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Thread: mortar under tub

  1. #1
    DIY Member duct tape pro's Avatar
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    Default mortar under tub

    Hi,
    I am about to install a new tub.
    The bottom of the soaker tub is 1.5" above the ground level.
    (apron of tub is lower than tub bottom by the 1.5". Why, I have no idea!)

    In the instructions it says;
    "Ensure the bathtub rests solidly7 on the installed supports. Shim tub as req'd to insure a level installation. It is recommended that a layer of mortar be applied and covered with a 2 ml polyethylene sheet to facilitate levelling.

    Now, what the heck kind of mortar would you use?
    Thinset or other?
    Maybe just cement?
    Is tile adhesive considered mortar?

    Thx in adv.

  2. #2
    In the Trades Gary Swart's Avatar
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    Default

    Mortar is mortar. It's not cement although it is related. It's what you use to make a brick wall. It can be found in most any big box store. The purpose of the mortar is to provide a solid base under the tub so that it does not flex. I say a solid base, but that does not mean you want solid mortar under the tub. You place piles around the area where the tub will set then set the tub in place. The weight of the tub will squish the mortar out. If you could see under the tub, you'd see there would be voids in the coverage, but there will be plenty of support. I'd be sure to get mortar placed around the drain area especially. You don't want any flex there for sure.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Set you ledgers at the proper height and level. Lay a good bed down there - it is better (unless it is cast iron, then it doesn't matter much) if you do support the entire bottom of the tub with the mortar. Squish it down until it is level both L-R and F-B and the edges are at the ledger boards. The edges are NOT designed to support the tub. If the tub has feet that are designed to sit on the floor, assuming your floor is level, you can push it down until they are on the floor. But, if it isn't perfectly level, as long as it is supported by the mortar - stop when it is level, it doesn't matter if they are on the floor or not. A fiberglass or acrylic tub will flex a little, and fully supporting it from below will make it feel much more substantial and make it last longer. the same is true for a shower base.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    Plumber jimbo's Avatar
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    Lightweight cement, brand names Structolite or Gypcrete works well. People have used drywall mud, but the set up time is long.

  5. #5
    DIY Member duct tape pro's Avatar
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    Default

    thanks for the tips.

    I'll go shopping tomorrow.

  6. #6
    DIY Junior Member Emma3's Avatar
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    Default more mortar bed questions

    I'm about to set a tub w/mortar bed and have some follow-on questions to this thread. Here's the background:

    -Acrylic drop-in tub, 3-wall alcove installation
    -Bottom of tub is 1/2" off floor at drain end and about 3 1/2" off floor at opposite end (all edges level and resting on stringers (ledgers?))

    I've estimated that I'll need about 1.5 cubic feet of mortar. Based on other posts, I was planning to use Structolite gypsum plaster for this.

    I've seen two contradictory posts about whether to put poly sheeting under the mortar in addition to on top of it. One said pour directly onto ply subfloor, the other said to use poly underneath to protect moisture from wicking out and drying mixture too quickly. Which is right? Does it really matter? Either way, I will put the poly between the mortar and tub itself.

    Also, are there any issues with using Structolite in a thick application like this (up to 3 1/2")? Seems like other tub sets only require an inch or so. Will it set properly? Can/should I add a little regular quickcrete mortar to the mix to help it set? Is Structolite the lightest weight product (more or less) that would be appropriate for this application, or is there something else I should consider?

    Thanks!

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    You'd have to read the spec sheet, but it is a setting compound as far as I know...that means it doens't dry, it chemically changes into a solid by chemical action. You might prevent the next guy from cursing you if you put something on the floor when they try to remodel.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #8

    Default

    I always put vinyl plastic 6 to 10 mil on the floor first. Mortor retains water that will rot the subfloor. Plastic over the top keeps things neat and tidy.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    The usual reason to put plastic on a wood subfloor prior to masonary products is that it draws too much out of the mix, weakening it before it gets a chance to cure, if the wood comes into direct contact. Some like to put a layer on top so the tub itself doesn't stick to it. Having it on both sides makes removal later on easier.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #10
    DIY Member duct tape pro's Avatar
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    Hi,
    I notice you have a slanted bottom to your tub too.
    What I did was layers of plywood under it before mortar mix (or drywall mix mentioned earlier).
    Plywood is solid and lense dense than mortar mix resulting in a thinner layer of mortar under your tub.
    End results looks like a series of a few steps.
    If you do a test in your kitchen (hard floor) you can see how far from back wall you can put each layer of your plywood which will vary with plywood thickness.
    (hopefully wifey doesn't get mad as you mess with the tub in middle of kitchen...nuff said on that one!) 8>)

    J

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