What length screw to use?



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.
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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
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Yes, in addition, the screw is now in shear rather then bending in the 1mm between the board and the wall. Sheetrock kinda messes this up, but...
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On 6/8/2010 8:31 AM John spake thus:

Right concept, but the correct term is "clearance hole" for the screw, or counterboring it into the piece being held.
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I'd use them. I'd also put a dab of construction adhesive in four or five spots too on the ones hanging the shelves. .
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Ed Pawlowski wrote:

I have not been paying attention- these are horizontal 2x4s, right? To act as a ledger for the shelves? Or vertical 2x4s to hold brackets? On a horizontal 2x4, you may wanna go with a wider board, to reduce the lever action that will dig into the drywall. Back in the stone age, for closets and utility rooms and such, I always had good luck with a pretty 1x8 band screwed into the studs, and metal brackets screwed into that wherever needed. Held a 12" shelf, supported by the brackets and the top edge of the 1x8, heavily loaded, with no problem.
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For closets, just run boards around the three sides and put a shelf on top. I despise those metal brackets.
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Let's do the math. 2 x 4's are actually 1 1/2" thick some 1 5/8". Drywall 1/2", less an eighth or so for recess. That gives you about an inch of penetration into the stud that will hold it all. I'd use them.
Steve
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