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Old 06-28-2019, 06:58 PM   #1
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Mortar under tub?


Hello,
We are installing a new air tub from the big box store.
It has five legs, with one in the middle, and sits three inches off the ground.
The instructions appear to be generic, not mentioning the legs, and call for a mortar base.
Do we really need to put a four inch high pile of mortar under it?
Thanks
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Old 06-28-2019, 07:24 PM   #2
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Re: Mortar under tub?


Always do as the instructions say, within reason.

When they wrote the instructions, they knew the legs were there, so no need to point them out.

The Mortar Base is there to help spread the load. Assuming you have a decent size tub that holds 40 gallons, that's 320 pounds. Add the weight of an adult, say 180 #'s that's 500 pounds of weight in a very small space.

If you don't have the mortar there, you have 5 points bearing 100 #'s each. Probably more on the center leg than the outer ones. By having a mortar bed you spread out the load.

The reason why you want 4" while the legs are 3" is to allow for spread once there is weight there. Wait until everyone is home before you put the mortar down. You want them to be in the tub with you. Unless you can attach the drain and fill it with water right away. And, put yourself in it. And stay in it for about 20-30 minutes assuming you are using quick set.
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Old 06-28-2019, 07:33 PM   #3
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Re: Mortar under tub?


Thanks for the reply!
Instructions seem to be for a wide array of tubs, one set of instructions for at least 6 or seven models, so didn’t know if a huge, thick mortar bed was necessary for this one.
Will do!
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Old 06-30-2019, 07:57 AM   #4
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Re: Mortar under tub?


TBH I would want to have enough mortar to support any part of the tub bottom that's close to horizontal.
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Old 06-30-2019, 08:26 AM   #5
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Re: Mortar under tub?


Quote:
Originally Posted by ktownskier View Post
Always do as the instructions say, within reason.

When they wrote the instructions, they knew the legs were there, so no need to point them out.

The Mortar Base is there to help spread the load. Assuming you have a decent size tub that holds 40 gallons, that's 320 pounds. Add the weight of an adult, say 180 #'s that's 500 pounds of weight in a very small space.

If you don't have the mortar there, you have 5 points bearing 100 #'s each. Probably more on the center leg than the outer ones. By having a mortar bed you spread out the load.

The reason why you want 4" while the legs are 3" is to allow for spread once there is weight there. Wait until everyone is home before you put the mortar down. You want them to be in the tub with you. Unless you can attach the drain and fill it with water right away. And, put yourself in it. And stay in it for about 20-30 minutes assuming you are using quick set.
and how much does all this concrete weight, all on 1 or 2 joists.

I would fill most of the space with built up foam board and use a few inches of mortar to fit the shape of the tub.
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Old 06-30-2019, 11:57 AM   #6
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Re: Mortar under tub?


Thanks for replying!
The weight if all that mortar was our big concern too.
Yesterday, we decided to cut two inches off each, pvc tube, leg.
This also brings the tub to a, much more comfortable, 21 inches in height.
Iím sure this voids the warranty, but, oh well😬 we already did that when we tested it for leaks, and repaired ourselves, several sections of air tubing that were not properly installed onto their nipples and leaking. Was a lot easier than trying to return and replace, or arrange for them to try sending a service technician.
Itís now firmly embedded in mortar!
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Old 06-30-2019, 12:11 PM   #7
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Re: Mortar under tub?


If you have access to the lower part of the tub, away from where the piping for the air is, or if you are confident the air won't leak any more, spray low expanding foam around the tub.

This will help keep the heat in the tub.


I do agree with @Nealtw though. The rigid foam boards not only minimize the amount of concrete you need, they also help provide a thermal break.

When I set my last tub. It was a whirlpool with a heater on the pump to maintain the water temperature. I put down a thin sheet of rigid foam. (That was all I had on hand.)

Then I sprayed the bottom of the tub with some spray foam and let it set while I mixed the mortar. I trimmed off the big bumps of foam and then I set the tub in the mortar. attached the drain and started filling the tub. I put some boards across the tub and set some boxes of tile on the boards to help it set.

Not sure if the foam helped in any way, but it made me feel good.
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Old 06-30-2019, 12:39 PM   #8
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Re: Mortar under tub?


Thanks again ktownskier!
We’re all done with the mortar, it’s set, after shortening the legs a bit.
Better height for us anyway.
Got the air tubing leaks fixed. Sure glad we leak tested before installation, the tubing was
Curled under where glued on several fittings and leaking water.
You couldn’t see them, they were all on the backside of the fittings!
Also on backside of tub where repair would have been impossible without total tub removal
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Old 06-30-2019, 07:02 PM   #9
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Re: Mortar under tub?


Sorry I'm too late for your project, but maybe someday someone will search and find this thread. Hopefully the ideas below may be of help:

Back in the "Day" when fiberglass tubs first came out, plumbers would often use Pearlite instead of mortar. It is much lighter, thus easier on the joists. You can lay a sheet of rosin paper, wax paper or roofing felt over the sub floor if desired.

I used it at my home and years later had to take out the fiberglass tub. The tub wasn't stuck to it and the pearlite bed came up in one piece that weighed about 20 pounds. (Rosin paper was under it)

Bonus- It can be crunched up easily into tiny pieces on the driveway with a 2x4. The fragments make a good soil amendment to modify clay soil and it will raise the pH in the garden.

You can get pearlite at plaster supply houses. It's used as a scratch coat under finish plaster. (If you're in Detroit area, Sabiston Supply on 8 & Groesbeck sold it when last I checked.)

Plan B-
A plumber I know uses spray foam. He lays a sheet of visqueen type plastic over the foam bed, then sets the tub before it cures. He uses the "Big Gap" type. The excess oozes up the side walls & can be cut away if desired. The plastic sheet is so the tub can be removed later.
The only downside is that foam out-gasses for a while, so it's not appropriate if you have sensitive people or pets (especially birds) in the house.

Hope The Rest Of Your Project Goes Well and is Fun!
Paul

Last edited by Bird Doo Head; 06-30-2019 at 07:10 PM. Reason: My Spelink Is Aufwl. I Reely shuld have payed beter attentin in schol.
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