Construction Question - Leveling SubFloor

[Tidybuoy]

I'm sure many of you have seen my kitchen remodel project. I'm about finished and I'm waiting on my cabinets and countertops.

I opened up a wall and expanded my kitchen to into a storage room next to it. Prior to the project, I never noticed the floor sloping but now that it is opened up, the old storage room slopes about 1 1/4 inch from the opening to the back wall. Since I have a month before the cabinets are done, I am going to shore up the space and level the floor using plywood and a self-leveling compound that is made to go over wood. I'm going to cover the entire space with 1/4" plywood, then add 3/4" ply against the far wall, then add 1/2" ply in the middle. Basically it will be a stepped covering that gradually ends up at 1/4". After that, I will cover the entire floor with a self leveling compound and feather into the remaining floor.

My Questions:
I've been watching numerous YouTubes and I have seen guys apply glue or contact cement between the mating boards, others use screws, and other use glue and screws. I'm seeking advice for the best method. The guy was using glue that came in a bucket and it was trowled on.

I'm wondering if I should put any other material between the sheets of plywood to cushion, quiet, insulate, seal, etc... This will be plywood on top of plywood after which I will use Henry Self Leveling Compound for wood.

Thanks

The back wall is 1 1/4" lower than the framed section.

[LWJ]

When I put hardwoods in my house, I believe I put heavy felt paper between the plywood and the oak. Don't remember why? The floor is tight and quiet, but I don't know if the felt contributed. I bet if you are generous with a screwgun there will not be any movement. Even more so as you are covering with self-leveling compound. That floor will be SOLID.

[john70t]

A pure glue might grip better, but liquid nails might provide a more solid base on top of the joists. IDK.

[mattdavis11]

Move the range, slope towards rear center, install drain. I'd knock a hole in the wall to join the two doors, and hit em with the hose when they came out the other door.

[908/930]

I used construction adhesive between layers of subfloor ply and then screws. When using a self levelling compound years ago I found that I had to use a 18" trowel to help it along.

[john70t]

The box store self-leveler appears to be concrete mix + latex primer + water based adhesive.

I once used it for a vinyl tile condo floor but primered with (hard Killz or 123?) and screwed luan over it with grouting the gaps and another layer of primer.
It did need to be hand smoothed and level checked.
It will crack when thicker than 1/2"-1", kinda chalky, and is not a great final surface for adhesion etc.

Maybe the top layer of plywood could go over that?

[gregpark]

I always PL glue and screw plywood. Run the sheets in the opposite direction of the subfloor sheets and overlap all joints. Henry self leveling is a great product. I use a long handle stand up concrete skreet to help it about but a 20" hand trowel works. The self leveling is super runny and doesn't need much persuasion manually. I always go around the room with a knife and a roll of paper towels to pack any holes or cracks it could drain into. The self leveling is so runny you can lose most of it down even a very small crack. I mix it in a 5 gallon bucket with my mixer drill and mud paddle. About 4" of water in the bucket and then add one 25 lb. bag of powder. After it sets in a couple of hours you can mix up a pail of Henry patch and level to fine tune. It's not nearly as runny and should be applied with a hand trowel. It's Portland based too and adheres to the self leveling. I'm a floor guy by the way and have been there a thousand times.

[A930Rocket]

^^^ What he said

[Tidybuoy]

What is PL Glue? I looked up Home Depot and there are 30 different types of glue.

My plan is to glue & screw and I will seal all edges of the room with foam sealant and that 3" foam membrain. I'm also going to seal all wood seams with Henry Feather Touch.

Thanks!

[Scott R]

Huh, I'm having a remodel done and had the same issues, the GC had the plywood replaced. and the joists fixed. Was really no big deal for his carpenters.

[908/930]

PL is a polyurethane based construction adhesive, ask for PL Premium construction adhesive should be on the shelf. Once it cures there is no easy way to separate the sheets, fit everything first. If you need to seal a partial tube I find that aluminium duct tape sealed over the end works pretty well.

[drcoastline]

Glue and screws, the trowel on type. The more glue the more chance of adhereing. Esentiall you are making a thicker piece of plywood gluing them togteher. The screws are really on there to hold tight until the glue sets.

You need the glue to ensure you don't get any movement and this squeeks. Just screws can loosen over time causing squeeks.

A barrier between the ply and the wood floor also prevents squeeks. Old school is 40lb felt/tar paper.

[gregpark]

Minimal polyurethane glue applied with a caulking gun. The stuff is crazy strong. When a subfloor squeaks it's almost always caused by the edges of two plywood sheets rubbing together. Felt is not going to help with noise reduction and not necessary otherwise when floating the floor with a self leveling compound.

[Tidybuoy]

Thanks All! I think I've got my weekend project all figured out. If I don't reply on Monday, it's because I'm stuck to the floor and can't reach my phone

[gregpark]

Trowel yourself out the door and not into a corner!

[908/930]

Just to give you a heads up, that PL premium does not clean off your hands that easy, it will clean off with mineral spirits, or will wear off in a couple of days. It also tends to drool out of the tube so have something to keep under the tip. Have fun.

[brp914]

I happened to have watched this the other day. Sal Diblasi has made youtubes for years. Here he describes how to level a floor with a laser. Might need a New Yawk to CA english translation.

[Zeke]

Zero to 1 /14" in that short of span (but a long span for mud) generally gets done with wedge cut sleepers and a new sheet of subfloor. Fast, cheap and strong.

[gregpark]

^ that works fine if you don't mind raising the elevation of the subfloor in that room ¾ of an inch. This can sometimes make the transition of the floor covering awkward at the doorway.

[gregpark]

To the OP
I've done self leveling fills as deep and deeper than yours without adding plywood. I add a liquid strengthening additive for concrete to the water when mixing (75% water 25% additive). I'm sure home depot sells this. I've never had any cracking in deep fills by doing this. I assume the guy in the above video is using additive because his fill is about as deep as yours. I use my laser but you can get it close with an 1¼" reference line snapped on the wall. After the pour has set, fine tune with Henry's portland based patch and skim coat checking with a straight edge. And duct tape stuck to floor and wall around the perimeter works fine as a leak dam.