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Unread 12-28-2016, 09:55 AM   #1
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Subfloor/Underlayment Advice

I am replacing my kitchen floor (11x9.5 ft) in my house (1940s) with 18x18 inch ceramic tile. The subfloor is 1x8 boards that are run diagonally over 9 3/4 with a span size of 14ft at 18" oc beams. Deflecto said I was good to proceed with ceramic

What I have done so far:
Screwed in previous 1x8 boards at every beam with 3 screws per intersection. Next I installed 3/4 OSB T&G subflooring with construction/wood screws (1 5/8) every 6 inches into only the 1st layer of 1x8 boards. This was installed perpendicular to the beams, all seams were offset and edges landed 4-6 inches from the beams.

I am assuming the floor isn't flat enough and will need to be flattened.


What should I use to determine if the floor is flat enough?
How would I flatten the floor and would it be flattened differently if using hardiebacker vs ditra?
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Unread 12-28-2016, 10:08 AM   #2
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Hi Spencer.
Use a 6' straight edge or level to check the floor.
If it has humps and valleys, hardibacker or Ditra
won't solve the issue. You'd need to fill in any low
spots and address the high spots.
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Unread 12-28-2016, 10:35 AM   #3
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What do you use as a 6' straight edge? I don't own anything that long that I trust to be straight. Is there something inexpensive I can buy?

I am also unsure about how I would go about flattening the floor.

If using hardiebacker (preferred due to cost) would I thinset the hardiebacker and screw it down and then pour SLC over the whole floor or pour SLC before I thinset and screw the hardibacker?

If using ditra, would I pour SLC over the whole floor and then thinset the ditra down?

Will I need a lath embedded in the SLC?
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Unread 12-28-2016, 10:49 AM   #4
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Welcome, Spencer.

You'd really want a ten-foot straight-edge to check that floor, but about the only reliable option is a tool made for the purpose. You could run a 1x4 over your jointer if you happen to have one of those sitting about.

The industry standard for substrate flatness for the size tile you have in mind is no deviation from intended plane of more than 1/8th-inch in ten feet nor 1/16th-inch in two feet. That's a very flat floor and your proposed SLC is one of the few methods to obtain that.

If you use a CBU for your substrate, you'd need to pour your SLC on top of that. If you use the Ditra or similar, you must flatten the floor before installing the substrate.

Most SLC manufacturers require a reinforcing lath with their product over wood framed floors and may have a minimum thickness. You'll need to read and follow the installation instructions for the product you elect to use.

Your joist structure as described meets the requirements for deflection only if they are of good grade and species and are actually 9 3/4 inches tall rather than the more common 9 1/4 inches. The spacing of 18 inches on center is a bit uncommon, too.

My opinion; worth price charged.

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Unread 12-28-2016, 11:34 AM   #5
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Michael and CX,

Thanks for your replies. Can I use a 6ft aluminum ruler on its edge as a straight edge for checking the floor? Or a 96 inch box level?

I was reading the instructions for quicklevel RS and found that it requires a lathe when used over wood, but it doesn't mention if I need it when I apply it over CBU. If I use CBU and pour the quicklevel RS over it, will I need the lathe? I don't want to screw something in every 6 inches again...Once was enough if possible.

As for the uncommon beam sizes and spacing...My house overall is really strange. Going from front to back of the house I have an "I-Beam" made out of a railroad track and that is what the beams sit on. This I beam is supported quite often (6-10ft) through the middle of the basement with walls.
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Unread 12-28-2016, 12:04 PM   #6
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You should not need lath if you install slc over cbu. In either case, you do want to use the required primer. One thing I've found is that trying to get slc to flow and level in a thin layer is MUCH harder to do than if it is a deeper pour. Think about what happens when you pour pancake batter into a pan...it doesn't flow all the way to the edges of the pan...it pools with a rounded edge. If you want to make something like crepes, you have to push it around to flatten it out. SLC is not super fluid...it's not bad, but it's not like pouring water...more like honey. Even water beads up on surfaces (like a newly waxed car - that shows the tendency much more dramatically, but it still happens on other surfaces - it doesn't sheet until you've flooded the area).

Anything you can find that is stiff enough to not bend can be used as a straight-edge. Over a large area, a laser can be useful, but not for a one-time purchase (maybe rent one?). To actually see if there's a gap underneath the straight-edge, you end up having to crawl around unless it's severe, but we're not talking fine tuning things to severe...it needs to be very flat, so the size of the gaps will be quite small.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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