Help! This is my first time on posting. Also first time with a real handyman project. My plywood subfloor not level. Unable to get underneath in crawl space. I have tried to level with quickdrying underlayment which I spread myself, but I make new areas uneven. Not sure why floor so uneven as at least several places have long dips and some are where boards meet. Frustrated with all the mess of underlayment and then attempting to sand underlayment. Still nowhere near level. Can see myself on this project in 2 months. One posting mentioned using fiberglass shingles for low spots.
What about self-leveling underlayment. I'll try almost anything, even toothpaste.
Thanks

Don't use a mortar (self leveler) if you plan on a nailed down installation!!

Nails don't penetrate 4000 PSI mortar very well!

Then lets talk about the added weight that mortar will add to the already sagging structure! Can you structure handle it?

I'm on one right now where the OSB is a peaked and vally, mess.

I'm raking the peaks with my circular saw, sideways and peeling some high edges. the sanding them flat, after setting the screws down.

Floorguy,
thanks for responding. good thoughts about support handling "cement" if it is already sagging. i'm wondering if i should remove it? break it up?
One thought was to use shingles in low spots and sand high spots.
Or, not sure would stay correct, but what about thin insulation foam that gives and is 1/2 inch thick?
tumblebeeman

When I have a problem wood subfloor the first thing I do is re-nail it with ring shank UL nails. That stiffens the floor and pulls down some of the high spots. I set my gun to countersink the heads for sanding.

Work on the high spots first using a belt sander or rent an edger if you have a lot to do.

On the low spots I use different things, 90lb roll roofing, 30lb felt, shingles, whatever works. Roll roofing and shingles will get gritty so I put a layer of felt between the granular surfaces to keep that from being a problem later on.

I am assuming you are installing a solid 3/4 nail down product?