I have some low spots that need to be leveled prior to installing 3/4" x 2-1/4" red oak unfinished hardwood. Shingles appear to be one of the generally agreed upon methods in this forum to level these low spots.
I have a few questions:

(1) Should the top gravely portion of the shingle be placed facing the subfloor (installed upside down) or facing the hardwood?

(2) Do the shingles go down before or after the 15 lb roofing paper? No additional subfloor is planned due to height differences of adjacent rooms.

(3) Must the shingles be fastened in place or can they lay loose and be held in place by the fastened hardwood?

(4) If they must be fastened, what is the best method?

(5) Obviously any loose "gravel" (for lack of a better term) should be brushed/vacuumed from the shingle, but won't additional "gravel" eventually come loose over time as the floor is subjected to walking and movement; and, won't this raise the risk of "crunching" being heard when walking in areas shimmed with shingles?

Any and all advice is greatly appreciated!

1) There exists no "formal" recommendation for this. I put the shingles in upside down, or, as you said, gravely side down.
2) Shingles first, the the felt over the top.
3) I fasten mine down securely with a hammer tacker/stapler.
4) You could use roofing nails if you don't have a hammer tacker.
5) Subfloors should not be flexing and allowing movement. If everything is tight, well nailed and solid, there should be no noise. When I do this, there never is a crunching noise.

Gary,
How many layers of shingles can you place in low spots without causing nailing problems?

I was told up to three layers of your average, cheap grade of fiberglass, asphalt roofing shingles. You know, the cheap three tab type. So, I don't exceed that amount. But maybe one could use four. I really do not know. If I have to go thicker, then I go to plywood or wood shake roofing. You would not believe just how bad some wood subfloors are Ray. I had one that dropped 2" in less than 6'! No shingles for that. I had to gradually build it up with plywood, tapering, sanding, etc. What a pain!

Just curious, how much build-up will 3 shingles provide (1/8", 3/16", 1/4" etc.)?

Depends on individual thickness. Approx. 3/16" to 1/4"

Gary,

What size nails/staples do you use when you have 3 layers of shingles under the hardwood?

Assuming the new wood flooring is 3/4", I use 2" staples, always. Unless the plywood is over a concrete slab.

i've seen some people recommend using a layer of the 15lb felt inbetween layers of shingles. Is this necessary?

I fail to see what this would accomplish. What is the purpose behind this recommendation? One is simply attempting to "fill in" a low area of a wood subfloor. One can use 15 or 30 lb. felt paper to "taper" the edges and I do this but I see no reason to "need to" install 15 lb. paper between each layer of asphalt shingles.

perhaps to reduce the chances of hearing the "crunch" of the shingles when stepped on but i suppose they should be secured by the nails in the hardwood which would make this issue moot.