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Thread: Mortar bed Question

  1. #1
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    Default Mortar bed Question

    I'm installing a Sterling Ensemble Whirlpool tub and the directions state to build a mortar bed and allow it to cure. once it has cured how ever it has began to crack an crack a lot. On this site I've seen recommendations to create pillars and then place the tub but I can only imagine this is while the mortar is still wet so which method is better because I find it difficult to believe that this cracked mortar is going to provide much support. Or am I wrong to worry about the cracks and should I press on with installation.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    What mix did you use, and how thick? It's also a good idea to put a sheet of plastic down on the (wood?) subflooring to help keep it from sucking too much moisture from it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    I used portland cement type II it was recommended by the guy at lowes, its about 2 inches thick but no plastic underneath, unfortunately I didn't seek advice until things seemed to be going wrong. But I can easily remove it all and start over no big deal just extra work, but better than ruining a tub and having to fix it later. Oh and yes it does have a wood subfloor.
    Last edited by tk0517; 08-31-2008 at 07:37 PM.

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    After reviewing other posts I'm convinced I need to start this process over, but before I begin first I should know the steps involved so correct me if I'm wrong.
    Step 1 Should be putting down plastic to prevent the wood from removing the moisture
    Step 2 Should be mortar itself, not what lowes recommends and a min. dept of 2 in. possibly with chicken wire or other mesh material to help strengthen the base and provide further support. (how many bags for a standard size tub would be recommended)
    Step 3 Allow it to Dry??? or not I don't know do I place the tub in wet mortar or allow it to dry. I don't know hopefully someone can tell me.
    Step 4. Place tub
    Any help would be greatly appreciated.

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    Engineer Furd's Avatar
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    1. Install the listings (ledges) to support the tub edges if you have a skirted tub. If you have a "drop in" tub then build the enclosure with the deck.

    2. If you have the skirted tub measure the depth from the bottom of the legs to the bottom of the tub. Remember this figure. If you are installing a drop in tub measure the distance from the bottom of the tub to the floor.

    3. Seal the sub floor or use a piece of plastic sheeting where the mortar is to be placed.

    4. Mix the mortar and place it where the bottom of the tub will be. Do not have any mortar where the legs will be as the legs must touch the floor. Pile the mortar in a way that the tub bottom will squish it down (i.e. leave space for squishing the mortar down) when you set the tub in place. The mortar is to support the bottom of the tub, reinforcing the tub bottom. This is in addition to the legs of the tub also supporting the tub. The height of the mortar mounds will be slightly more than the measurement taken in step 2.

    If this is a skirted tub then before placing the tub into position run a thick bead of 100% silicone caulk/adhesive on the listings. If this is a drop in tub make some 1/8 inch shims to hold the tub edge above the deck surface. Set the tub into the mortar and press the bottom into firm contact with the mortar making sure that the legs touch the floor (skirted tub) or the edge of the tub is firmly touching the shims all around the perimeter. If necessary, trowel some more mortar into place for full support under the tub. You can walk in the tub (bare or stocking feet) to ensure full contact with the mortar.

    5. Let the mortar set up for at least 24 hours before doing anything else with the tub installation. After this time if you are installing a drop in tub run a thick bead of 100% silicone caulk under the edge.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    The mortar has two goals: provide support and to level (if required). If the floor isn't flat, then one or more of the legs may not set on the floor. If the tub is properly supported, that should be okay.

    The tub must be set in the wet mortar. There are several things that can be used, but straight portland cement is not one of them! You can use a sand mix (aka deck mud - the mix that is used to make a shower base) which is mostly sand with some portland cement (a 3-5:1 sand to cement mix). This will not be runny...mix it until a clump holds together when squeezed together by hand, it should not be dripping wet - you just want to get all of the sand and cement mixed well then wetted (i.e., damp). It will be sort of like wet beach sand.

    You can also use plaster of paris, but mixing up a big batch and laying it out before it starts to harden can be tricky - it starts to set quickly.

    Some prefer structolite.

    When setting the tub, you want to settle it down in the mortar until it is level. If you ledgers are set properly and the lip is not warped, you can push it down until the lip hits the ledgers.

    You can, but it isn't necessary, also put a sheet of plastic right under the tub. It will make it easier to take out if desired, at some later date. It's not there to bond the tub to the floor, but to support it. Over a wood subfloor, you absolutely do want something to prevent the wood from sucking the moisture out of the mix.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    thank you very much for your responses you've saved me thousands I'm sure, plus an upset wife which is worth much more.

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    DIY Member topshot's Avatar
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    Will be setting my tub next Thur. Went to the big box and they have these to chose from - which one do I need?
    Stone Mortar Mix
    Type S Masonry Mix
    Mortar Mix
    Sand Mix
    Concrete Mix

    I assume you basically leave the square foot around the drain alone since the subfloor is cut out?

    Also, do you typically do underlayment under the tub rather than stopping at the tub edge so if water gets on it it won't bubble up?
    Last edited by topshot; 08-07-2009 at 09:27 AM.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    I'd use the sand mix...it doesn't need to look pretty. Most of the other choices would end up being stickier or flow. Sand mix is closer to wet beach sand. It should be mixed so when you grab a clump, you can compress it into a ball that will stay together but not drip water out. You need to put some plastic or roofing felt on the wooden subflooring to prevent it from wicking moisture out of the mortar so it can achieve its strength.

    Assuming you are going to tile the walls, install a vapor barrier, lap it over the tiling flange of the tub. Then put up cbu (cement board). You can then trim the vapor barrier so it will be hidden when you tile. if your tile are fairly large, you can stop the board just above the lip. If they are small (you want more than 1/2 of the tile supported by the cbu), you may need to notch the studs or fir them out to allow the cbu to overlap the lip. Stop the cbu about 1/4" from the tub top finished surface if you overlap the lip. Caulk that joint and the vertical corners (no grout there). You can buy color-matched caulk in both sanded and unsanded configurations it not using say white.
    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 topshot's Avatar
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    Quote Originally Posted by jadnashua View Post
    I'd use the sand mix...it doesn't need to look pretty. Most of the other choices would end up being stickier or flow.
    Sand Mix it is then. Thank you.

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