Shall we assume you have already investigated the structural reason for the slope?

thanks for your advice/questions.

The picture looks way more out of level than really is. reason for slope along the wall where the two power outlets are is due to the weight of the wall. However, its fairly level horizontally until about a foot from that wall. the joists have sagged being 2x8's from normal weight of floor. im able to crawl underneath and the joists look in good shape otherwise.

This section was an add-on build over original garage so i plan on adding additional support. I feel jacking up would cause more misalignment and drywall repairs

im worried about crunching sound from shingles and/or "kissing" sound from the 30# felt. (or so i heard) Think i could forgo the shingles and put 1/4 or 3/8 ply in center of floor and taper with felt? Does 15# make less noise? But i would have to use several layers if i did.

i almost want to put down the 3/4 ply then self leveler and omit the possibility of strange sounds. Is this a better idea? Since ill be adding support underneath?

ugh.. thank again!

You can use a skim coat compound like WWHenry's feather finish. Screed in the low spot with a straight edge. Allow it to cure, then sandwich it with a layer of pre-sanded plywood for your vinyl.

i appreciate your advice. thank you. Are you saying to screw through the compound?

Three layers of shingles in the middle and tapered to ends seems to level things out. Perhaps once things are screwed down tight there shouldnt be much movement, so no crunch sound. i already have what i need so this would be the cheaper option. hmmm...

It doesn't have to be level - just flat.

If you want it level I'd use 1/4" plywood. Figure out where it's a 1/4" off and lay that first. Then another further back - like you're making two thin steps. Fasten these down - then pour the SLC. Lay the 3/4" plywood, and pilot drill through the SLC.

Backer board screws will go right through SLC. And it’s non-corrosive.

your gonna have a hard time finding an SLC spec that approves feather edged slc sandwhiched and screwed thru rather sandwhiched period... Good luck. :)

in floor assemblies Plywood of 1/2" + increments is actually rated for structural integrity not 1/4 and 3/8" so skip that unless you already have met deflection.

Clean floor screw or ring shank nail whats there now. Use 5/8" EXP1 plywood stagger sheets crossgrain to joists and make sure every short seem is split on a joist. Glue the new layer with full contact liquid glue (I.E. Titebond in gallon jugs) Screw thae new subfloor every 6" and 4" on edges/seems. Prime it and pour an SLC that allows for the MINIMAL thickness required. DO READ SLC MFG instructions. Tis way you can go up or down with plywood to meet SLC specs I.E 1/2", 5/8" or 3/4 ply.

Featheredging slc over joist/ply is never the smart way so use wood first at a thickness that will allow for the thicker SLC pour at its thinnest spot.

If its my clients home thats my method, an industry approved method.

Method B would be felt/lath Mud screed. But id need to verify structure span and integrity and need a minimum of 1-1/4" thick at thinnest point.....now that would get me a perfect flat floor for the least $ and steps. But requires a skillset to screed.

Yeah I wouldn't screw through SLC. Yes the screws will do fine, but the SLC will not, you will break it up in thin spots and have crunching.

Shingles are......a pain in the ass to do well. What I did to use shingles is get an 8 ft level. You shim the end of the level up 1/2" on the low spot, then sit and slide shingles under it until they stop in 1, 2, 3, etc depths until you are up to 1/2 inch.. You do this every 4 inches across the floor moving perpendicular to studs. It is a slow, painful, messy process, and shingles are technically not approved for indoor use due to offgassing and odors.

SLC, if you have not worked with it, is also a PITA. SLC does not in any manner "self level." You need to do as Mint Tile said, get your new plywood down first. Then you want to dam off the room border and pour all at once. You will have to trowel it out to feather the edges, and still smooth the surface. You will need a LOT more SLC than you think you do, probably 4 - 6 50 lb bags minimum if nor more for your room size. Do you even have a method of mixing and pouring 300 lbs of SLC at once?

For what it is worth, I'd go with the SLC. Maybe consider hiring just that done if you can find someone willing to do it. Reason being: you can lay tile on it in the future if you want. You can't lay tile over plywood that is screwed over shingles, it is not solid enough. If you use shingles and want a different floor in the future you may have to rip up your leveling job.

If, you’re going to pour SLC , apply a bond coat to prevent bond breaking.

Feather finish does not require a bond coat. The product will not crack or fllake off if, the subfloor is prep properly. you do have to work it smooth. It is not self-leveling.