subfloor advice

Discussion in 'Remodel Forum & Blog' started by Emma3, Oct 31, 2007.

  1. Emma3

    Emma3 New Member

    Messages:
    20
    Location:
    Washington
    We are in the midst of a bathroom remodel and trying to prep the room for our plumber to do the rough-in. We removed the old tub and drum trap assembly, and the vinyl flooring, then salvaged a layer of fir under that for use elsewhere (ok, my husband did all this, but he's out of town so go easy on me :)). Anyhow, it turned out that the subfloor under the fir was rather cut up in places, though it feels sound everywhere else, despite how it looks in the attached pics.

    Here's what we need to know:
    -do we leave the existing subfloor in place and put additional layers over it despite the holes?
    -what do we install over it? A layer of ply and then hardibacker? (the new floor will be tile). What grade/thicknesses of each? Screw to joists or to subfloor between joists?
    -The new tub is a drop-in style, though it will be installed in a 3-wall application. Should the substrate under the tub be the same as the rest of the floor (hardibacker) or does the mortar bed go on the ply? Does poly go both under and over the mortar? (BTW - I think we're clear on how to install stringers for tub support, just that they are dependent on floor height, so need to address that first).
    -Assuming we put ply and/or hardibacker over the existing subfloor, should we cut holes in advance to allow for the drain? How big a hole? Although the basement below is finished, there is an access panel in the ceiling near the drain.
    -What happens to the mortar bed in the area of the drain? I assume the drain won't be there yet when the tub is set because they want to set the tub before roughing in any valves. Does that make sense?

    That's all I can think of at the moment. Thanks!

    Attached Files:

  2. statjunk

    statjunk DIY Senior Member

    Messages:
    542
    I think most of these questions would be better answered at the John Bridge forum. www.johnbridge.com. Check it out.

    In the meantime here is some of my .02.

    Consider that you'll need to put ply and hardi over the plank and what it will do to your floor height once installed. I'd consider pulling all of that up and lay brand new 3/4" ply with 1/4" hardi over that. The tub gets mortored in around the drain area and yes you lay the mortor right on the ply. No Hardi under the tub. If you use ply it goes over the mortor under the tub to stop the mortor from adhereing to the tub. Yes you should cut your holes for sewer services to be installed later. Consider the size of your toilet when doing this.

    However you'll likely need to run your joist type, span and flooring type through the deflecto meter on the John Bridge website to see if maybe you need to sister some joists and or add additional layers to the floor to stiffen it.

    Tom
  3. Emma3

    Emma3 New Member

    Messages:
    20
    Location:
    Washington
    Thanks for the link - looks like that site has a ton of information. Couldn't find the deflecto meter though... I'll keep looking.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,809
    Location:
    New England
    Deflecto is in the blue tool bar. You will need to know the size, length, spacing and wood type. Length is the length between supports, not necessarily the length of the board and is rarely the size of the room. A support can be the rim joist, a beam, or a support wall underneath. Many people make the mistake of using the room size.
  5. Emma3

    Emma3 New Member

    Messages:
    20
    Location:
    Washington
    Aha - thanks. Here's my score:

    Thank you for using the John Bridge Forums Deflect-O-Lator :)

    For joists that are SYP or Douglas Fir, in good condition, 9.25 inches tall, 1.5 inches wide, 16 inches on center, and 9.5 feet long between supports, the deflection calculated is 0.119 inches.

    This translates to a deflection of L / 962.

    Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile or Natural stone, Congratulations!

    So it sounds like I don't need to do any extra reinforcement, but the recommendation of removing the planks and then using 3/4" ply + 1/4" backer would still apply. Do I have that right?

    Are there any other pros/cons to leaving the existing planks in place other than finished floor height? Right now the adjoining hallway floor is about 3/4" above the bathroom subfloor planks, and the planks themselves are about 3/4", so if I remove them and start from scratch I'll have 1 1/2" of height to make up. We're planning to add heating mats under the tile, so I guess that puts us pretty close to flush by the time we're done with ply-hardi-heat-tile. Am I missing anything?
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,809
    Location:
    New England
    You can't install tile on dimmensional lumber...it moves too much, even if it may have good deflection numbers. As long as the planks are in decent shape and you've got them screwed down to the joists well so they don't deflect, then they still add to the strength of the end subflooring. 3/4" of good ply is okay to meet the minimum requirements of tile all on its own (although people like more, and more is required for stone), so that on top of the planks will make a very sturdy floor for tiling.

    Personally, unless I needed the height, I'd use Ditra instead of cement board. Much lighter, and easier to install. Check out www.schluter.com to see a video of it being installed and check out the properties. CBU does not add strength to the floor for all practical purposes, but it does create a good decoupling layer that bonds the tile well. Ditra does it better.
  7. Emma3

    Emma3 New Member

    Messages:
    20
    Location:
    Washington
    By "extra" reinforcement, I meant in addition to 3/4 ply + backer ;-)

    So here's my dilemma: If I leave the existing 3/4" planks in place, I only have 3/4" of height remaining to accomodate additional ply, ditra or cement board, heating mats, and tile. In that case, wouldn't I be better off removing the planks and installing 3/4" ply, rather than leaving them and adding a very thin ply?
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,809
    Location:
    New England
    Yes. The minimum ply that can be added on top of planks that will support tile is 1/2", and if you have some voids in the existing subflooring, that wouldn't do.

    So, if you remove the planks, you can use 3/4" T&G ply, heating mats, a membrane, then your tile and you'll be at the minimum. Well, if you want, (it is a lot of work), you could get things lower by putting cleats on the joists, installing the first layer of ply between the joists on the cleats, then installing at least 1/2" of ply over the whole thing. This would save you 1/4". Note, you'd be installing 4' wide pieces...the grain must run between the joists, not parallel to it. Most people don't worry too much about a height transition between rooms...it is fairly common. A good tranistion makes it all work out. Make the tiled floor strong enough to last, then worry about how to tranistion. The easiest is to a wood floor - a short wooden ramp connecting the two looks and works well. If the area is carpeted, there are several methods that work as well.
  9. Emma3

    Emma3 New Member

    Messages:
    20
    Location:
    Washington
    Thanks a million! Seems to all make sense now. The planks are definitely coming out, especially since we just ripped a few more out while the plumber was here taking a look-see :)
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