laying tile floor - what is proper subfloor?

Discussion in 'Remodel Forum & Blog' started by kavita, Jan 15, 2005.

  1. kavita

    kavita New Member

    Messages:
    41
    Location:
    vermont
    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
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Ceramic tile should have 1¼" minimum substrate. A good base would be to put down ¼" Hardibacker using screws on every "dot" and thinset mortar underneath. Use an isolation membrane on tob of the Hardi.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    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.
  4. kavita

    kavita New Member

    Messages:
    41
    Location:
    vermont
    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
  5. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    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.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    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.
  7. Don Zorn

    Don Zorn New Member

    Messages:
    33
    Location:
    Alberta
    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
  8. GregO

    GregO New Member

    Messages:
    73
    Location:
    Virginia
    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...
  9. kavita

    kavita New Member

    Messages:
    41
    Location:
    vermont
    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. GregO

    GregO New Member

    Messages:
    73
    Location:
    Virginia
    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.
  11. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    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. Don Zorn

    Don Zorn New Member

    Messages:
    33
    Location:
    Alberta
    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. kavita

    kavita New Member

    Messages:
    41
    Location:
    vermont
    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 :eek: :)

    best to all,

    kavita
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    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.
  15. GregO

    GregO New Member

    Messages:
    73
    Location:
    Virginia
    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.
  16. Don Zorn

    Don Zorn New Member

    Messages:
    33
    Location:
    Alberta
    Kavita - This only applies under tiles.

    I too thought this was strange and counter intuitive - but learned this tip from a couple of professional tile mechanics. Here's the reason:

    If you screw the top layer of ply into the joists and there is any movement in the joists, it is more likely to transmit that movement to the tiles - which can lead to tile/grout cracking. In tile setting any movement of the subfloor is considered bad.

    If you only screw the top layer of ply to the subfloor below, movement in the joists is not quite as likely to get transmitted to the upper layer of plywood - in effect decoupling the joists from the top ply. Albeit the bottom layer is still screwed into the joists and the top layer is screwed into the bottom layer so they are still coupled indirectly - but at least not directly.

    I personally believe that if you don't follow this advice, it is not a catastrophe, but if you do follow it, it is just one more step that will only help to make your tile installation more successful. Good tile meachanics subscribe to this methodology - feel free to ask on the Floorstransformed ceramic tile forum.

    In my opinion, you should be OK without waterproofing you bathroom floor before tiling as long as you don't leave standing water for long periods of time on the tiles.

    My $.02 - worth the price charged!

    Good Luck.

    Don
  17. Don Zorn

    Don Zorn New Member

    Messages:
    33
    Location:
    Alberta
  18. GregO

    GregO New Member

    Messages:
    73
    Location:
    Virginia
    I understand what Don is saying, but Don, I somewhat disagree with you. Please hear me out... Atop of plank flooring, firmly securing the plywood is critical to ensure a stable substrate. Atop of that plywood, a tile underlayment should then be placed (e.g., hardibacker, durock, wonderboard, etc.) The specific tile underlayment is what needs NOT be secured to the joist, but obviously to the subfloor only the depth of the subfloor.

    There is no way I would only secure plywood only to the plank flooring and not the structural timbers; that would be asking for trouble.
  19. Don Zorn

    Don Zorn New Member

    Messages:
    33
    Location:
    Alberta
    Greg - I hear ya. I thought the same thing - screw the underlayment right through the subfloor into the joists = nice solid floor = less tile cracking. But apparently I was wrong. It is even listed in the TCA handbook - only screw the underlayment into the subfloor. :)

    Now this is for a plywood underlayment with plywood sub-floor. Maybe with plank subfloor it is different. That I am not sure about. :)

    Don
  20. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    Prior to putting down the plywood, make sure that the planks are firmly attached to the joists - it wouldn't hurt to add some screws, then nail (or screw) the plywood to the planks, avoiding the joists. If nailing, then use ring-shanked nails. If you hit a joist when doing the plywood, don't worry about it, but try to avoid them.
Similar Threads: laying tile
Forum Title Date
Remodel Forum & Blog laying down vinyl tile on painted cement basement floor Apr 13, 2009
Remodel Forum & Blog The ART of laying it out Jan 24, 2012
Remodel Forum & Blog Leveling a low spot AFTER laying HardieBacker Board Jan 29, 2008
Remodel Forum & Blog Advice on laying out recessed lights Oct 9, 2006
Remodel Forum & Blog best tile to concrete cement Jul 1, 2014

Share This Page