Confused about mortar under tubs...

Discussion in 'Shower & Bathtub Forum & Blog' started by Charly, Aug 15, 2008.

  1. Charly

    Charly New Member

    Messages:
    4
    Location:
    Texas
    I have only recently heard about this and am confused about a couple of things. I am installing a fiberglass tub for a neighbor and when you look at the bottom it has a large piece of what I call Chip board (chips of wood glued and pressed together) and the legs are 4 x 4 wood. When I've stood in the tub, there is no flex whatsoever. I replaced the floor with 3/4" plywood and plan to lay vinyl linoleum.

    1. Why would this be necessary?

    2. With the majority of the wood not covered by fiberglass wouldn't it soak up moisture from the mortar, thus weakening it to some degree?

    3. If it was absolutely necessary to support the bottom, even with the chip board wouldn't using a 4" or so wide board down the center of the tub be enough if anything was needed at all?

    No instructions came with the tub or surround (both fiberglass) and not in any boxes either... Gawd I so enjoy remodeling mobile homes! NOT! lol
  2. Master Plumber 101

    Master Plumber 101 In the Trades

    Messages:
    268
    Location:
    Wisconsin
    I just responded to this same question. Use a gypsom plaster, it's soft setting and the end result is great(my opinion). Why is it necessary? It gives the base a more firm setting,not necessarly to the bottom of the tub but at most the supports on the bottom of the tub.
  3. lylec

    lylec Landscape Designer and Contractor; handyman repair

    Messages:
    6
    Location:
    Atlanta, GA
    I will do something to help explain this for everyone, and for future reference. Since I have the side of my tub open, without any mortar bed, it is very easy to see the bottom flex when a person steps in it. I will take a few photos with a tape measure next to the bottom . I will take a photo of the empty tub, and then with a person it it. It is quite easy to see the entire bottom flex. The chipboard provides almost no strength. Go lay a piece of plywood 3/8"(which is stronger than chipboard) out on 2 2x4's spread 5' apart and then stand in the middle. I guarantee that the board will flex. I believe the chipboard is just for shipping/crate purposes, but it is not support for the tub bottom. According to my whirlpool specs, the weight with water is 554 pounds. Without the bottom fully supported, this weight is being held by the rim of the tub, which is not good.
  4. Charly

    Charly New Member

    Messages:
    4
    Location:
    Texas
    I can certainly understand with a 5' bottom needing major support, but this tub is only 27" wide, maybe a little less. I get nervous when it's suggested to use anything with moisture, no matter how little, when it's around wood is all. That's why I ask my questions and would prefer using wood if it was absolutely necessary to support the chip board underneath.

    Granted I only weigh about 170lbs., but when standing in the tub and flexing my knees I feel no movement at all. I originally was going to put a strip of wood down the center for added support, as I know someone about 300lbs. will be using the shower. Then I heard about using a mortar or the like....

    I've never dealt with gypsum plaster, but am presuming it is like what's in sheetrock. If that's the case, it doesn't break down at all over time nor would any moisture get into the chip board, floor or legs of tub?

    Thanks!
    Last edited: Aug 15, 2008
  5. toolaholic

    toolaholic General Contractor Carpenter

    Messages:
    874
    Location:
    Marin Co. Ca.
    Charly, You came here because You don't know what Your doing. And that's why the pros help You. Now You 2nd guess the pros. I've been setting plastic in motar for many years without a problem. I first staple down chicken wire
    to reinforce the motar. It keeps the motar from breaking up later.
  6. Charly

    Charly New Member

    Messages:
    4
    Location:
    Texas

    My dad is a retired general contractor and I’ve been building houses and remodeling for over 30 yrs. Of all the tubs I’ve put in, I just never read instructions about using mortar, sand or anything else and this tub came without instructions.

    I came here to gain knowledge and ask questions about something I’d never heard of doing on the information highway. No offense was meant or was second guessing implied nor rudeness expected… I cannot apologize for wanting to know all the whys, pros and cons of something I’m unfamiliar with for the good of the customer.

    Guess I shouldn’t have come here to ask my questions…. It won’t happen again toolaholic.
  7. Terry

    Terry Administrator Staff Member

    Messages:
    15,416
    Location:
    Bothell, Washington
    Charly,
    Not all tubs need support.
    If the tub is on the floor, not needed.

    If it's stiff enough, not needed.

    Sometimes mortar is used when there is flex.
    Most deck mounted tubs need some sort of support.
    Mortar, or something that sets up hard works.

    I like the chicken wire idea. See, I learned something new today.
    I've used steel mesh in driveways, so it makes sense.

    I tried a time or two trying to get the exact level on the support below, and somehow got too much in.
    I found that small piles or someone said ridges of mortar have worked, the tub squishes the mortar down to size, and there is space between the piles for it to spread, only as much as it needs.
  8. leejosepho

    leejosepho DIY scratch-pad engineer

    I have a one-piece fibreglass shower/tub with a flat and solid piece of 3/4" plywood on the bottom, and neither could I feel it flex prior to installation. Nevertheless, I installed it with a tapered-to-fit 2X4 wooden frame (rectangular box) underneath it just to be sure some time ago, and now I can feel the bottom flex whenever I take a shower. I have easy access behind the tub and at its ends at the moment, so now I plan to add the mortar base I should have done in the first place. Foam failed me long ago, and now wood has done the same ... and that leaves us dependent upon mortar even if I do not completely understand why!
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,317
    Location:
    New England
    Many older tubs were made of cast iron (and many still are). A CI tub is strong enough to not worry about flex, but even one of those can benefit from something under it if the floor is not both flat and level. No tub is designed to be completely supported by hanging from the rim. A few tubs have a structure that is fully supportive and works fine IF the floor is both flat and level. If your tub is not set level, you'll have problems with moisture running where you don't want it.

    Any moisture used in mortar or plaster is chemically incorporated into the mix when it cures...there is NO water left. After that, the only way moisture gets under there is if something fails or you play water games in the bathroom. To help keep the wood dry while the stuff cures, a sheet of plastic works fine.
  10. lylec

    lylec Landscape Designer and Contractor; handyman repair

    Messages:
    6
    Location:
    Atlanta, GA
    To follow up Terry's point about how to best place mortar before setting the base, here are some thoughts. If you have ever set tile, you know that you use a notched trowel. The reason for the notches is to allow you to press the tiles down. If you try to press the tiles into a solid even spread of mortar/thinset/adhesive, there is no where for the excess to go. Same idea applies here, with several mounds or rows, or zig-zags, you can start high, and then as you press the tub down to the desired height, the mortar can spread. This is also why you do not put any weight on it for days, as there are voids that the partially cured mortar could still ooze, thus lowering the tub and removing the support.
  11. Master Plumber 101

    Master Plumber 101 In the Trades

    Messages:
    268
    Location:
    Wisconsin
    If you never used gypsom plaster to set a tub(best used on whirlpools and shower bases) you should try,its the cat's you know what. cast iron tubs are probably not necessary to do so but works great with everything else. Try it once it work just as good as mortar and cets up nice and hard.::D
  12. Terry

    Terry Administrator Staff Member

    Messages:
    15,416
    Location:
    Bothell, Washington
    And that is really the reason.
    After time goes by, and you have cycle after cycle of people stepping in and out of the tub or shower, you start to break down the fiberglass and the wood.
  13. yoyoma

    yoyoma New Member

    Messages:
    12
    My plumber's assistant said he was not going to do a mortar bed when I asked him. Said it's not necessary since it's going to be supported by the 2x4s on the long side of the alcove. I checked the installation manual and nowhere did it mention mortar beds. So I called Maax up and they said it's "recommended" and they were going to update their manuals with this information. The guy at Maax also said if people using it are under 300 pounds then a mortar bed isn't really necessary.

    I told the plumber's assistant to hold off on the tub installation. Then I called my GC and told him I wanted a mortar bed. Lucky I was there to see what they were doing. My GC said I shouldn't have to be there, and that it doesn't matter because you could tell the difference afterwards and make them do it over the right way with the mortar bed. He said to let them do it their way, don't watch them, and only later make them fix it. I thought it was better to catch them while doing it and save everyone time.

    I also told my GC that I wanted the 2x4s on the ends too because that's what the manual states, and he said it's not necessary, just the long side, but if it'll make me happy then he'll do it. I fell bad pushing things but I'm the one who has to live with this, not them.
  14. lylec

    lylec Landscape Designer and Contractor; handyman repair

    Messages:
    6
    Location:
    Atlanta, GA
    This is on Page 2 of the installation instructions for the American Standard Plebe Whirlpool that is in my house, (incorrectly installed by the master plumber that installed it before I bought the house).

    "This bath must be supported along its entire bottom in all types of installations. We recommend the use of mortar of plaster as bedding material (sand is not recommended). Apply enough bedding to support the complete bottom of the bath. After the bedding has been poured, and before it sets, lower the bath into place until the rim is leveled against the leveling stringers. The rim of the bath MUST NOT support weight. Allow the bedding material to completely harden before applying weight to the rim or bottom fo the bath. Any finish material such as tile or wall board must be self-supporting if it contacts the deck of the bath."
  15. krow

    krow Plumber

    Messages:
    906
    Location:
    Ontario, Canada
    I always tell my clients

    "At the end of the day, I get to go home, so you (the client), has to live with the end product."
    I often ask if the client has a preference of how they would like a product installed because I don't want to be called back and be accused of not installing it how they want. Most of the time, the client will ask me to do it the "best way possible", so then I will install a mortar/cement pad under the acrylic tubs
  16. yoyoma

    yoyoma New Member

    Messages:
    12
    The question is how do I know the good plumber from the bad one? This plumber came on the recommendation of my GC so I used him. He's going to be back tomorrow to do the tub install and I won't be there unfortunately. I hope I'll be able to determine whether a mortar pad was installed even though I won't be able to see it. Based on today's experience, I don't trust them at all.

    The plumber's assistant also installed a Toto Carlyle incorrectly. He flushed it, and it was making a trickling noise well after it stopped flushing. He was about to calk the base when I said, "What!? No way, fix it please". 10 minutes later he leveled it better and the trickling was gone. Now tell me why did I have to babysit them? I think a professional would get it right from the beginning instead of taking shortcuts hoping I don't notice.
  17. Terry

    Terry Administrator Staff Member

    Messages:
    15,416
    Location:
    Bothell, Washington
    They do that when water is added to the bowl, or at the end of the flush. It takes a little while before the capillary action quits pulling the water down the trapway. It will stop fairly quickly though.

    Your tub/shower faucet will drip for 35 to 40 minutes after taking a shower. I've had complaints on those too.
  18. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,317
    Location:
    New England
    NO tubs are designed to be held up by their rims on the ledgers...the ledgers are to hold the edges in case you sit on them and help hold things in place while the mortar bed sets up. The only tubs that don't need it are those with an engineered flat bottom that is already fully supported, or a cast iron one, and then, only if all of its feet are sitting evenly on the floor. Water weighs about 8#/gallon. Add say 30 gallons (and some can hold more than twice that much) and an average American and you've got easily over 400# hanging on that 1/4" thick lip of plastic...NOT a great idea. The tub WILL flex. They're fairly resilient, but in the process, it will stress the drain line and possibly squeak as it rubs against anything it touches.

    I don't see the issue - this is not a corner to cut. Mortar is cheap AND it lets you get the tub perfectly level.
  19. yoyoma

    yoyoma New Member

    Messages:
    12
    Well, there is a plywood-type bottom plank underneath the bottom of the tub, but maybe that's just done for packaging. Not sure. I think this may be a misunderstanding because the plumber's assistant doesn't speak English well. It's times like these that I wish my GC used native English speaking subs.
  20. leejosepho

    leejosepho DIY scratch-pad engineer

    Probably nobody knows for sure about that in every case, but my guess is that the old standards have been generally forgotten over time.

    As far as I know, plastic showers and tubs first began appearing in mobile homes around 50 years ago or so while cast iron and steel were still being used in regular houses. At that time, a plywood bottom with short stilts to support the flexible tub probably seemed (or was deemed) solid enough to match the quality and expense of a trailer house ... and with maunufactured housing now confusing things even more, everybody has forgotten (or has never heard) about the great differences between a rigid cast-iron tub sitting on stubby legs and a flexible plastic tub sitting on short wooden stilts.

    Overall, my guess is that the plywood bottom was originally there to simply hold the bottom of the tub in place (flat and sloping toward one end) until the mortar placed under it had set, and that the stubby legs were there so the tub could be pressed down onto the mortar without being pushed too far ... but I doubt that procedure was ever actually followed in very many mobile homes.
    Last edited: Aug 17, 2008
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