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Thread: How to construct my subfloor for ceramic tile

  1. #1
    DIY Junior Member DIY_Steve's Avatar
    Join Date
    Nov 2012
    Portland, OR

    Default How to construct my subfloor for ceramic tile


    I'm hoping some of you with experience with remodeling bathrooms can help me plan how to build my floor system to include a heated floor, ceramic tile, and doorless zero threshold shower with trench drain.

    I am remodeling a small master bathroom. I removed all the floor and subfloor down to the joists in order to update plumbing and electric. Now its time to put in the new subfloor and floor. I removed two layers of what looked to be 5/8" plywood under carpet (nasty and lots of rot underneath), so I have roughly 1.25" available to be flush with the adjacent hardwood floor. There is a small shower that is about 36" square that I want to drain towards the bathroom into a trench style drain, zero threshold, such as the Proline Drain from QuickdrainUSA.com. Anyone have experience with these? I also want to include a heated floor such as Suntouch. Anyone have experience with these?

    So my root question then is what to put under all of this...I will finish with large ceramic tiles, say 12" square. I have heard two schools of thought on this from various people I have asked and DIY books from the library:
    1. One layer of 3/4" plywood, then cement board tile backer 1/4", then heated floor, then tile, and done - this should come close to flush with the hardwood floor adjacent
    2. One layer of 1/2" plywood attached to joists, another layer of 1/2" plywood attached to first layer but not to joists, then cement board tile backer 1/4", then heated floor, then tile, and done - this will put me above the hardwood floor adjacent

    Within the shower I would follow the manuf. instructions for the trench drain.

    One more question - some of the existing 2x10 joists have damage from demolition - should I sister them at the top edge with a 2x6 to bridge the damaged areas, and glue/screw the new 2x6 to existing 2x10? Or is the plywood good enough to bridge these areas?

    Your advice is appreciated! And don't assume I know anything, tell me everything I need to know...

    DIY SG

  2. #2
    Licensed Building Contractor dtherrien's Avatar
    Join Date
    Jan 2010


    Hi DIY SG,
    I would use 3/4"t&g plywood. Screw it down. I like to use decking screws.
    The 1/4" hardi backer board will work if you install it in thin set. Use good quality thin set...Like Mapei or Latacrete..and not from a big box store. Again, you need to put some screws in the cement board to make sure it is well bonded to the plywood. I take it that the heat will be the electric tile warmers? It can be installed on top of the cement board.
    It depends on how bad the damage is on the joist...If it is...then you will be fine with what you are planning.
    Also...I would insulate the floor while you have the joist exposed...If it is on the second floor.


  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    The first thing you need to do is to determine if the joists are suitable for tiling at all! This requires info on the species, size, spacing, and span of the joists (this is NOT the size of the room in most cases, it is from one supported end to the other). IF they are suitable, then it depends on the type of tile, either ceramic or natural stone. Natural stone requires addition strength on both the joists AND on the subfloor. Industry standards call for a MINIMUM of 5/8" ply (no 'D' faces, and exposure I glue) on the floor. Most pros do not like the minimums, so 3/4" would be their minimum. It is more important to have the floor strong enough so it lasts than to have a perfectly even floor heights between rooms. It's not hard to make a functional, good looking transition between tile and hardwood. Do not let the height be a major factor.

    Instead of cbu on the floor, you could use something like Ditra from www.schluter.com. This is thinner and easier to install. The heating would go underneath the Ditra. Schluter has a fairly comprehensive installation manual available for download (free) on their website to discuss the requirements and benefits.

    Making a curbless shower still requires the floor to be sloped at 1/4" per foot within the wet area to the drain. Getting that slope without raising the whole bathroom floor generally means recessing the shower area. This requires those joists to be stronger to maintain the required strength with their shallower depth, or more of them. If you build the shower using a conventional liner (not my first choice), that depression would need to be probably more than you could achieve and still maintain the needed strength and have room for the drain and trap. This would push you towards one of the surface membrane waterproofing systems from Schluter, Noble, or others.

    There are lots of discussions on this subject here...spend some time reading those over, then ask some specific questions based on your individual requirements. The search function is your friend here.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    ACO Shower Drain Sales johnfrwhipple's Avatar
    Join Date
    Jul 2009
    North Vancouver, BC
    Blog Entries


    Steve the first step is always a "Tile Layout" and a "Lighting Plan".

    Have you done these?

    This should be decide on before the demolition. When the job is demo'd or built you can see how the framing is installed, what shape it's in and what options are available in drains.

    We like to start with a tileable substrate in the beginning. Often for us this is a double layer of 5/8" plywood. When we can we add a 1/4 or 1/2 layer of cement board over top.

    Now your working off of a tank.

    Other considerations are entry height build up.

    Recessed framing.

    Divot method drain installs.

    Backup drains.




    Heated benches.

    All play a role in selecting the waterproofing system.

    Always plan on adding waterproofing (sheet membrane) over cable heat. Avoid the heating mats - they suck.

    Always flood test your showers!


    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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