Mortar bed under 1-piece Aquaglass fiberglass tub/shower

[smsucigarguy]

I am about ready to install my 1-piec Aquaglass tub/shower as part of a complete bathroom remodel. I know this has probably been done to death, but I'd like to solicit some advice about whether this tub/shower needs a mortar bed underneath. It is supported by (2) 2x4's on the underside that are attached to a piece of what looks to be 3/4" OSB. It appears that the entire weight of the tub rests on these 2x4's, hence the only place that looks to benefit from mortar would be under them. The instructions that came with the unit are silent about using anything underneath. I have attached some photos of the underside (tub laying on side). Please let me know what you all think would work best a trouble-free install. Thanks!

[dlarrivee]

It's up to you.

I would use mortar anyways. Some of the tubs don't sit level when resting on the apron and the lumber feet they're built with, and may require some shimming.

I prefer a less hollow sounding tub, with less flex in it, so I would try to use mortar under the tub regardless.

[Terry]

If the tub sets flat on the floor, it can be slide into place without anything.

If not, then mortar for sure. And while you are at it, it doesn't hurt to support the plywood sections with a few piles of mortar.

Some homes are level, and some aren't. In an older home, I check the floor and walls with a level, and shim as needed. That may be with something like mortar or with shims.

[jadnashua]

It's misleading from the angles...when sitting upright, do the wooden supports sit flat on the floor? If not, I'd want to support them and maybe at least part of the bottom. Otherwise, you're relying on the strength of the fiberglass resin to hold that wooden structure to the tub. Keep in mind, a gallon of water weighs about 8#, then throw in a good sized body or two, and you're talking about a fair amount of weight. Doesn't hurt to support it fully down to the floor.

[smsucigarguy]

It's difficult to see underneath the unit to tell if the supports are flat on the floor. There is not any movement when it's put into position and I step inside. However, I certainly understand the considerable stress caused by the water weight + user's weight that is only transferred to the floor through those two piece of lumber. I'll put some thinset down and rest a little easier.

I've been working on renovating the only bathroom in our house since the 23rd of December. We've been staying with my in-laws since then. Certainly, the motivation to get this over with is an understatement. I appreciate everyone's advice!

[jadnashua]

If you use anything, thinset isn't the thing to use! By its design, it is only intended to be a thin layer, and to bond things. You need something structural, either deck mud (sand mix) or structolite, or some people use plaster of paris (I've never used that).

While it is sitting on its side, it should be fairly easy to tell with a straightedge if the wooden structure is designed to be sitting flat on the floor or not. Also, if there is any deflection in the fiberglass, eventually, it will get micro fractures and start to look funky. Eventually, it could fracture, but that often takes a long time, if ever.