On 2010-06-08, Joe J wrote:

I'm attaching 2 x 4s to the studs in a finished garage. Finished
with 1/2" drywall. Are 3" screws long enough, I have a bunch
leftover from a different project?

Here's what the National Design Specifications for Wood has to say on
the matter:

For laterally loaded wood screws, the minimum embedment is 4 times the
shank diameter, and to develop the full lateral strength, you need an
embedment of 7 times the shank diameter. You also need to drill a
pilot hole at around 90% of the root diameter.

Withdrawal capacity is linear in embedment, so the more the merrier.
This becomes an issue if you are applying an eccentric lateral load,
e.g. your 2x4s are going to be support shelves, where the weight is
some distance from the wall. Then the top set of screws is loaded in
tension.

As an example a #10 screw has a shank diameter of 0.190" and a root
diameter of 0.130". So to develop the full lateral capacity, you'd
want an embedment of 1.33" and a 7/64" or 1/8" pilot hole. With a 3"
screw, you could achieve the necessary embedment in your situation by
counterboring the 2x4s by 1/2".

Cheers, Wayne