I'm installing 6 3/8" wide, 3/4" thick tongue and groove engineered hardwood, over 5/8 OSB at 12" off center.

I plan to nail down the hardwood since this seems a stronger bond than glue down planks.

There is a 3/8 inch plywood on top of that. All the plywood has been screwed to joists to remedy squeaks, and I've moved on to flattening the floor.

The floor has many valleys averaging about 3/16" to an 1/8" that happen to also be in high traffic areas.

My first option was to use a leveling compound or featherlite to build up to those areas, then glue down the boards that are above the leveler.

The problem?... Over 50% of the floor would be covered in leveling compound if I went this route.

A second option was to use plywood screwed and glued down in the high spots, then use cedar shims on the edges, then featherlite those edges. To limit the amount of compound.

The problem?... This works fine for deep valleys over 1/8" but lower than that I'm at a loss, and there are larger areas with an 1/8" dips.

Finally my questions...

1) Can I use something like ramboard to raise the level of those smaller gaps?

2) How many layers would likely be too many?

3) Could moisture/humidity cause mold, or cause the ramboard to expand, leading to the staples popping/ breaking the tongue?

4) Would felt or roofers paper work better?

5) Would overfilling with plywood, and then sanding down a small beach worth of sawdust into my home be a better option?

Two final notes, the customer (my wife) is not thrilled with the prospect of smelling tar from the roofers paper. I'm also planning to use wax paper on top of the finished subfloor to help with installation.

Some good advice is why I'm here, and it will be greatly appreciated!

Jim

I know that roofing paper was not made to help flatten a floor, but it is what I have been using for the past 40 years.Asphalt covered streets do not compress, so I decided to use it for building up low spots, long ago. Asphalt saturated felt, or now its fiberglass, does not move with change of moisture content, or flatten out unless it gets very hot, enough to melt. Ram board was not made for floor flattening.
The thickness of roll roofing, or three tab shingles is approximately an eighth inch, which is the tolerance for flatness of a wood floor. It would be much easier to get the floor flat before any underlayment is installed by grinding the high spots, then using roof felt to build up the low spots so no further flattening would be necessary after it's fastened. I have a limit of a quarter inch of tarpaper thickness and then use quarter inch plywood (or thicker), but felt does not compress, so more layers would be fine if you would be floating the floor. I don't worry about the difference between the tarpaper thickness and to floor since the flooring will bridge the one eighth inch taper at the edges of a shimmed area. Fasteners usually need to penetrate an inch to be effective for flooring, so it depends on how long they are.
When the Kraft paper covers the roof felt, there will be less smell, but after the flooring has been laid there will not be any smell.
If you try to use leveling compound, you will still need to sand it to get everything flat, in my experience. It's possible to nail through it, but I don't recommend it. Too bad the three eighths inch plywood is already fastened down.
I've laid a lot of 3/4" floors over 5/8ths inch plywood and no problems. I always try to nail into a floor joist, and don't worry about whether I'm parallel to the joists or cross them.

First and foremost thank you for the reply. I think we're going to end up going with a two pronged approach. Using tar paper on the large areas where there is no sunlight, and patch in other areas.

I do however have another question related to this one since we stumbled onto another option this week. I found some very thick hardwood underlayment ... Mono Serra Hardwood Underlayment Paper... sold at Lowe's.

This is a silicone vapour shield or "SVS" underlay, and due to its construction it offers a VERY sturdy support after multiple layers, matching and maybe even surpassing tar paper. I've stacked multiple layers of craft paper in the past, all be it in smaller areas.

The issue of course is I'm weary of stacking multiple vapour barriers. However, since this will be a stapled down floor, will this be acceptable? I'm thinking of using it near some vents to our furnace, since the only other option for this area is tar paper, I'd rather not spread that smell through our house unnecessarily.

I guess the gist of my question is, is it ok to use multiple layers of a paper that has a vapour barrier, if it will be full of holes, and the area in question is above grade and very dry.?

Additionally, would spiking multiple small holes through the layers be a good idea?

Also noteworthy, I thought of using some general purpose spray on glue "3m 77" between the layers. I tested it out on a small area. The results weren't great as the glue didn't adhere very well to the shiny side, and it felt that I was maybe adding more moisture than I was preventing.

Thanks again for your help, this will be great experience to take along with me since I'm trying things I haven't tried before.

I use tarpaper that is an eighth inch thick and staple it down with a hammer tacker stapler and three eighths inch staples. I can't afford to fiddle around with thinner products to build up a low area. Did you take the high spots down with an electric planer or edger? This makes flattening the floor easier and faster. Just set the sub-floor fasteners with a quarter inch punch and a big hammer.
The tar paper smell has never been an issue after it has been covered with flooring.