# Can I support a fiberglass tub with a layer of 1" thick foam board between a concrete subfloor and mortar base?

I'm trying to maximize the headroom in my basement shower. I originally planned on placing the tub on top of the subfloor and sleepers that make up the rest of the floor, but then I realized I can add about 3 inches by not doing that. The pipes actually run half in-half-out of the slab, which I "framed around" with sleepers - see here. The tub needs a mortar base, so I thought I can just put down some plastic to separate the mortar from the concrete floor and install my tub mortar base and tub directly on top. But now I'm thinking that the concrete floor is about 65 degrees in the winter, that's not exactly ideal to step on with the heat transfer.

I'm thinking I can "frame around" the pipes on the floor with 1" foam insulation (on top the 6mil plastic) and then apply my mortar directly on top of that layer to support the tub.

I think the thickness of the mortar (3" at least) should distribute weight sufficiently to support the tub while walking in it. It should be relatively solid, not like I'm walking on foam. The foam is quite rigid anyways. Under the load of a full bath tub though, should that be fine? Or should I add 1" sleepers between the insulation and the mortar, to provide a solid path to the ground? Would I need a layer of OSB over the sleepers for them to be effective, or is the mortar good enough? Or maybe I should just skip the foam all together?

## 1 Answer

A typical bathtub holds around 40 gallons, which equates to 8 pints of water, and since a pints a pound the world around, that means 320 pounds of water+you in the tub plus whatever part of you is above the waterline.

If we estimate the surface area of the bottom of the tub at approximately 4' by 1.5' that's about 6 square feet or 864 square inches.

That means that at most the pounds per square inch on the bottom of the tub will be about .4 - and that's ignoring that part of the weight will be carried by the sides of the tub around to the rim.

Then there's the mortar. 1 cubic inch of mortar comes in at about .08 pounds and 3 of those vertically means .24 pounds bringing the total pounds per square inch on the foam, maximum, to about .64.

So the concern here is - will the foam take the weight - and the only way to find out is to determine if the foam is able to withstand pressure of .64 pounds per square inch without compressing. And also - will the foam's strength deteriorate over time.

These are answers you'll have to find out from the manufacturer. (Although you can test it yourself - use weigh the head of a hammer resting on a scale while the handle rests off the scale and that is probably > 1 pound... then rest it on the foam) But if the foam is rigid enough to take that kind of weight, you'll probably be ok.

• I never even thought to apply math to the problem. I actually thought more of "Do people do this or am I crazy?" - but your post puts it in perspective. Considering some people use foam and 2 layers of plywood as their entire subfloor, I think I should be more than alright doing this structurally. Maybe I'll add some plywood over the foam to prevent the mortar from cracking as extra precaution. Commented Aug 9, 2014 at 18:57