Self Leveling Compound Unusual Installation Question

[PGB1]

Hello All!

I'm working on a room at home to receive floor tiles where the floor is 3/4" exterior plywood. Unfortunately, the floor is not only out-of-level in two directions, toward one corner, but also is wash-boardy because the floor joists are not on the same plane. (To better explain- Some joists are higher than others.) I'd like to end up with a level floor and at the height of the adjoining floor.

The overall dive of the floor is about 5/8" North-South and 3/8 East - West. There is plenty of height to level it and meet the adjoining room's floor height.

The existing wood is screwed and glued to the joists, so removing it would be difficult & may damage the joists.

The washboard of the floor will make the normal long, tapered shims not support the underlayment plywood well. There will be gaps.

I hate to resort to self leveling compound, but here's my Idea:
Put self leveling compound on the existing plywood, then new plywood on top of the leveling compound. The compound will be trapped by the walls and the rise of the adjoining room's floor.

The new plywood would be screwed through the self leveling compound & into the existing plywood. (But not the joists, as I understand this is a bad method to screw underlayment layer to joists.)

I'm not great at explaining with words, so a quick sketch is attached.

Is this a good plan or do you all know a better method?
Thank You for helping & sharing your knowledge. I appreciate it very much.
Paul

[cx]

First, Paul, you do understand that your tiles don't care a whit whether your floor is level, right? They care only about flat; the larger the tiles, the more they care.

If you just want the floor to be level, that's a different issue.

Have you evaluated your floor structure to determine if it qualifies for a ceramic tile installation without any structural changes? And you don't indicate the joist size or span or spacing below that subflooring.

If I had a sufficient subfloor structure and was not concerned about raising the finished floor height, and wanted a floor that was really flat and level, I'd use a mortar bed on top of my existing subfloor. Nothing makes a better substrate for a ceramic tile installation.

My opinion; worth price charged.

[jadnashua]

You can't successfully screw through a layer of SLC, so it's either all SLC, or leave it as is with the waves, or tear out the subflooring.

When I remodeled my first floor, after consulting an engineer, I ended up stripping the subflooring, and plaining and shimming the joist tops to get them all in plane, then, installing new subflooring. In my case, I was installing an engineered wood flooring, but the flatness requirements are similar to those needed for tile. A power plane and laser level made that fairly straightforward.

FWIW, my out of plain errors were greater than yours.

[PGB1]

Thank You CX & Jadnashua for replying with helpful knowledge.

I never knew one can't screw through self-leveling compound. Thanks for mentioning it, Jadhasnua. Does it crumble or is it simply too hard of a material for the screws to pierce?


I'd prefer the floor be level in case (for when...) things spill. I suppose it isn't critical. I do have to get rid of the waves that are evident when a long straight edge is placed on the sub floor. They are probably within thinset range. (About 1/4" maximum gap of straight edge to floor- all short spans of dip- about 6" max.)

The floor structure is doubled 2 x 6 on 16" centers. The joists are nailed & glued together. The glue is construction adhesive (not PVA), so I don't know how much that adds to strength, if at all.

The span of the 2 x 6's is 60 inches between supports. (North is the concrete block building wall and south is an I-Beam upon which the joists rest before continuing another 4 feet to a main beam.)

Your mortar bed plan sounds good CX. When I was young in the late 1960's, I worked for a tile setter and was the mortar mixer & broom pusher guy. We used to do lots of sand beds & lead pans. If only I remembered all that I learned from him!

If I lay a mortar bed, is Ditra still allowed on top? I'd like the water retention capabilities and the protection of de-coupling. Or, should I put PVC shower pan liner material?

[cx]

You certainly can screw through SLC, but it's the potential for crumbling, 'specially at the thinnest sections, that is the likely problem.

You want the floor level, level it, say I. But it's not required.

If the joist pairs are both supported at the ends, gluing is not necessary, but always a good idea.

Good that you have some experience on the mortar beds. Forget all you ever learned about lead pans. If you'll add a geographic location to your User Profile we won't hafta try to guess that you're somewhere in the vicinity of Boston.

Ditra would be an option on top of your mortar bed, but not a requirement. PVC shower pan material would not, unless you plan to build a shower and install the second mortar bed. I thought this was just a floor installation.

My opinion; worth price charged.

[PGB1]

If you'll add a geographic location to your User Profile we won't hafta try to guess
Detroit (Sorry it isn't there. I'll try to add it again. For some reason, it didn't sick. I also don't get notifications of replies. Guess my computer is wonky.)

I thought this was just a floor installation.
Yup- Just a floor. The shower pan liner came into mind, as did the Ditra, to keep spilled water from going through the floor. Dog washing = Lots of water on the floor.

[cx]

Lordy! That ain't even close to Boston, is it, now?

No rule against putting a waterproof membrane on top of your mortar floor if you want to do that, Paul.

My opinion; worth price charged.

[Dan Marvin]

Leveler over the plywood and a layer of Ditra should have you good to go. I echo the sentiment that you wouldn't want to try to put more plywood over the leveler, that's asking for trouble. Follow the waterproofing instruction in the Ditra Handbook and you should be in good shape for washing the dog as well.

[PGB1]

Lordy! That ain't even close to Boston, is it, now?

=CX
Just a hop, skip and really big jump! (Plan to visit family there soon.)

Thanks Again CX, Jadnashua and Dan for helping.

I read as much as I could find on line & in the tile handbook, but am not positive about the subfloor & underlayment requirements.

Existing: 3/4" exterior plywood on doubled 2 x 4 joists 16" on center. Plywood is screwed & glued with unknown adhesive to the joists. Span of joists is 60 inches.
Floor area = 60" x 62". Deflectolator showed L/2505 for the joists.

From what you all wrote, it sounds like I'm safe to leave only one layer of the 3/4" plywood with a mortar bed (or leveler) on top? Or did I misunderstand and need a second layer of plywood for strength?

Thank You for clarifying if I'm correct.
Paul

[cx]

Yes, if your plywood subflooring is at lest a C grade on both faces and has T&G edges, that is technically sufficient subfloor for all manufacturers of tile installation substrates.

My opinion; worth price charged.