Ivan Vegvary wrote:
I own two stud finders and neither is giving me consistent results.
Need to find all of the ceiling joists in my kitchen so I can install
2 light tubes and 18 can lights. Symmetry is important. Would like
to put in 14" diameter daylight tubes so joist placement is
important.
My problem is that the ceiling consists of 1/2 inch drywall under 1/2
inch plywood. This is throughout the house and is probably why the
stud finders are not consistent. Complicating this is the fact that
the joist are all over the place and not necessarily parallel. I
have a few 13" gaps and a few 17" gaps etc. I know this from
measuring up in the attic. Further difficulty is that I have about
24" of blown in insulation. If I had drywall only (you lucky
*******s) I would simply go in the attic and shove an awl down along
side each joist, and then run chalk-lines down below. Can't shove
the awl through the plywood! So I'm working down below on a ladder
with a yogurt can attached to my portable drill (wife doesn't like
dust) and it takes me about 6-7 holes to locate a single edge of a
joist. Needs to be repeated at the other end which takes just as
many tries. The next joist can be measured out 16 inches and still
takes about 5 holes due to irregular placement. All together I have
about 48 locations to do. Way to slow.

All alternative ideas appreciated. You're welcome to call me an
idiot if it pleases you. Just help me save some time and effort.

You need a very powerful small magnet - like the ones I took out of old 5 1/4"
harddrives. They will quickly find the nails into the joists. They will probably
stick to them. I use them to find the joists on my lath and plaster walls and
ceilings. Of course, if the drywall is not nailed at the joists, this will
confuse things, but the magnets should still pull at least gently on the nails
1/2" away. If it sticks hard, that's probably a drywall nail. If it pulls just a
little, that's a plywood nail.

It seems you could just measure below, since with the plywood, you can hang
lights anywhere.