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Thread: Subfloor question for tiling a bathroom floor with heat mat under the tiles

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    DIY Member Drewski123's Avatar
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    Default Subfloor question for tiling a bathroom floor with heat mat under the tiles

    My bathroom's subfloor is an old tongue and groove sub-floor 7/8" thick. It seems to be in "OK" condition but I am little bit concerned of my plan.

    Here are the layers:
    1. Current tongue and groove 7/8" thick subfloor
    2. A layer of modified thinset cement based mortar (using 1/4" notch trowel)
    3. 1/4" Wedi building panel
    4. SunTouch heat mat
    5. at least 1/4" modified thinset cement based mortar (or maybe SLC) to cover the mats (let it dry)
    6. 1/4" modified thinset cement based mortar
    7. 3/8" ceramic tiles 12"x24"

    Do you think I should strengthen the subfloor? The joists are spaced 16 OC.

    Please advice,

    Thank you.
    Last edited by Drewski123; 09-02-2011 at 11:58 AM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Big hiccup...none of the tile substrates are rated for application directly over dimmensional lumber - you need at least a 1/2" of ply, C-faces or better (i.e., no D side) installed across the joists, and exposure I or exterior glue holding the ply together. Then, lots of screws to hold it down (screw any loose planks down before you start). Then, and only then, IF the joists are sufficient, can you tile things. You can use the Wedi, but are you sure it is rated for having heat applied over it? The amount of insulation 1/4" would provide is marginal, and probably not worth the extra cost. You have a bunch of choices on decoupling, you could install cbu-mat-tile, or tack the mats to the ply, put slc over it, then a decoupling membrane like Ditra, and then the tile. You can also check out www.johnbridge.com for some ideas on tiling. SLC over ply generally needs to be at least 1/2" thick over the highest point. If you install cbu first, you can put a thinner layer of slc if you wish to go that way. It's MUCH easier for a first-timer to get a good slc pour if it is thicker rather than thinner. Just like your pancake batter doesn't flow to fill the pan keeping a certain thickness, slc does too. Only if you get enough down, break the surface tension, does it actually flow, and then, only if it is thick enough. But, for a novice, getting a nice flat floor by trying to spread thinset out over mats without nicking wires is not the easiest thing, either.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member Drewski123's Avatar
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    Thanks for your advice. According to Wedi spokeperson, a heat mat can be applied on top of the Wedi board.

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    DIY Member Drewski123's Avatar
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    Is the Green Glue noiseproofing compound good enough to glue the ply to the subfloor?

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    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Post(s) deleted by John Whipple
    Last edited by johnfrwhipple; 03-18-2014 at 07:51 AM.


    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    You do NOT want to glue ply to your dimensional lumber! Or, generally, to a layer of ply underneath. You need 100% support between layers, and a thick glue you squeeze out of a tube will NOT give you a nice, consistent, complete layer - you'll end up with some voids, and tile hates voids. With stuff like cbu, the idea is decoupling, and glue just doesn't work. If you were trying to obtain maximum stiffness from ply, you can laminate (not simply glue) two layers of ply together if you use a full spread wood glue like Titebond II (it's easier and cheaper to just use thicker ply - exception, stone requires a two layer subfloor). You need a LOT of it, gallons on any sized room), a spreader to get a good coat everywhere, and speed so you can lay the sheet of ply into the wet glue before it starts to skin over, then the proper method of sinking the hundreds of screws needed. Do it wrong, and you'll end up with a much worse structure for tile than you start with. Generally, it's not for the feint of heart. The industry standards just call for screws. Plus, if you glue layers of ply together, you'll destroy the structure if you ever do decide to remodel!

    If you go with Wedi, you MUST follow their instructions, which call for it to be set in a bed of thinset, sort of like a huge tile. If you use cbu, the same thing, except you also need to add a bunch of screws. In the first case with Wedi, the thinset actually holds it, but for the cbu, the purpose of the thinset is to actually fill in any minor imperfections and provide 100% support - the screws hold it in place. You'd likely use a different thinset to set the Wedi to the ply verses cbu to the ply, as you could use a dryset thinset for the cbu, but would likely need a modified thinset to embed the Wedi to plywood.

    You need to read the installation instructions for the products you are contemplating. Another good item is the TCNA handbook (Tile Council of North America). This handbook describes the tested, known to work methods of doing nearly anything tiled. It is the basis of many of the building codes. There is often more than one way to do something right, but many more ways to mess up. It's good you're doing some research, but verify your resources, too.
    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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    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Post(s) deleted by John Whipple
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    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The whole reason for ply over dimensional lumber is that that base moves too much. You don't want to glue to it. If you want to laminate sheets of ply together, you can and it is stronger, but if not done right, you'll have a mess. If you hit a void when screwing the ply down, move the screw! The TCNA handbook, from my last look, does not advocate gluing anything to planks. They do that for a reason. IF you do glue anything, other than say a construction adhesive on the top of the joists, you want a full-spread glue. This is almost impossible with anything you squeeze out of a tube. IF the layer you are putting down is stiff enough to bridge any gaps or voids, you may not notice, but the object is to get intimate contact, and a thick glue you squeeze out of a tube will NOT give you 100%, even coverage with intimate contact.
    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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    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Post(s) deleted by John Whipple
    Last edited by johnfrwhipple; 03-18-2014 at 07:51 AM.


    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    They do not advocate installation of Ditra over sawn lumber unless you add at least a nominal 1/2" of ply, just like I suggested. See page 9 http://www.schluter.com/media/DitraHandbook.pdf.
    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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    DIY Member Drewski123's Avatar
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    I just calculated the deflection and I got L/318. Looks like not enough for a ceramic tiles which I want to install.

    I am attaching a couple of sketches of the bathroom floor right above the basement. The sketch is not in scale but the dimensions that are shown in text are right dimensions. Could you take a look at it and let me know if there is a way to have the tiles installed even though the deflection is below the recommendation?
    The area in red will have tiles installed. The entrance to the bathroom (not shown) is between the cabinet and the tub.

    Thank you,
    Andy
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    ACO Shower Drain Sales johnfrwhipple's Avatar
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    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

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    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

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    DIY Member Drewski123's Avatar
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    Thanks John. I already have blocking in the center of joists.
    Also, on John Bridge forum somebody mentioned that the measurement of the joists should be taken at the unsupported joists only, is that true? If it is, then in my case the joists would be about 14 '25". According to deflecto calculator (using 14' joists) I am getting a deflection of L / 380 (good for ceramic tiles).

    If it is not true, my second option would be to add sister joists. In this case I could easily add at least half of additional sister joists.

    And lastly (third option), I may add a supporting wall in the basement (basement is unfinished). There is an area right under the current bathroom that I plan to convert to a basement bathroom (there are already installed drainage / vent pipes).

  15. #15
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by johnfrwhipple View Post
    Andy (sic) blocking could help you with your final deflection numbers. I would consult a local structural engineer and ask for a new blocking layout. Perhaps you may need to double up all the joists.

    JW
    All blocking does is transfer load to an adjacent joist and keep them from twisting, which maintains whatever vertical strength they have; blocking does NOT decrease the deflection - only shortening or strengthening the joist vertically can do that. Preventing the joists from being able to twist maintains their strength, it does NOT increase it, nor does blocking (except by helping to maintain what strength they have). Keep in mind that the subflooring provides the same general load sharing, and they do not rely on increased strength subflooring to account for joist deflection issues - it is required to provide strength BETWEEN the joists, not along them (especially since the max strength is directional on most subflooring, it is quite a bit weaker along the joist than between them).

    Re glue between ply and planks...the whole reason you want the ply there in the first place is to decouple the tile from the seasonal movement of the planks...the planks are likely strong enough in deflection between the joists in the first place. Each stage of decoupling progresses to the maximum stability required for the tile. Gluing the ply to the planks just helps ensure that that movement is transferred through the floor, negating most of the purpose of the ply in the first place. They omit saying to glue on purpose. Same idea with the cbu...testing proves that over time, the screw anchor ends up crushing the cbu since it is tied to the less stable subflooring, but the cbu, if installed properly with the thinset and tape, remains monolithic (albeit with some microscopic crushed holes around the anchors). The thinset underneath cbu is not there to hold it down, it is there to hold it up with 100% support (i.e., vertical deflection). Glue is good between the subflooring and the joists. It is okay between like materials (i.e., ply), but takes careful application with the right materials and technique. All of the TCNA subflooring prep is based on no glue, but, as said, gluing the subfloor to the joists is a good practice - the TCNA generally takes over from a properly installed general application subfloor and converts it to one that can support tile. Laminating ply to ply is a lot of work for not that much improvement (yes, a laminated set of ply IS stronger than those just screwed together), but if you do it wrong, it's worse. And, it makes it a major effort on a remodel down the road as you'd likely have to cut and chop the whole shebang out and start over, increasing costs by a lot. If you had two idential floors, one with blocking and one without and put identical weights in the middle of the span, they would both deflect the same amount assuming that the joists were held properly to the subflooring. The blocking helps to keep them vertical and stable.

    Re how to measure for joist deflection - yes, you must measure the joists between structural support members UNDERNEATH the floor (i.e., what is holding it up). Those structures could be the rim joist, a beam, or a structural wall (note, not all walls are built to be structural!). Take a worst cast, a very small room in the middle of the joists. Say the joists can deflect 1/2" under a 'normal' load. Take that same floor and shorten the total length, even though the only area that is tiled is still in the middle, and the deflection may drop to 1/8"...much more tile friendly. Tile is very brittle, nearly any movement will either break the bond, or break the tile, or break the grout, whichever is the weakest. A joist tends to bow - point deflections don't generally happen, so you need to take into consideration the whole length, regardless of where the tile is. Now, if you're only tiling around the perimeter, say right next to the support, you might get away with it since at the supported end it doesn't move much, but that is an atypical situation.
    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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