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In a bathroom remodeling project, I noticed the newly installed plywood sub-floor is flat but not level, which may cause issues when installing the shower and the sink cabinet (water evacuation and shower door and cabinet not properly aligned with the walls).

Now, Internet tells me I should level my floor with self-leveling compound (after having caulked plywood joints and applied a coat of primer). Also, Internet tells me that putting backer-board over that (when settled) will cause it to break when I'll add the screws to fix it to the sub-floor. And Internet also tells me I could self-level the sub-floor and NOT use backer-board.

I'm confused.

What is the proper way to level my sub-floor before installing my tiles?

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You could do either:

  • level the floor or
  • level the individual parts (shower enclosure, vanity, toilet)

Which you choose will depend on the degree of out-of-level

Self Leveler: self leveler does not need to stay intact after the backer board install. It can fracture and still completely support the backer board.. its got nowhere to go and it doesn't migrate or degrade.

My personal preference is to use self leveler and then use a crack isolation membrane.
(Ditra is my preference, comes in 1/8, 1/4 and heating wire support styles)

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    +1 answer. The only problem with floor leveler is that it wants to be level to the earth without regard to the autonomy of the space you're working in. If the far side of the room is the high point, the threshold might end up being to high to keep your floors on plane. Then you have a little step up to get into your bathroom and then your mother-in-law trips going to the bathroom and sprains her knee. And then she's staying with you. No bueno.
    – user23534
    Oct 8, 2014 at 5:18
  • @paperstreet Yeah, this is one of my concerns. The previous installation had only one layer of plywood sub-floor and tile was installed directly onto that. Now, with the self leveling cement and cement backer-board, it's going to add a difference in my floors of at least 3/4". I still have to figure out how I'll make the transition.
    – user25447
    Oct 8, 2014 at 14:11
  • Better start TiVo-ing Mama's stories and fluff the pillows in the guest room. ;)
    – user23534
    Oct 8, 2014 at 15:50
  • I think I'll go with the full room. It's my first tiling project, so I'll try to remove all the risks I can. @paperstreet Fortunately, this is master's bathroom so she won't get near that doorstep often :P
    – user25447
    Oct 9, 2014 at 14:19
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The answer is screed. Screed is a portland based mix of 1 part portland cement and 3 parts sand (not masons sand, you want to use coarser sand, check your local stucco supplier and as a last resort use the bagged sand sold for pavers at the box stores).

Dry mix the 2 products together in a wheelbarrow, mud box or on a sheet of plywood. Mix twice dry then twice wet (usually 5 gallons of water to a mix of one 94lbs bag of portland/32-34 FULL shovels of sand). When the screed can be balled up and hold together it is good, you do NOT want WET screed.

Over a nailed or stapled wire lath, run 'ribbons' (6 or so inches wide) parallel to your longest run. Use a straight edge and occasionally check for level. When no gap is between the straight edge and screed you have a flat plane. As you run the straight edge (the actual screed, the term is general) you want it to be firm, meaning you shouldn't be able to push a finger into it. When you have 2 parallel screeds run (and level to each other), fill in the middle and screed as above (a flat trowel helps).

Note- you should have what looks like way too much screed on the ribbons and in the middle; moving the product back and forth helps achieve compaction. As a rule of thumb, you don't want to be any less than a 1/2 inch on top of the lath. Also, using screws through backerboard into self-leveler tends to create 'anthills' as the dust has nowhere to go.

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  • Why do you want course sand and not mason's?
    – Mazura
    Nov 16, 2014 at 3:11
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    mason sand is too fine,coarse sand in the screed acts as a second straight-edge,if you will,it work with you to cut the screed level.also,finer sand will compress as you run you finish trowel over it,coarser sand will bind up on itself,think of quick-sand...it is round and therefore you sink,like being in a beanbag chair,no sharp edges offer no resistance
    – user27976
    Nov 16, 2014 at 6:02

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