First the background. I plan on installing 3/4" 3" wide prefinished hardwood flooring in my family room & a short hallway. I'm the original owner & the house is 25 yrs old. I've removed the particle board underlayment (room was carpeted) and have a 1/2" plywood sub floor. I replace 3 sheets of the subfloor with new 1/2" due to soft spots at the seams. Then I went through & deck scrrewed all the original subfloor to the joists. Now I am adding another 1/2" layer of new plywood, also deck screwd to the joists every 8" & screwed to the orginal sub floor 8" in the field & 6" on the perimeter. I'm almost done, everthing going well and finally hit a snag. I started at the outside wall, laying the underlayment, & now as I approach the opposite inside wall (load bearing) I notice the subfloor slopes or dips at the interior wall. So, I took a rigid straight edge & cantilvered it off the new 1/2" underlayment & "mapped" the low area, taking readings at each joist, every 8" along the joist (read "existing subfloor"). From the "map" I created what could be best described as a jigswaw puzzle / topographical map made of 5/8", 1/2", & 1/4" plywood. This"creation" consists of 3 sections & is about 8 feet long & 3 feet wide. The final step is to transition the different thickness of plywood with roofing shingles & felt paper. Then the final 1/2" underlayment will be attached.
Now, I'm not even sure of what my question is, other than "am I nuts?" or "Have I lost it?". My buddies said maybe I should ask a flooring guy to look at, but I'm trying this website instead. I know this area needs some type of correction, but I'm not sure of how or to what extent. Therefore, I'm looking for guidance, suggestions, or comments.

I don't believe the house has settled (no cracked drywall). The joists all look ok, as does the subfloor from underneath. I don't think the cause is even that important, only the corrective action. The point at which this slope starts is at the I-beam in the crawl space. Maybe the I- beam was set a liitle too high. However, the whole floor does not dip at the load bearing wall, only an ~ 8 foot length. Oh, as this area breaks into the short hallway, the hall way also pitches down sideays to this low area. Do I have everyone confused now? Any help would be much appreciated.
Thanks, FJK

Your plan to shim up under your final 1/2" ply is the way I would approach it and have done the exact same thing in the past. The plywood/roofing felt is the way to go, IMO. Sounds like you got a good handle on what needs to be done. Only tip I have is to use longer screws in this area so you can screw the whole thing down solid to the joists and bottom subfloor (but you probably already thought of that). Good plan and good luck!

Gary: Thanks for your input. I appreciate your guidance, in this case confirming I am using the correct approach to resolve the problem. Out of curiosity, what is the maximum layers of shingles have you used to straighten out a floor? Also, does it become an issue, relative to nailing the hardwood floor? Conceivably, you would only have the 1/2" of ply for holding the nail/staple.
Thanks, FJK

I was thinking the same thing but the amount of shimming required is not clear to me in your description. There comes a point where I would use plywood. 3/8" plywood is readily available and I'd look around for some 1/4" stock as well. At some point you could just switch to 3/4" underlayment. I dont like to shim more than 2-3 layers of 15# felt. The staples or cleats extend <1-1/4" below the flooring.

Actually, if they are driven at the right angle as they should be, they only protrude ¾"(measuring straight down at a 90º to the board) with a 2" fastener. and ¾" flooring.

Commonly done, your forgetting the angle.

We're both wrong. I went out and drove 3 cleats with the manual powernailer. All measured in at 1"- 1-1/8". Thats why we shorten to a 1-3/4" fastener when nailing to 3/4 ply on concrete.
Also, 1/4" of the tip of the cleat is tipped and has no holding power, so there is only about 3/4' of cleated holding power, all concentrated near the flooring.