Hello all,

I'm installing new hardwood flooring in my house. My goal in no particular order is to:

Add 1/2" CDX ply for height and strong surface, perp to joists

Install roughly 650sqft of 3/4" thick, 2 1/4" white oak (perp to joists) -- the hardwood flooring will be nailed down (blind/finish)

Raise low spots as deep as 1/2" and lay plywood flat on top

Insert underlayment such as a paper moisture shield poly SVS, aquabar "B", or rosin paper

Here's the current arrangement:

1951 construction house

Wood plank subfloor (3/4" thick by 3") running north-to-south

Joists are 16" OC, 2x8's, 12' resting on center beam running east-to-west

Original hardwood (3/4" nail down red oak) ran perpendicular to joists

Subfloor has been screwed into joists (2 1/2" torq deck screws) every other board

My issue is filling the low spots. I've combed every dark corner of the internet and it seems most use asphalt shingles combined with #15 roofing felt or #30.

Something feels quite dirty about both using asphalt shingles as well as having random objects such as vinyl tiles under the floor in my home. I'd prefer something different if it exists.

Given it's a nail-down hardwood flooring, self leveling compound is out of the question.

Are there any alternatives or is this really the only practical option? I don't want to remove the subfloor nor alter the structural integrity.

Thank you for your help, pros!
Raoul

Nothing wrong with asphalt roll roofing or three tab asphalt shingles.
You need to have your sub-floor within 1/8th of an inch in six feet for the best r esults, and some flooring requires this for the warrantee. You will be covering the shingles with aqua bar asphalt/wax paper so you won't be seeing it after the smoothing process. You don't need to make it level.
We drive over asphalt all day and the roads don't squish out from pressure,
unless they get over the melting point of the asphalt, so no worries.
After sanding or planing the high spots, take a six foot straight-edge to mark the low spots with a pencil or marker with a line for each increment of height. Take a corner of the asphalt paper with one thickness and make a contour type of line where it touches the bottom of the straight-edge, another line where two thicknesses touch and so on until you have marked the depression. After you have the contours marked out you cut the shapes for your individual marks, starting with the thickest level. Staple the roll roofing down so it won't move around with a hammer tacker using 3/8th inch long staples, or longer.
The only problem I have had with the asphalt paper is that it has become expensive in the last few years. When I started using this technique in the 1980's the roofing was $10 a square/ 100 square feet. Now it costs over $50 per roll.
Cover the roofing with your plywood fastened down with 8d nails into the joists, if possible.

Thanks for responding.

Do you have any sources for there being nothing wrong with asphalt roll roofing? If I have an exposed basement to the subfloor above, I'd imagine I'd have fumes going above and below.

Thanks!

If you are sensitive to smells you may put down a layer of kraft paper before your roll roofing. The kraft paper/aquabar has both paper faces with wax/asphalt in between. You may be able to smell something when it is in a roll, but very little when it is covered with flooring. If there are gaps between floor boards in your sub-floor and no covering of the ceiling in the basement you might notice a smell for a week or two, nothing like a tar and gravel roof smell when it is being installed.