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? The 2 x 4s will be holding 5/8 plywood for shelves. Or,
should I just bite the bullet and go with 3 1/2?
He didn't say he had drywall screws. He's saying that he has to go
through a 2x4 and drywall to reach the stud. With three inch screws
he's going to get about an inch into the stud. With 3 1/2 it will be
a bit more. If I had the 3" laying around I might just use a few more
of them and consider it to be good enough.
When choosing length, try to have about 1/3 of the screw in the piece
being fastened and the remaining 2/3 of the screw threaded into the
piece being fastened to. As the thickness of the lumber increases
though, this ratio can be reduced to 1/2 if absolutely necessary.
That's why only 2/3 of the shank is typically threaded on our 1" and
longer wood screws.
In your example the preferred screw length would be ~4-1/2" to 5"
A Liberal is a person who will give away everything he doesn't own.
No, the shank of a wood screw is smooth so that the screw will slip through
the material being held and snug it up to the base material.
Adequate screw length depends on the load. If screwing to a wall and the
load is straight down, you can get away with a MUCH shorter screw than if
the load is horizontal, or nearly so, to the wall. It's the difference
between hanging a picture and an L-bracket for a shelf. A thumb-tack may
hold a 30# picture but not a shelf bracket.
Here's what the National Design Specifications for Wood has to say on
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
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".
If you want to use your 3" screws, you're going to need to drill
shanks in your 2x4's (shanks are holes that are wide enough so that
your screw can slide through without the thread catching -- you don't
want the thread catching both the 2x4 and the wall), so if you use a
counter-sync/shank combo, you should be be able to embed the 3" screws
a 1/2" into the wood easily enough. If you don't have one of those,
you might want to consider buying some 3 1/2 - 4" screws that have no
thread the top 1 1/2". That way you won't have to do any shanking,
and the shelves go up faster.
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