We've all done this: you get down into the unfinished
basement, snake a few CATV or phone wires around
the ductwork and pipes, then drill up through the
subfloor into a pocket between two studs, and
cut into the wall to put an electrical box for
your new TV connection (or whatever).
But, how do you locate the hole, from below,
so that it comes up in the right place?
I had several ideas, all bad.
(1) measure from the structural boards in the
subfloor, accounting for 2x4 stud and other wall
thickness. Isn't accurate if the house has brick
veneer and I don't know the exterior wall construction.
(2) find a through-the-floor member and measure from that.
Works in the kitchen, with drain or water pipes, but not
across the house in a bedroom.
(3) drill a small pilot hole in an inconspicuous place and
use that as a reference on both above-floor and below-floor
measurements. Didn't have a long enough bit,
at the time (can go a little over an inch with
the small sizes - maybe I could've started with
1/8" and finished with a bit of clotheshanger wire?)
I've drilled into the top of a 2x10 in previous attempts
(4) put a magnet under the floor, and fish for it
above with a pendulum. This one almost worked,
but I was finding nails because the magnets were
too short (ideally, want over 2" length between N and
S poles for this kind of thing).
So, what DOES work?