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Leveling a bathroom floor for tile and tub

Bindee's picture

I'm trying to figure out the best way to level out my old bathroom floor and at the same time prepare it for bathtub placement and tiling. The entire bathroom is about 8x5 feet and from the back left corner to the front door it slopes 1 and 3/8ths of an inch. I can only heighten the floor a maximum of ¾ of an inch (including tile height) to match the existing maple flooring in the next room and to come to the bottom of the cast iron toilet flange so I won’t be able to completely remove the slope but I thought I could level it out a little. To level out the entire room would you use asphalt shingles sandwiched between the plywood subfloor and 1/4 inch backer board (permabase)? Or would you just recommend pouring floor leveling compound over the plywood subfloor (prior to the pour I’ll use the primer and I’ll staple hexagonal wire on top of the subfloor per manufacturers directions) then applying hydraflex over the leveling compound to prepare it for the tile and tub.

SLC (post #211063, reply #1 of 9)

With proper perparation, SLC would be your better bet.

It'll find cracks and crevices and run where it can, so plug or dam any escape routes. 


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check on TCNA specs (post #211063, reply #2 of 9)

Do you know the structurall specifications of your floor system (i.e. joist size, spacing, underlayment)? I recommend that you detail your tile job with TCNA standard F180-05 or FF185-05 (poured gypsum & SLU respectively) in mind.

Most importantly, according to the Tile Council of North America, "design floor areas over which tile is to be applied to have a deflection not greater than 1/360 of tfhe span for ceramic tile whene measured under a 300# concentrated load". You may also want to incorporant a decoupling membrane (like Schluter's Ditra) into your installation as well.

Thanks for your input. The (post #211063, reply #3 of 9)

Thanks for your input. The house is old; it was built in 1920. The joists are 2x10. I sistered one joist because the original plumber had cut through it causing the floor to sink. I raised up that area with a floor jack - but the whole house is just crooked. The joists are spaced 16 inches on center and I added a LOT of blocking between the joists where I will be laying the tile. The area that will be tiled has 3/4" green treated plywood as the subfloor. The area where the bath tub will go has the original subfloor 3/4" thick with 1/4" spacing between the boards.

Would you use backerboard/ditra membrane on top of the plywood subfloor and then cover with floor leveler or would you put the floor leveler down first and then the backerboard/ditra membrane on top of that (this is what was recommended at Menards). Or, since I have very limited height (maximum of 3/4" to meet the toilet flange), could I just use floor leveler right on the subfloor without backerboard/ditra membrane?

other options... (post #211063, reply #9 of 9)

A better plywood sublfoor grade is CDX or BC. However, if you're set on using the existing pressure treated, at least  make sure that it has a moisture content of 8% or less. Sounds like it's going to be hard to get the floor straight and properly bonded to this though. If you're going to try to use a thickened SLC to float out of level, I would first pull a tight skim coat of latex modified thinset over the P.T. plywood to promote a good bond. Let it dry a full day at least. Then tack your wire in the thicker areas to be floated "straight" and go at it with the SLC. Remember that your SLC is not going to give you any real degree of structural integrity to the floor. You're pretty much limited to the 3/4" P.T. underlayment strength which is questionable from a TCNA perspective.

Two other options I would consider if I was fighting the build-up height restriction you indicate:

1. Remove the plywood and set up for a traditional mud set base. A mud set base should generally have a thickness of 1" or more. Your probably not going to get that with your existing P.T. subfloor in place. Many tranditional mudset floor in older homes utilitze mud set bases that are screeded over dropped panels between the joists in order to gain thickness for the mud pack. Usually the tops of joists are shaved to a point or "ridge line" along the span. This serves to  reduce sheare stress in the mud pack and distribute the compressive weight along the joists better. It's just good structural geometry. The thickness at dropped panels (which are simple plywood panels screwed to cleats where are, in turn, screwed to sides of joists) can be anywhere from 2"-3". Chicken wire mess is used over the entire field. Over this mud base I would still install Ditra using non modified thinset like Ditraset by Bostik. It's only a build up of 1/4" for this additional substrate and your getting a waterproof membrane and a decoupling membrane in one. Thinset your tile to the Ditra, again, with non-modified thinset.

2. Remove the treated plywood and shim the tops of your joists to plane out. Use a string or straightedge to get what you need. You can either shim the tops with 2x rips or sister 2x4's to the sides of existing joists with structural screws. Then install 3/4" CDX glued and screwed to shimmed joists or sisters. Lastly, Install 1/2" BC screwed 8" O.C. and thinset your tile directly to this substrate with latex modified thinset.

Good luck. Have fun.

Tile base need to be FLAT (post #211063, reply #4 of 9)

Since you can only raise the the floor 3/4 inch, and the existing floor slopes more, the SLC will not give you the results you are looking for. It will level the lower portion up to the 3/4 inch depth, but there will be a kink in the surface, and a different slope up the the 1 3/8 level. Packing a mud floor will allow you to get a flat surface that can be tiled.

Would you "pack a mud floor" (post #211063, reply #5 of 9)

Would you "pack a mud floor" right on top of the plywood subfloor or would you put backerboard down first? Thanks

No backer board, mud floor (post #211063, reply #7 of 9)

No backer board, mud floor goes on the subloor.

Suggested order of operations:

1.  Check subfloor and repair as required.  You have a small area, so deflection is not likely a problem, but  always a good idea to check floor deflection.  Add attional joists, bracing, as required.  Large floor tiles need a much stiffer floor than smaller tiles.   Add additional screws to firmly attach sub floor to joists.

2.  Mark the desired height of the packed mud at several locations along the wall.  These marks will be used to set the screed lines.  Take your time and get this right.  This is easy if you are setting a level floor, (use a laser ).  A little more difficult if you are setting a sloped floor.  The marks must be set so that they give a flat floor surface.

2.  Isolation member on top of subfloor.  15# tar paper is traditional, newer crack isolation members might be preferable (need to check with the tile experts, I am an amateur at tile work)  Also seals off the subfloor from the damp mud.

3. Expanded metal lath.  Stapled or nailed to subloor.  Provides reinforcment to the mud floor.

4.  Make the screed lines.  Dump mud along the walls, pack down and level with the desired height markings. These should be wide enough to guide a screed for the next step. 6 to 8 inches wide should work.

5. Fill in the middle, pack down a little higher than the sidewall screed lines. Then screed the surface with a straight edge so that the floor is even with the screed lines. Trowel smooth and repair any missed spots.  

6. Wait.  Let the mud cure at least 24 hour.  Longer if you have the time.

7. Tile.

Lots of videos on YouTube.   Search for "dry pack mud floor"

A trick I used once, for (post #211063, reply #6 of 9)

A trick I used once, for getting a layer of concrete (not packed mud) level, was to drive drywall screws into the floor at several points, with the heads lined up with the eventual top of the mortar.  You can, eg, throw a level or straight-edge across the screws to get them level/flat.  This worked out well in a corner situation where access was awkward.

The screws can, of course, be removed after the mortar has partialy set.


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Thanks for the information. (post #211063, reply #8 of 9)

Thanks for the information. I found info online regarding mud floors which said that the mud floor should be 1 and 1/4 inch thick. Unfortunately, I only have 3/4" (including tile height) to work with. Given my height restrictions I am not sure the mud floor will work for me. Is it possible to apply the SLU to backer board in the lower areas of the room and then fan out the edge so that the transition will be smooth. I am using 1 inch hexagonal tiles so I think smooth transitions would be okay.