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Thread: laying tile floor - what is proper subfloor?

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    DIY Member kavita's Avatar
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    Default laying tile floor - what is proper subfloor?

    greetings folks,

    best new year's wishes to you all ...

    i'm going to lay a tile floor in the bathroom. it's about 10' x 10'.

    the original subfloor is rough-cut 1" x 6" plank (so it's true 1"). i've pulled up the various years-worth of decaying add-on subfloors (lauan, ply, vinyl, etc) so the rough-cut plank is all that's left.

    some of the boards are slightly cupped, but the floor is quite level overall.

    what are your suggestions for the best foundation for ceramic tile, in addition to the solid subfloor there now?

    many thanks,

    kavita

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    Plumber jimbo's Avatar
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    Ceramic tile should have 1" minimum substrate. A good base would be to put down " Hardibacker using screws on every "dot" and thinset mortar underneath. Use an isolation membrane on tob of the Hardi.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I've spent a bunch of time reading at www.johnbridge.com which is pretty much dedicated to tiling. They've got a structural engineer over there that can help. Double check, but from what I've read there, on top of plain lumber subfloor, before tile, they want a minimum of 3/8" plywood (exterior grade with no "D" sides, ac, ab, or bc). The subfloor is only part of the equation - you need to know what the joist size and maximum unsupported span is. Also, are you considering ceramic or stone tile? A stone tile needs twice the stiffness of ceramic. Check it out.
    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 kavita's Avatar
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    Default tile talk

    thanks for the input ... now that i'm visiting the john bridge tile website, i'll learn the language - an isolation membrane doesn't sound desirable in my current vacabulary, but i'm sure it IS for tiling purposes!

    i think i'll be using very simple mosaic tiles backed with netting for this particular bathroom.

    this is a small job within a rather huge building project and i hope to accomplish it fairly quickly (for me, anyway) so i can move into the house and have one partially completed bathroom to use.

    it would seem that mosiac tile is one of the easier bathroom applications.

    i'm also considering pergo (thanks also for your great laminate-in-bathrooms considerations in an earlier thread) but don't know if installing pergo with glue is more challenging than mosaic tiling for a fledgling installer.

    any thoughts on that one?

    thanks again - kavita

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    Plumber jimbo's Avatar
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    Most of the Pergo and other brands of laminate flooring today use a snap together rather than a glue joint. They aslo are rated by their manufacturers for use in bathrooms. Most of us have seen much too much water on a bathroom floor sooner or later from leaks, overflows,or even just normal usage to be comfortable with a wood based product. But it is done often, and often with great success.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The solid wood subfloor, while strong, expands and contracts quite alot between seasons, that is why you at a minimum need some plywood on the floor. 3/8" is the minimum, since none of the 1/4" stuff is structural nor does it have the exterior grade glue necessary for under tile. For a mosaic, you wouldn't want to use Ditra, but other brands of isolation membranes could be used. The membrane helps to prevent the movement of the subfloor from messing with the tile. The little towers on the Ditra would make laying mosaic nearly impossible without some extra work, so probably isn't a good choice. Keeping all of the mosaic tiles level takes practice, plus alot of grout lines, so any flex will cause cracks if not done right.
    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 Don Zorn's Avatar
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    You will need a minimum of 3/8" ext. grade ply on top of the plank flooring, then 1/4" CBU thinsetted to the plywood with non-modified thinset.

    Recommend that you take Jim's advice and visit the John Bridge tile forum or the floorstransformed tile forum at http://floorstransformed.com. There are professional tile mechanics who frequent both of those forums who can guide you through what you need for a subfloor prior to installing the tile.

    Don Zorn

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    DIY Member GregO's Avatar
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    Kavita,

    If it were mine, I'd add screw down 1/2" BC plywood and then thinset mortar (NOT premixed) 1/4" hardibacker, and then thinset (again, not premixed) the tile. If you want to save an extra 1/8", you could install Ditra matting atop the 1/2" plywood instead of hardibacker. Good luck and have fun...

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    DIY Member kavita's Avatar
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    Default so many ways!

    thanks all, for the input.

    just visited the john bridge website again - my head is swimming with the myriad methods for properly laying a tile floor in a bathroom! there are certain points upon which no one seems to entirely concur.

    i've already laid the 5/8" ply over existing subfloor, and have puchased 1/2" Durock (hardibacker) and not-pre-mixed thinset.

    i'm perplexed (even after, or maybe especially after!) reading numerous posts on the john bridge website about this isolated membrane aspect.

    some folks are adamant that one must be used and others eschew them completely.

    some folks use ply + hardibacker + a membrane, some folks use only the ply + membrane OR hardibacker.

    some folks insist Ditra must not be used with mosaic tiles, others question that wisdom.

    some folks won't use ply at all.

    so ... i'm a bit perplexed.

    i want to lay a floor that will be compatible with tile, and have checked the joist spans and original subfloor specs, added sufficient plywood laid with room to expand / contract, and purchased hardibacker probably in excess of the thickness i really require.

    however ... am i missing the point if i do NOT include a membrane in the equation, since this is a bathroom / damp setting?

    [yes, i'll also post this over at johnbridge.com ...]

    thank you all so much for your time and info,

    kavita

  10. #10
    DIY Member GregO's Avatar
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    Kavita,

    I know it can all be very confusing, but don't let it bother you. For a 10 x 10 bathroom, installing 5/8" plywood over your plank flooring and mortaring 1/2" Durock to that is plenty for a room that size; you do not need an isolation membrane. Something like that would be good for questionable applications where subfloor movement/flex is likely to occur. I always recommend a high quality thinset mortar such as Custom Building Product's Flexbond. It's a little more expensive than basic setting mortars, but it equals pennies per square foot extra.

    Do take a look at the Q&A section of www.ceramic-tile.com as well. That site has very good free advice for general applications like yours - you can search the archives for specific advice. Do it once, Do it right. Good luck and have fun. Keep us posted.

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    Plumber jimbo's Avatar
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    I agree with the last post. My original suggestion to use a membrane was not well thought out. A membrane is an isolation device to prevent slight cracks or movements in the substrate from coming through into the tile. It is useful when putting ceramic on a concrete slab, which is subject to hairline cracks, and sometimes has anti-fracture joints in it.

    Over a wood subfloor, the main criteria is to develop enough total thickness to prevent any flexing under load.

    In addition to the other references mentioned in this thread, the Tile Council of America ( www.tileusa.com ) put out an excellent handbook which can be ordered on line at reasonable cost.

  12. #12
    DIY Member Don Zorn's Avatar
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    Kavita - I am only a DIYer with four tile projects under my belt - currently working on the fifth - so take my advice with a grain of salt. Forget the isolation membrane - that is primarily for setting over cracks in a concrete floor or if you are tight for headroom and don't want the extra height that CBU will add to your overall height.

    The 5/8" exterior grade plywood on top of your subfloor should be gapped by 1/8" in the field and by 1/4" around the perimeter of the room. Lay the 5/8" ply at 90 degrees to your subfloor and stagger the joints so that you don't have four corners meeting in one spot.

    Screw the 5/8" ply only to the subfloor - not into the joists - every 8" in the field and 6" around the edges. Use deck screws or plated for corrosion resistance - not drywall screws. Thinset the 1/2" CBU with un-modifed thinset (the cheap stuff = $8/bag) - the purpose is to fill voids below the CBU, sao that there is no movement later on - NOT to adhere the CBU to the plywood. You can use a 1/4 x 1/4 square notch trowel for this step.

    Immediately screw or nail the CBU as per the mfgr's recommendations while the thinset is wet. Leave 1/8" gap in the field and 1/4" around the perimeter with the CBU. After the CBU is set, stay off the floor for 24 hours. Caulk the perimeter of the room before setting tile.

    To set the tile, use a good quality latex modified thinset - don't cheap out here. While setting the tile, work the latex modified thinset into the gaps in the CBU and use a margin trowel to push fibreglass mesh tape (special stuff for CBU) over the CBU joints as you are setting the tile.

    Good luck!

    Don

  13. #13
    DIY Member kavita's Avatar
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    Default thanks, and a question about screws / ply ...

    thanks guys, i accept your invitation to relax and keep it simple here!

    your clarifications are most helpful. it seems much less esoteric now.

    i'm curious about your suggestions re: screwing down the ply, don. could you further clarify something for me?

    >> Screw the 5/8" ply only to the subfloor - not into the joists - <<

    does this pertain only to subflooring beneath a tiling job, or is this what you also recommend for ALL ply subflooring over planks?

    i.e., when you lay a subfloor that'll be covered by carpeting, hardwood, laminate, etc do you also screw the ply only into the base floor?

    i'd imagined it would be ideal to grab the joists to the ply for extra hold, so i'm really interested to hear your reasoning. makes me think about the movement (over time) of joists, and how that movement effects the ply thus the tilework (in this case).

    lastly, i'm still perplexed about the water resistance factor if i skip the membrane. does the durock provide a measure of moisture resistance? some folks talk about a moisture/vapor barrier ... anybody use these on a wood subfloor application?

    i so appreciate everyone's help! i've blazed DIY projects solo for too long and learned the hard / frustrating / disappointing way enough now to really appreciate asking for advice. some of us are slow learners

    best to all,

    kavita

  14. #14
    Retired Defense Industry Engineer jadnashua's Avatar
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    For areas that get wet, a membrane is not a bad idea, especially with a wood subfloor. If you wipe up spills, it shouldn't be a problem on a normal floor. Moisture does get through the grout but it takes awhile. If you have kids that slosh water all over the floor each time they shower or bathe, then it may be a good idea. Tile should stay until you tire of it, not because of a failure. So, depending on the circumstances, waterproofing the subsurface helps. An isolation membrane is not necessarily waterproof - you need to follow the procedure to seal the butted up seams. It does provide crack resistance, regardless of whether you seal the joints or not, and is often used without sealing them.
    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 GregO's Avatar
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    Hi Kavita,

    As for screwing the plywood down, I would definitely screw into the joists and to the plank flooring to firm things up. Don't go too crazy with it, but make sure you DO NOT use drywall screws, but coated screws like prime guard decking screws.

    While backerboard is not waterproof per se, it is greatly water and rot resistant (it is cement-based), so it will withstand any normal wear and tear that a bathroom goes through on a daily basis. When installing the backerboard, screwing it down atop of the thinset mortar with specific screws (e.g., Hardibacker or Rock-On brands) all over as recommended (usually 6" along edges and 8" in the field) is best. Then embed backerboard tape and tile away with the same or better thinset.

    So, yes the Durock and your thinset mortar will provide plenty sufficient moisture resistance. In addition, if this bathroom is over a crawlspace, I would make sure you have 6mil or thicker poly plastic sheeting throughout the entire crawlspace to prevent excess moisture that could affect other areas of your house as well.

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