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Thread: Mortared shower pan on slab foundation

  1. #1

    Default Mortared shower pan on slab foundation

    I am remoldeling my master bath and making it all shower. I have everything out and the walls done with durock. I want to make a mortared shower pan since the shower is an odd size. Everything I read about it is talking about doing it over a subfloor. I need to know the procedure for a slab floor.

  2. #2
    Plumber jimbo's Avatar
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    You still need to builed a preslope foundation, covered with a membrane layer, then the tile.

    For best advice, try this forum: www.johnbridge.com

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    Homeowner geniescience's Avatar
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    only that. not deck mud.

    You need only a slope, a liner and tile.

    Therefore you could (even) have a sunken shower floor, by cutting away some of the slab concrete. You do not need to raise the floor by adding any cement product.

    If you build the slope on top of the slab instead of cutting the slab down, all the height (or thickness) of "new" cement you need is from ZERO at the drain, and from there UP with the slight slope to whatever thickness is needed by the slope. Two feet from the drain, you need a half inch height.

    You do not need to start with any greater thickness at the drain. My memories of the jb forum are not happy, with this subject: nobody ever said what i have just said above, and yet they claim to know a lot. Maybe today someone over there will confirm the above...

    The point is, you asked about a slab and how it differs from a wooden subfloor. This is how it is different. It doesn't need any "deck mud" or thick layer that is designed to hold together and be stiff enough... All that is for wood buildings.

    A slab has enough strength already.

    The concrete product that you will use to make your slope will be whatever the local concrete product manufacturer sells for this purpose. I cannot name product names, since they vary a lot from place to place.

    David

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    Retired Defense Industry Engineer jadnashua's Avatar
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    You want to bond the deckmud to the slab. There are several ways to do this...you can mix some thinset runny and almost paint it on, then get the deck mud on top of it quickly before it skins over. You can sprinkle dry cement on the floor then sprinkle some water on it then dump the deck mud, and there are other techniques.

    Deck mud is sharp sand and portland cement mix in the 4-5:1 ratio. Think sand castle that hardens. You can taper it to zero at the drain, but that will often mean digging a bigger hole to recess the drain and getting it supported is harder. It may be worth it if your goal is an absolute minimum height floor. A Kerdi shower would provide that as well, since you only need one layer - the slope, then the waterproofing membrane, rather then the preslope, membrane, top slope, then tile.

    SO, preslope, liner, then final slope, then tile. Unless you use something like Kerdi (you should check this out at www.schluter.com). The advantage of using Kerdi is that there is no thick cement layer on the walls or floor to get saturated...the waterproofing layer is right under the tile. You can also cover your walls and curb with normal drywall, which sticks the Kerdi in place well and provides a fine surface for the waterproof membrane.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Homeowner geniescience's Avatar
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    Quote Originally Posted by jadnashua
    Deck mud is sharp sand and portland cement mix in the 4-5:1 ratio. Think sand castle that hardens. You can taper it to zero at the drain, but that will often mean digging a bigger hole to recess the drain and getting it supported is harder....
    all wrong here.

    Also, not true that using Kerdi is the way by which
    Quote Originally Posted by jadnashua
    you only need one layer - the slope, then the waterproofing membrane, rather then the preslope, membrane, top slope, then tile
    also not true is
    Quote Originally Posted by jadnashua
    The advantage of using Kerdi is that there is no thick cement layer on the walls or floor to get saturated...
    also no good is
    Quote Originally Posted by jadnashua
    You want to bond the deckmud to the slab
    Sorry to be so clear about it. Over at the world's friendliest web site, they would fudge the issues so no-one would ever know.


    David

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    David,
    If you have a gripe with them over there, fine. The information comes from the Tile Council of America. They set the standards for the codes that dictate best practices. Many locales abide by their recommendations. They spend big money to test these and have a proven track record. There is often more than one way to skin a cat, but saying one is all wrong is plain disregarding long term testing and proven methods.

    You build yours your way, and I'll build it mine. They both may last a very long time, but I can sleep with the knowledge that I'm not reinventing the wheel, and that it has been life-tested.

    Building a sloped floor with stuff like concrete is tough. Plus, the goal is not a nearly waterproof layer, but one that will allow any moisture to percolate to the weep holes, assuming you are doing a liner and clamping drain.

    Kerdi is a whole other concept, where you actually make the tileable surface totally waterproof. Therefore, there isn't any need for the deckmud underneath, but as said, it is far easier to pack deckmud into the shape you need than most other products.

    the thickness needed to be able to maintain a monolitic shape will depend on the substrate - a slab requires less, a wooden subfloor more.

    The standards are set to ensure a 300-pound point load will not damage the surface.

    If you can point to a nationally recognized, published standard, then fine. But to say outright the whole concept is wrong, is misguided. Please keep your personal gripes to yourself.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    Homeowner geniescience's Avatar
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    huh? waterproof?? Why bring that up?

    Jim, I think it is great that you mention the TCNA. I will look at what they have to say in regards to all you have written and all I have written. It is good to have an objective source for information.
    Quote Originally Posted by jadnashua
    ... the goal is not a nearly waterproof layer, but one that will allow any moisture to percolate to the weep holes, assuming you are doing a liner and clamping drain.

    Kerdi is a whole other concept, where you actually make the tileable surface totally waterproof. Therefore, there isn't any need for the deckmud underneath, but as said, it is far easier to pack deckmud into the shape you need than most other products.

    the thickness needed to be able to maintain a monolitic shape will depend on the substrate - a slab requires less, a wooden subfloor more.

    The standards are set to ensure a 300-pound point load will not damage the surface....
    Please note that "deck mud" i.e. sand castle mix, is not designed to be laid down at Zero-to-1/4"-to-1/2" thickness, not even on a slab. Too much sand in the mix. To build a "preslope" as thin as that, you use a mix that has a lot more cement. (Not because I say so.)

    Then, on that preslope, you can put any one of many many types of liners, not just KErdi. Not just the two types mentioned above, the liner-and-clamping-drain, or the KErdi. There are other shower waterproofings out there too. Example: many liquid membranes.

    If we accept that the preslope is a separate concern from the waterproofing system we can begin to discuss them independently from each other. Is anyone going to tell me that a slope on a slab, or how it is made, will depend on the waterproofing laid on top or on the drain used in the hole...?

    Let's give this some thought, everybody.

    David

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    The preslope could be anything hard enough to stand up. About 1/2" at the drain on a slab is probably the thinnest you'd want to go with deckmud. Deck mud is an inexpensive and easy way to form substance, and if you are going to use a clamping drain and a liner, you need deck mud on the top of the liner. If you choose to use Kerdi then you can do it in one layer...just the preslope with the waterproof layer of Kerdi on top. This would likely produce the lowest tileable floor unless the slab was recessed, which is unlikely to happen on a remodel or new construction as an afterthought.

    Hotmopping, RedGard, Kerdi, Latticrete, Noble, and others make systems that allow for an acceptable shower construction. Wedi makes some kits with their panels that are real slick, and if it fits your situation, a good choice (but pricey - more than Kerdi, but faster to install).

    There isn't one correct way to build a shower, but there a numerous wrong ways.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9

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    I really appreciate all the help. Especially you geniescience. What I'm needing now is what is going to be the best concrete to use. My drain will be at zero so that is not a problem. I found some concrete adhesive today at HD. Would that be a good thing to put down before I pour concrete? Or do I really need to put plastic instead? My concern there is that the slab could move under the plastic. I just want to do this thing so it is right the first time so I don't have to redo it in a few years. Also would it be a good idea to use some lath as a reinforcement to help hold it all together and prevent cracking?

  10. #10

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    I also want to mention that I don't plan on tiling the floor. We want to stain it or color the concrete. I'm doing the walls in marble so I don't really want that on the floor. I have looked into using a latex additive to help with water resistance.

  11. #11
    Homeowner geniescience's Avatar
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    Concrete Weld is good to use. Thinset is good also. Bonding new concrete to old concrete is the right thing to do.


    2.) No to plastic. As you mentioned it, in your description.


    3.) Based on your questions, I must say now that you may be oversimplifying things. It is -- in most residential showers -- important to arrange for a continuous liner. As an example, check out Redgard, a liquid that makes a rubbery seal everywhere. It bonds to concrete or thinset, so after you put this liquid membrane on the walls and floors, you then install the wall tiles (or marble) and the floor tiles or new concrete mix.

    3.a.) Continuity is the concept. Behind the marble and down onto the floor and over to the drain, and including the seam with the drain.

    3.b.) There are cases where continuous membrane is NOT required by code. (e.g. residential shower on a ground floor slab, in Arizona), This may not be your situation.



    4.) The single most important thing is the slope to the drain.

    4.a) The worst situation -- which you will read about often -- is a flat liner, laid flat on a flat floor (or slab), which doesn't drain water but holds it on a flat surface, and then a thick thick bed of sand castle mix laid on top with a slope too so that the surface tiles get installed sloped to the drain. This holds a ton of stagnant water after a few years. This is the enemy that most people warn against. The floor looks sloped, and is sloped, but is in fact built as a container holding stale moisture under the tiles.



    5.a.) Waterproofing is to manage those h20 molecules that seep through and create a bad environment underneath the visible surface.

    5.b.) H20 does migrate into your walls and floor. Grout is porous. Cement is porous. Concrete is porous. In your case, with marble, you will have walls made of a material that only looks and feels like it is impervious to water, but is actually porous.

    5.c.) If someone only took two showers a week in an un-membraned shower, it might dry out under the surface sufficiently not to let h20 build up. Once usage surpasses a certain threshold, the undersurface moisture never diminshes, and keeps building up. This is the reason why it may take a year or two before the threshold is reached. Once that threshold is reached, your shower walls and floor are an in-concrete mold factory. Mold can grow inside concrete. Concrete has enough room in it to let mold grow in its micrometer air spaces. Mold offgases.



    Bryan, you need to find a floor surface material that serves the necessary purpose. Then you need to make it match your marble walls.



    David

  12. #12
    Homeowner geniescience's Avatar
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    slope of the liner, the membrane, the waterproofing.

    The single most important thing is the slope to the drain, of the liner, the membrane, the waterproofing, the thing that prevents water from getting into the slab.

    However, there is a chance that in your case you will use a product, material, procedure that is not commonly used. Based on what you wrote above.

    -david
    Last edited by geniescience; 09-10-2007 at 05:32 AM.

  13. #13
    Homeowner geniescience's Avatar
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    Default concrete floor in a shower

    Quote Originally Posted by BryanG
    ... floor... stain it or color the concrete. ... using a latex additive to help with water resistance.
    another post, to clarify things.

    first, the possible addition of a latex additive, is not going to please anyone here in this site, or at any other web site or discussion forum, where people who build showers discuss topics like "how-to". The fact that your "thinset" (that is what it is called) has latex in it, won't please anyone who knows that shower walls need a waterproofing membrane. Whether or not the thinset is this or that, is not a replacement for the membrane.

    Perhaps you are currently under the spell of a charming remodeler who has build a shower without any membrane, and "hasn't had any problems" so far. I happen to know many people like that, so I can imagine it is a common occurrence everywhere else too. You still do need and ought to plan for a membrane. I have seen showers built with a membrane only applied in the corners and on the bottom half of the walls, and one recent thread here mentioned that happening. It was a complaint thread, and the person posting later reported that the guys building the shower "were going to" run the membrane higher up the walls. Perhaps they always intended to, but just ran out after they did the corners and half the walls. Yeah, sure. .

    The floor needs it even more.

    If you don't do it, you can end up with locker room smell, stale urine smell, mold smell, etc. And other problems including structural.

    I know people who say, "there is NO water going to get past this epoxy grout," and other such things, and they don't put liners (membranes) on a floor sloped to a drain. Their confidence is based on their common sense and experience, but it really isn't a good idea to rely on that alone, for a shower that will see regular usage.


    Building a concrete floor in a shower is a rare thing, so it will be hard to get advice from anyone who has done it. With or without a membrane.



    David

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    Jack of all trades frenchie's Avatar
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    Quote Originally Posted by geniescience
    Building a concrete floor in a shower is a rare thing, so it will be hard to get advice from anyone who has done it. With or without a membrane.

    You forgot to mention why - because it's a really, really, really bad idea. Nobody reputable recommends concrete floors in showers, everyone warns against it.

    I agree that it sounds like Bryan is getting some really bad advice from somewhere. The antidote's simple: keep posting here, post also at John Bridge's, pay attention to the answers. Maybe pick up the TCNA handbook, too.

    Just to reiterate:

    You need a membrane. It must be sloped.

    Latex does not make thinset waterproof, it just makes it more elastic / less brittle.

    Concrete floors in showers are a good way to guarantee bacterial colonies.
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    Homeowner geniescience's Avatar
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    Quote Originally Posted by frenchie
    ...a membrane. It must be sloped. ....
    you never know, if someone has found a way that has worked reliably for years, etc etc,,,, and I am still eager to know whether the expensive epoxy coating you see on old concrete floors in high-end commercial retail, can be used for shower floors, and if anyone is going to try it, or has done it and can tell use what the experience was like.

    But that is not "tried and true".

    True, that there appear to be no documented cases of concrete floors in showers being so successful as to be "proof" making its way into the TCNA handbook or any other impartial third party organization's documentation. That will be years from now.

    david

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