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