Pulled up the original cast iron tub from the 1st floor bath. A hole was busted into the slab to insert the drain. Not sure if original owners had a leak or if slab was busted to accommodate the tub drain. Hole is about 12 x 10 and it seems as if I were to put mud down before seating tub that this mud would simply settle into drain hole which, I don't think I want.

Can I simply use adhesive under tub feet?

A lot of people use drywall mud. Spread a 1/2" coat on the floor and set the tub in it. Get in the tub to put pressure and set the tub. It doesn't run very much so it shouldn't run into the hole. Can't you fill the hole with something? Dirt, gravel, quik-crete?

Thanks! That's doable. As for filling the hole with something... It's the only life-line to the drain - no other access for this drain - so putting quik-crete in it doesn't seem like the right thing to do... pee-gravel or sand might work.
Thanks again!

There are all kinds of different substrates to set your fiberglass tub in. My favorite, is STRUCTOLITE perlited gypsum plaster... stuff dries overnight to rock hard.

Brian (ma0641), I don't think most people use the drywall mud from a bucket... they use drywall mud mixed from a bag, right? The stuff from the bucket never hardens completely and can create a mold covering over it so just want booismycat to either use drywall mud from a bag or structolite. The pea gravel in the hole is a great idea!

Finally, booismycat, be sure to check with manufacturer instructions as some tubs, like the american standard americast tub, will actually void the warranty of the tub/shower if a setting substrate is used... ;)

Questions? Let us know...

Mark

Sorry 064 but I just have to disagree with your method of bedding a tub.
But first let me address Mycats 12 X 10 hole. This is called a tub dap-out and has to be there to connect, see image) the tub waste to the trap raiser. He is bang on about the hole.
As for bedding a plastic tub first check under for factory supports but you don't Spread a 1/2" coat on the floor and set the tub in it.
You first set and connect the tub. Then you puddle mortar or cement under the tub from the floor to the base of the tub being sure to puddle around the drain,

Get in the tub to put pressure and set the tub
Do not get into the tub!!
Let the cement cure and set up. You want as much support as you can get and squishing down the cement with your weight will leave a gap. More questions? I'm as close as a click, v Tom

I may be missing something but I thought the question was solely what to do with the dap out and never about how to set a tub. Booismycat please restate your specific question. PS I use Structolite or thinset to set shower pans, I think ma0641 may have seen Structolite and confused it with drywall compound, not the same stuff and Marks is correct, drywall powder from a bag might be used by some folks. This stuff drys rock hard in 20 or 45 minutes and can be confused with lightweight drywall compound

Last one I saw done was from a 5 gallon pail. This was a corner jetted tub. 1/2" will dry fairly fast

Hi,

U need to bust slab open to rough in the drain for waste and overflow. What I would suggest is backfill the hole with drain rocks to within 1 to 2" from top of slab then mortar the rest to top so if there is a leak on your waste and overflow it does not get saturated around the hole. If u decide down the road to jog the drain u should be able to break the mortar easily.

If u want a solid tub surface, lay mortar not unless u want to feel like it is sinking or bouncing.

The question is whether I need to set a fiberglass tub in some type of mud when it is installed directly on the slab - 1st floor. Am looking for clarification - sounds like mud is required unless specified.

Drywall mud from a bag can be used.

Thinset can be used (only takes long time to dry).

Structolite perlited gypsum can be used.

The way I bed a tub is to measure the space from the bottom of the tub to the floor... can be between 1-3" depending on the tub.

Then I trace out the "footprint" of the tub onto the floor and distribute the bedding substrate in the footprint of the tub. Here, add an inch more of bedding substrate than you measured between floor and bottom of tub (the end closer to the drain will need less, right?).

Then install the tub and squish the tub down into place. Level the tub from front to back and side to side and secure as recommended by the manufacturer. Any excess bedding substrate that squeezed out can be pulled up the sides for increased sound resistance and insulating (keeps hot water in tub hotter...longer).

As stated earlier, be sure to check the manufacturer's installation instructions.

That's how I do all my fiberglass tubs...

Mark

PS: Brian, I have never seen a tub that only needed a 1/2" of bedding material and anything thicker using drywall mud from a bucket and the substrate will never dry... like putting it back in a bucket, really!

Currently, there is no access to puddle mud after tub is installed. I agree with your "don't squish tub down". Question on mud is more clarification on preferred method. Instructions list two different methods - glue or mud.

It isn't necessary to fill a tub dap-out with anything. If as open hole disturbs you take roof cement and coat the sides and botom. Good luck, Tom

When I removed the old cast iron tub... I was amazed that hole was not square or formed. Looked like a hack job to me but, I guess this is common in the industry.

Thanks! Need to address plumbing in dap-out.. then will fill with pee-gravel. Tub instructions provide two install methods - mud base or glue feet only. Seems like mud is the preferred. Thanks to all of you!

Once tub is in place again... won't disturb me in the least. Was just surprised that it looked the way it does but, guess this is common for the industry. You guys have put me straight - Thanks!