Level (flatten) subfloor for hardwood installation?


Hello everyone.
I feel like I have read every single post on the internet / new groups about leveling subfloors for hardwood floor installation. I feel like I've heard it all. . .from leveling compound, to asphalt shingle shims, to felt paper shims, to ripping up the floor and adjusting the joists. Trouble is, I don't know if I need to "flatten" my subfloor. I keep seeing posta about certain spacing over 4, 6, and 8 foot spans. . .to determine whether it's in fact needed. I have no clue on what to do if the dip is 15 feet accross. I will attempt to describe my situation.
The room in question is a second story bedroom, in a 15 year old house, over a finished garage with 2X10 douglas fir on 12inch spacing. The overall space is ~16' X 16'. Now imagind that in the center of this room ther is appoximatly a 1/2" gradual dip (w/ a radius of approximatly 15 feet). This dip is NOT noticable when using a 4 foot level/straight edge, in that when you run the 4 foot level accross the area with the dip, I never see more than a couple milimeters or so of space under the straight edge. However, if I use the string method (a peice of string at the far ends of the room accross the dip and make it taught) there is ~1/2 inch dip at it's max. Now to make things more interesting, imagine that there is a interior wall that bifercates the romm into two sections (think of the floor plan as a "C" shape), and the interior wall extends right into the center of the dip!!!! What horried me, as that this dip appears to be a result of the origional construction, because the builder clearly "shimed" up the interior wall to compensate for the dip!!! Having the house inspected appears to show that this is not the result of setteling or defective joists, but simple poor workmanship. . . <tear>. . . .
I plan on installing 3/4" thick Plank (3.5" wide) Kempas Hardwood, using nail down method.
Hopefully, if what I wrote makes sense, you can understand why I am unwilling to rip up the floor (would require removal of a wall), and can't get it from below as I would have to rip out a celing below.
So getting back to it. What I have is a very gradual dip in a floor that is NOT perceptable when using a 4 foot straight edge. Do I need to flatten the floor? If so, what does the collective wisdom of the group recomend? 30lb felt to shim up a bit? If so, how many layers can I build up, before I risk the nails not having enough bite. Best regards, Jon
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I am in the middle of levelling a floor for hardwood. My problem is a combination of a room that spans an addition built 20 years ago. The old foundation wall runs in an "L" shape through the room causing a high ridge along the "L", combined with some settling in the older portion. I have been belt sanding the ridge down about 1/8" to lower it, plus I will have to use some floor levelling compound about 9/16" deep in the worst area to bring it up to reasonably level.
I am using engineered flooring, which has a plywood base that has saw cuts across it, so it can bend to follow gentle curves. My problem was the ridge was too abrupt for it to follow without risking the staples pulling out or the curve being visible in daylight. I am covering my subfloor and levelling compound with 1/4" firply glued down to give a good base for the hardwood.
You are using solid wood, that stuff won't bend at all. But if the longest length is not excessive, the shorter lengths will probably follow your curve. Get your wood and do a dry installation in the middle and see how it lays down without fasteners. You may have to do a little "adjusting" only, if you find any dips within your curve. I would not use anything with tar inside the house, nor use any paper products as they will eventually compress and cause squeaks.

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You'll be fine.
I installed 1500+ s/f of Kempas solid plank 3/4" flooring in my house and it looks great. There wasn't a level floor to be found, and the hallway had many undulations. Never more than 5/8", but repeated. The Kempas plank WILL bend, despite previous poster's comments and I have had no problems with nail pulls, etc over the last 3 years. about 500 s/f was installed 3 years ago, another 1000 installed last year and the remaining I finished installing tonight.
I like the stuff and the price is right (Bruce ABC511) at lowes.
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