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.