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?
Thanks
It's not the length, it's how you use them :-)
Ditch the drywall screws for the purpose entirely.
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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"
Gordon Shumway A Liberal is a person who will give away everything he doesn't own.
Since he suggested using a 3" I believe he is going through the narrow dimension of a 2x4. Which is around 1 1/2" plus 1/2" of wall board.
Correct, 1 5/8 to be exact, plus the drywall at 1/2". So, 2 1/8" total. If I drive them through the 2x4 hard, they'll countersink about an 1/8, so I would have about 1" into the stud.
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
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.
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.
he never said anything about 'drywall' screws. And i don't think drywall screws come 3 or 3 1/2 inches long anyway.
Of course you don't think (they indeed do).
Nice. Insult me, when it was YOU that made the mistake.
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
Wrong again, dope.
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...
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.
Yes, 1/3 of the shank is smooth and the other 2/3 of the shank is threaded.
Yes again, but if he NEEDS to use a 2x4 then he NEEDS screws with similar holding power.
Gordon Shumway A Liberal is a person who will give away everything he doesn't own.
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