Bathtub installation - mortar bed vs. adhesive method

formosalilyJune 19, 2007

Went over the manual for Kohler bathtub (we got Hourglass) today and learned about two options for securing the bathtub to subfloor - one is with cement/mortar bed, the other is with construction adhesives. Would appreciate feedback/insights on the pros and cons for either installation method and recommendation on which method is preferred.

Thanks in advance!!

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I prefer the mortar bed method. My view is the mortar pile squishes and spreads out supporting a larger area, which is what you want, IMO. Did this with 2 Kohler showers and a Sanijet whirlpool tub and the support is rock-solid, no flex at all. Just plan the support framing to give the tub maybe a 1" gap underneath for the mortar bed.

    Bookmark   June 20, 2007 at 9:12AM
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I always used mortar. If your floor is level, you do not need very much.

    Bookmark   June 20, 2007 at 1:16PM
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Mortar. Full bed support. Easy to level the tub even if the floor is out of whack. You can lay a thicker bed if you need to raise the tub a bit, use a thinner bed if minimial height is desired. For drop-in tubs, set your bed, drop in the tub, then depress the tub until it settles to the deck. reason I can recommend it.'s easy. That's one.


    Bookmark   June 20, 2007 at 6:20PM
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A friend mentioned "Flexbond Thinset Mortar" to me. Can this type of mortar be used to prepare mortar bed for bathtub?

    Bookmark   June 25, 2007 at 10:14PM
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It could, but boy would that be expensive!!! You can use either a sand mix or mortar mix for about 3-5.00 a bag, when the Flexbond is about 35.00 a bag.

    Bookmark   June 25, 2007 at 11:02PM
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Thanks Bill. Is there any advantage using Flexbond instead of regular mortar mix since the price is so different?

    Bookmark   June 26, 2007 at 9:11AM
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I'll jump in because I think you may be under a time crunch.

No advantage. All you're looking for is for a fully conforming yet rigid material that will support the base of the tub.

Flexbond has additives that just are not required in this application.

You can use something like this type of Quikrete. It's a bagged material, a sand/cement mix, that will do the job at a fraction of the price. You'll need a few bags, and it's about $25-$30 less per bag than the Flexbond.


    Bookmark   June 26, 2007 at 10:27AM
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If your floor is level, you need very little. Read the installation instructions, most new tubs have 2 or 3 feet running the length of the tub that fully support the tub along with the framing along the top flange. They actually require no mud bed, but it is recommended because of the reality no floor is perfectly level. If your floor is level, you can trowel down a base of thinset or mortar to set your tub into. It will squish down until it rests on your framing.

    Bookmark   June 26, 2007 at 1:58PM
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The arcylic tubs I install are self supporting on the base and require no mud bed. The mud bed for me is usually only a notched trowel layed on a tad bit heavy. This compensates for any unlevelness. I then set the tub. This method works if you level the floor prior if out of wack. I really never load up the floor with bags of mortar to level. But that is just my way. There are many ways to do the same thing.

    Bookmark   June 26, 2007 at 4:33PM
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Tom and Pete-- some tubs will need to be bedded, and some won't. The ones that DO need it, there's no way around it. They're usually drop in tubs that if not bedded, the flange on which they end up resting will flex everytime someone fills and gets in the tub, and sooner or later that flange will end up weakening and ultimately, snapping off, if the tub isn't properly bedded.

    Bookmark   June 26, 2007 at 4:46PM
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Most better tubs are self supporting. You can easily tell the better ones from the cheap ones buy the way the bottom is constructed. Many acrylic tubs made now are self supporting.

    Bookmark   June 26, 2007 at 8:26PM
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Are the tubs with "support blocks" considered self-supporting? I would appreciate if someone can elaborate on what makes a tub "self-supporting".

The Kohler Hourglass bathrub we got does come with 5 support blocks at the bottom.


    Bookmark   June 26, 2007 at 9:40PM
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You need to read the manufacturers installation instructions....this will tell you if it is self supporting or not. In my new Bain tub I am installing, it tells you in the manual that the tub can sit directly on the floor and that a mortar bed is not required.

    Bookmark   June 26, 2007 at 10:57PM
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Like those that have said it before when in doubt, USE MORTAR. These plumbers don't want to do it because it takes time or because they had to cut a hole in your subfloor to put in the drain on some of these drop in models. Depending on how you plan to finish the edge around your tub is when I advocate for mortar. If your tub is a drop in & you plan on using "ceramic tile" around it, definately MORTAR in the tub. Ceramic tile pieces do not have flex built into their vocabulary. When they flex, they will BREAK.
If there is a a small hole in your subfloor then you will have to use 6 Mil plastic under your mortar bed put the mortar in & then put layer of 6 Mil plastic over it & then put tub over top the plastic & cement.
This will accomplish two things, it will support the tub & if you EVER need to remove the tub for ANY reason [usuallly a plumbling leak] or work years down the road, you will be able to accomplish the removal with relative ease because of your foresight.
Plastic laminate surrounds will pretty much flex so you can use glue on the feet to get these in. Jacuzzi recommends mortar on their drop in models its in the directions !! Marble is generally immovable but depending on its thickness is where I want to make a judgment.If it is less than 1 inch thick go with 59+ a foot in cost you'll be sorry if you don't :>(

    Bookmark   July 11, 2007 at 10:40AM
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Follow-up question (considering the same install)...
My current tub has the subfloor (2nd floor bathroom) totally cut out between two joists where the drain is located for the entire length of the 5' alcove tub (with no insulation such that I can look down to the garage ceiling on the first floor).

How do I fix this? Insulate and do what to the subfloor?
Plumbing runs below the subfloor height.
What are my options to fix this right with the same kind of Kohler Proflex airtub replacement? Thanks in advance, Jeff.

    Bookmark   October 1, 2008 at 11:06AM
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My tub has supporting blocks and i will go for the adhesive method.
1.the mortar is very heavy and i need 2'' thick because of the blocks; i don't want to add more weight to the structure since the tub is a big one.
2. the mortar under the tub will cool the water very fast; with the mortar i will probably have 30 minutes until the water get cold.

Here is a link that might be useful: trimarts

    Bookmark   November 15, 2008 at 10:50PM
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I have a Kohler cast iron 5' x3' Steeping Whirlpool Tub that I need to remove to install my new 5'x3' Jacuzzi fiberglass tub. My question is; Can I somehow break this cast iron tub to remove it so we don't break our backs while in the process of removing it? We really enjoyed having the Steeping Tub but the porcelain finish has began to chip off & we're to the point where it looks bad so we'll be replacing the tub during the holidays.

    Bookmark   December 24, 2014 at 2:28PM
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It'll be easiest if you can cut it up with a sawzall. You can also create score lines with a grinder and break it up with a sledge.

If you hit it with anything hard...a sledge, etc...cover it with a tarp, protect the floor, and wear protection. You can send shards of cast iron in the most unusual of places. Through a window. Into your thigh. Into your eye.

You really need to assess your abilities and if you proceed, proceed with caution.

If you can get it loose, you may be able to sell. Some salvage or resell places will cart them away without trashing your house in the process.

Merry Christmas!

    Bookmark   December 24, 2014 at 10:40PM
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Thanks mongoct. For those of you that are interested in knowing the progress of my project having to do with the demolition of my cast iron tub..... what a mess. busted three wooden sledge hammer handles & it took me the biggest part of one afternoon. I wasn't in any hurry & because I'm 66 years of age I rested as I managed to break off pieces of the tub. As mentioned earlier by mongoct, shards of cast iron did fly everywhere almost like shrapnel. The tub's out now and it's Jan.19th & we've yet to install our replacement tub. A bigger issue came up & we're having to tent our home for termites, after that we'll install the tub.

    Bookmark   January 19, 2015 at 9:05AM
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