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I am making some shelves and workshop benches out of 2x4 and OSB, just the standard type you see in a lot of garages/workshops. I want them to be sturdy and support a good amount of weight.

My question is what type of screws should I use, drywall screws, decking screws, or something else; what about torx vs phillips? Would I need to predrill holes (OSB/2x4s), ideally I wouldn't so I could make quick work of the assembly. Also what length is good, some places have screws going through two 2x4, would 3 inch screws protrude the other side in that case?

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Do not use drywall screws. They are thin bodied and brittle. They hold because there is virtually no dynamic load and the load is spread over many screws on many studs.

I would favor construction screws, although deck screws would be fine. You can find both types with drill point tips that make predrilling unnecessary except in harder materials (such as hardwood or mdf).

Most carpenters have moved to star (Torx is a brand) over phillips, although some modified phillips (such as Pozidriv or Supadriv) have advocates, but they need special bits and screwdrivers to work at their best.

Length of screws should be determined by the thickness of the wood you are attaching. It is bad form (and dangerous) to leave tips protruding because of overlong screws. If you need to go through one 2x in places and two 2x in others, use different length screws. Usually if there is a pair of studs that are already attached, any OSB panels would likely only need to go through the stud it is facing, not the sister stud. Screws can probably be the thickness of the OSB + 1 to 1 1/4 inches. The screws to sister 2 2x studs should be 2 1/2 inches (2 3/4 would be fine, but an uncommon size)

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I have abandoned the use of Philips screws in favor of torx. You should not have to predrill osb.

1 3/4" or 2" should be sufficient for attaching the osb to the 2x4s. 2x4 are 1 1/2" thick so 2 1/2" for securing the 2x4 framing together unless the shelf's are going to moved around then i would use carriage bolts. i would predrill on the ends of the 2x4 to avoid splitting but not necessary more than 3" from the ends.

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A company called McFeely's makes 2 7/8" screws called "Promax" specifically designed to join standard lumber.

1.5" is flat shank, 1 3/8" is threaded - exactly what you want. They self-drill and then really crank the lumber together. Amazingly I've never found anything else like them in the big box stores.

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