Hi all,

First time poster here -- so giving my thanks in advance for any advice y'all can provide.

I'm preparing for a 1 1/2" x 3/4" red oak strip flooring install (to match the home's 1970's addition with the original 1950's hardwoods). The subfloor is 3/4' T&G planks (varying from 4" to 6" wide) installed 45deg to the 24" o.c. 2x6 joists -- which means I have a joist span that is actually wider than 24". My plan is to add 15/32" T&G plywood underlayment but I need to address a long running dip that at is about 1/2" below plane at its lowest point. Essentially the trough, if you will, runs through the center of the room perpendicular from the back wall all the way through the doorway and then through the adjacent hallway (~16').

I have read elsewhere about the use of roofing shingles and felt to build up low spots, and I think this is workable in my situation. What I'm curious to know is if it would be better to build up the low spots between the subfloor and the underlayment or between the underlayment and the hardwood flooring. However naive, I like the idea of the former because it'll give me a uniform surface when I start nailing down the flooring -- but perhaps there are structural arguments for the latter approach.

Another consideration would be to run a plywood strip + shims down the center of the trough to get the center in plane, and then build up around it with whatever materials are appropriate, and then follow with the underlayment.

Again, many thanks in advance. Cheers!

After further reading, it seems like I would technically be out of spec with my joist spacing and 3/4" subfloor. With this current configuration, the subfloor would need to be at least 7/8" plywood -- correct? Am I going to have to rip up the existing plank subfloor to proceed?

If I could get away with the existing subfloor, I would still be interested in guidance on my original question pertaining to the flattening process.

OK, apologies for spamming my own post -- but it seems like my original plan for a "double layer subfloor" (NWFA parlance) would get me up to spec so long as the second layer is at least 19/32". See Part V: Solid-Board Subflooring of the NWFA Installation Guide 2019 (p. 56) for the reference I'm using.

Now, back to the question of flattening...

Asphalt roofing shingles or thick roll roofing is about an eighth inch thick. You can make a "map" outline where the different thickness can be outlined like a contour map. Start by marking the lowest area with a single layer, then work up to the thickest. The single layer will be the largest area. Then add two layer area inside the large area with another contour line, etc. I staple down the layers so they don't move as another layer is added on top of where the lowest are is outlined in eighth inch increments. You need a straight-edge to be able to see how much you need to add the layers of roofing to get to flat. At the end you ought to be within an eighth inch. Use small pieces of the roofing as a gauge so as you move the gauge around to help mark the line, you don't need to worry about how much the marked area will compress if the roofing is not perfectly flat when you are done adding all the layers necessary. The underlayment will flatten out the asphalt saturated felt/fiberglass when it is fastened down according to the correct nailing schedule. An underlayment over the shingles may not be perfect, but close to within an eighth inch in 6 feet. I have a seven inch floor grinder called an edger to make the area as flat as possible using 16 grit sand paper, which can grind the surface, after nails or other fasteners are set below the surface with a punch. You can rent an edger and buy paper where you rent it. Before starting with the asphalt saturated felt, I use the edger to grind any high areas to get them flat first, which lessens the amount of felt that is needed.

Pete -- thanks for taking the time for a thorough reply. The build-up method you described is what I envisioned. More importantly, you confirmed that my preference for building up to flat between the subfloor and underlayment is a viable option (in lieu of building up between underlayment and flooring).

Cheers!