On Jun 8, 3:43*pm, keith wrote:
On Jun 8, 2:31*pm, "Joe J" wrote:



"John" wrote in message

...

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.

John

Why wouldn't I want the screw to catch both the 2x4 and the wall stud?
Seems to me that would make a nice tight fit.

When you snug the 2x up to the wall there is no gap. *If the threads
are sunk into both boards, one or the other has to strip out to close
the gap. *You don't want that force stripping the threads out of the
hole in wall stud.

Sometimes you get worse, and you can't strip out the threads from
either the wall, or the board, and that's worse.

Lets say you were putting a screw through your 2x4 to the wall, and
that the screw thread caught tightly on both. Also, lets say when you
were doing this, there was a 1mm gap between your board and the wall,
because you weren't holding it just right. You screw the screw in as
far as it goes, but there's still 1mm gap. Picture that you manage to
do one last turn of the screw, the screw moves 1mm further into the
wall -- well, it also moved 1mm further into the board, but because of
this, the board does not get tighter to the wall).

If, on the other hand for some reason the screw thread does not catch
on the board, because you either drilled a shank hole, or because the
screw has no thread for the depth of the board, then turning that
screw an extra turn pulls the board 1mm closer into the wall. This
creates tension and friction between the wall and the board, and that
is where you get your real strength. If you're having trouble
picturing it, fasten two boards to the wall -- one with a shank, and
one without, and hit the top of both boards with a sledge hammer. The
screws will likely break in the one without the shank holes, but
you'll likely just dent the board/wall in the other.

John