Sometimes there is some confusion about which size screws to choose from. Typically, you have 1”, 1 ¼”, 1 5/8”, 2”, 2 ½” and 3” screws.
First, you should know what boards are being screwed together and in what direction. Makes sense.
Always screw through the lesser board into the greater board. In other words, screw through the thinner width to connect the thicker board. For example, if you were to screw a 1×4 into a 2×4, you should screw through the 1×4 to enter into the 2×4.
This is not only structural, but very smart.
If you are screwing into a 2×4, then you want to use 3” screws.
In this case, the 3” screw goes through the 2×4 on the 2” side (or more precisely, the 1 ½” side.)
If you want to screw together two 1×4 boards, then you use a 1 58” screws. In this case, the 1 5/8” screw goes through the 1×4 on the 1” side (or more precisely, the ¾” side.)
A carpenter should always choose a screw that is at least twice the width of the board. 3” screws should be used to connect 2x4s and 2x6s. 1 5/8” screws should be used to connect 1x4s and 1x6s. 1 ¼” screws should be used to screw through something that is ½” thick. 1” screws should be used to connect anything less than that.
And as always, make sure the screw is not too long so that it pokes out from the back. A pointy screw would be unsafe to precious hands. It would have to be grinded off, which would make the screw useless in the long run and will also reduce your structural integrity for whatever project.
But you also don’t want to use a screw that is too terribly long. If you use a 3” screw to attach a ½” board to a 2×4, the smooth head will be the only part holding the ½” board together.
Look at a 3” screw. There is a long portion of it that is smooth and has no threads towards the screw’s head.
You don’t want that. You want those threads to be a part of keeping the boards tight. So choose something smaller.
I certainly hope this helps. Keep building!