I'm dealing with surface prep. I will be laying a foam backed click lock floating laminate (forgive me, it is not hardwood) floor. Flooring is not my specialty but this one is my floor so I'm game to deal with it.

The surface, except for a few dings @ 1"x 10" max, a 6' seam between old and new where the underlayment was replaced with new plywood, and a 26"x18" repair (floor vent removal), the floor is flat and vapor proofed. The vapor proofing appears to have been layed as a baking to a vinyl floor decades ago. Yes, that material might indeed be comprised of asbestos, hence I do not wish to disturb it, nor do I need to. It looks to be tar based, which offers the vapor barrier feature, a plus. I simply want to "fill" in the few dings, seam, and vent repair piece, to flatten all to match the rest of the surface.

The subfloor and underlayment are of wood. Using a concrete type levelor is not a good solution.

I was thinking perhaps of filling the gaps with something like Rustoleum LeakSeal Flexible Rubber Coating. (I could use the leftover product on my leaky gutters!) Perhaps there are better solutions including do nothing, just lay the darned flooring. Any suggestions would be appreciated.

The repair piece can be seen at
https://www.flickr.com/photos/pdbois[img]

My go to floor leveler is tar paper. A roll of 15, or 30 pound roofing felt will build up a low area and you can lay the flooring immediately. I use 90 lb. roll roofing which is usually about an eighth inch thick so it is relatively easy to get within an eighth inch. You only need to use a straight-edge to see how many layers is necessary. It takes more layers with the thin material. When making a repair of a heater vent, the shim should be installed under the patching material where it rests on the framing. If you would like to make the patch stronger, latex sub-floor adhesive can be used to glue the layers of tarpaper.

For a full floor project indeed roofing paper works great, it is quick and easy. But these are small areas and all but the vent filler are extremely uneven. They are areas where the tar paper ripped out along with the vinyl along the walls. Trying to cut new paper to fill that would be impossible. Hence, my thought of using a liquid tar product. I wonder if anybody else has tried it.

Other fillers will work. The best will set up and stay hard. I like the self leveling latex cement and fine sand mix which can stick to existing tarpaper. It will not completely flatten out at the edges. Feathering out with a trowel helps make it smooth after it has stopped flowing. It will be ready for flooring the next day.

Reminds me of a job I did at a downtown historical building - they wanted new chevron floors which require a very flat subfloor, and while it was a plywood subfloor, it was horribly uneven. We edged and screwed but it wasnt enough and we couldn't convince the owner to replace some of the plywood, so it was left to us to make it work. So I called in my concrete levelling subs and had them take a run at it with some self leveller and planipatch. They had to reinforce with mesh but were able to make it work and it didn't add too much weight to the subfloor which was my only concern. The install was great; the building is a commercial space and as far as I know the floor has held up nicely since install.

So you can use concrete self-levelling solutions over plywood. Just be careful and spot patch only - no flooding rooms!

Thanks to all for your input!