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Here is my kitchen rennovation. About 240 sqft, 5/8 plywood subfloor. I Put in almost 500 flooring screws beside the nails, not glued down. 36 year old home, dimentional lumber in Brampton Ontario. Trying to decide how to proceed to level my floor for 6.5mm SPC vynl plank. I know it just has to be flat at least, if not level. Plan was to to put floor leveler directly on the subfloor, building up bigger dips with thinner plywood. The manufacture recomends at least 1 ¼” to use the leveler though. Maybe that’s for tile? Would the deflection could crack the leveler on 5/8 subfloor? I’m trying to avoid having a big step up into the kitchen, the other room has ¼ hard wood. Could I grind off maybe 1/8” or so of the high points in the room, laminate with ½ plywood with screws or staples? then self leveler? If there must be a big step up then I’m willing to settle, I just want a solid floor. I drew a little diagram showing the constuction and deviation of level in the floor. How should I approach!? Having a hard time on how to proceed. Thanks enter image description here

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  • a friend of mine used polymeric sand instead of SLC and it's held up well after about 3 years. it was a lot simpler to apply and tweak (not level, but flat) with a 6' level and brooms before misting it up to lock it in place.
    – dandavis
    Commented Nov 8, 2022 at 4:29
  • Please Edit your question to reattach your diagram.
    – brhans
    Commented Nov 8, 2022 at 10:20
  • Please edit your question to include some formatting. It's very difficult to read and parse what you're saying. Also, it seems that there are about 5 questions in here - this site (take the tour, look at the "how to ask" section in the help center) work on the "one question per post" principle.
    – FreeMan
    Commented Nov 8, 2022 at 12:33

1 Answer 1

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1 1/4" is the subfloor thickness of floor recommended for tile by TCNA. This is primarily to avoid grout cracking overtime due to deflection.

5/8 is fine for LVT or engineered hardwood. You can sand down high spots particularly high spots over beams but I wouldn't go more than an 1/8 out of the 5/8 and keep it to small portions of the floor. You don't want the equivalent of 1/2" plywood being your subfloor but generally high spots are small areas.

SLC can generally be applied in feather thin lifts - equal to the width of the leading edge of a feather. You need the correct primer over plywood ( different than concrete and different than what you use if you are doing multiple lifts ).

SLC is a pain in the butt. It doesn't self level, you need spike roller, a trowel, and a squeegee. I've done it a few times but typically need a few lifts. Worst thing is to add too much and have to grind it back down. It is pretty soft at first though so I find when pushing it into a corner or wall if there is build up you can use a chisel to sand down the corner build up as long as you do it fairly soon after it initially sets up.

I recommend different techniques depending on the usage of the area. If this is a hallway it will have lots of traffic and you want it flat. If this is a large open area then maybe you don't care about certain areas as much since they'll be under furniture or rugs.

If your floor isn't flat you can notice bounce or deflection in the LVP when walking - some people don't mind this too much. If the SLC cracks and you have the deflection it can lead to the popcorn sound when walking. Generally underlay - even the attached underlay will tend to absorb this sound either by preventing the cracked SLC from moving or preventing it from contacting a rigid surface and only contacting the underlay. If your planks are mechanically joined the tongue and grove can separate and break.

I have a laser that shoots out the line 1/8" above the floor surface so you can easily see the high spots once you position it in a low spot - Huepar FL360G - it seems to work well.

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