Mortar under cast iron tub?

Hi all,

I saw a home improvement show where they were doing a tub install. After the tub was in place on the plywood subfloor and against the studs, they nested the tub in mortar by spreading it underneath the tub (the hollow part that doesn't show). The instructions for our tub installation does not mention this. Is this because my tub is cast iron? I have read a couple of threads where mortar was put underneath a steel or fiberglass tub, but I saw no mention of it under cast iron. Should I put mortar around mine? Is there an advantage?

Julie

The mortar does two things...it supports less than CI tubs so they feel more substantial (yours doesn't need it) and, it allows you to level the thing perfectly (yours might). It depends on what you need.

tub

Your tub probably has "feet" under it that set on the floor, but you also need a "ledger" board under the back rim to give protection against wall movement.

hj,
Yes, my tub does have feet and it has flanges for the three alcove walls. It is a Kohler Memoirs CI. I don't understand what you mean by putting a ledger board in. Where would it go and how do I install it? I am clueless. Thanks!

Jim,
Thanks for the input! It is close to level, but not quite. Do I shim the tub up and then put the mortar underneath?

With a CI tub, you could probably shim it...problem is, most stuff you could use would eventually compress. The mortar won't. I'd wait for someone else to give you some more info...I don't want to lead you wrong. My guess is the mortar would be sufficient to hold things up if you had enough surface area of the bottom covered. The leg that needed to be longer, would be supported by the mortar under the whole tub. Basically you lay the stuff down on the floor, set the tub down on top of it A tough feat since it weighs so much!), then smush it down until it is level and (knowing) as many feet as possible are on the subfloor and still have it stay level.

The ledger boards are a piece of wood - maybe a 1x2 or as big as a 2x4 if you can fit it in there, that is screwed to the wall across the studs and supports the edges of the tub from underneath. If you get them set up at the right height and level (and assuming the edges are actually straight), you could then smush the tub down until it just hit the ledgers all around, knowing you installed them level, the tub should be, too.

ledger

It is a 1x4 or 2x4 nailed to the studs right under the rear edge of the tub. Which you use depends on how wide the back ledge is. Most tubs will accomodate the 2x4, but some have a shallower lip and a 2x4 hits the curvature of the tub. If you put mortar down, which is basically useless with a cast iron tub, you don't have to worry about "smushing" it down because the weight of the tub will do that. If you just set the tub on its legs, it will be as level as the floor is. If it is not level, the ledger board will help considerably along with shims.

We are finally going to do this now. We are going to shim the tub to level and install the ledger board (a 2x4 fits fine). I have almost a full bag of regular thinset mortar (unmodified) that I have no use for. I was wondering if I could use this to setup under the tub or do I need to get a different type of mortar? This probably sounds really elementary. Thanks for being around to give me advice--it is invaluable!

Thanks again!