled. In those cases, the tiles would also clamp the tub.

Now my questions:

a) Which of the two options (Styrodur/drywall) is really more advisable here, regardless of cost?

b) Would the Styrodur option be stable enough so that if someone sits on the rim at the head end on an empty tub, the thin rim won’t compress the Styrodur and cause the tub to tilt?

c) Since the tub is clamped on three sides by tiles, but I read that when the tub is full, forces can occur that might slightly change its width – wouldn’t this cause the silicone seal on the long side to tear?

d) On the exposed side under the tub rim, a Wedi panel will be clamped and tiled – is damage to be expected if a person of normal weight (60-80kg) sits on the tub rim there?

The bathtub supports made of polystyrene foam or similar materials are perfectly suitable for sitting on the edge of the tub, and as you have already been told... nowadays this is rather the norm.

Uh, sorry, who told me that a bathtub support frame is suitable for this? That was never mentioned; it should stand on feet and *not* be installed on a support frame. The bathroom showroom explicitly advised against using a bathtub support frame for acrylic tubs. They have occasionally renovated bathrooms for customers and must have seen some really poor installations... They place the tubs on feet with a drywall support structure around them if needed, otherwise, the tubs are fixed directly to the wall with mounting rails, and since then, their customers haven’t had any problems for years.
Therefore, I would like to return to my questions, especially the first one.

rabudde schrieb:
Uh, sorry, who told me that a bathtub support frame is suitable for that? That wasn’t mentioned at all; it’s supposed to rest on feet and *not* be placed in a support frame.

The one here:

rabudde schrieb:
Today I asked the plumber and the drywall installer on site. They couldn’t understand it at all, as they have only seen versions with Styrofoam supports for years, which were then tiled. Even there, the tiles would clamp the bathtub.

Apparently, Mycraft designed it that way, and that’s how I understand the statement as well.

Regarding your questions:
a) We have Styrofoam-type supports and no issues so far; which is better, no idea.
b) I weigh about 90kg (200 lbs) and haven’t caused the bathtub edge to bend or tip yet.
c) On our installation, only two sides are clamped; so far no cracks in the silicone. And if there are cracks, it’s just a maintenance joint that will eventually crack anyway due to settlement in the new building.
d) Sitting on the bathtub edge hasn’t damaged the tiles either.

If you absolutely have to choose acrylic, I would generally recommend installing acrylic tubs on tub feet with tub anchors.

You see, that’s the problem. Anchors would be ideal, but since there is contact with the wall only at the base side, it is suboptimal.