I am making a planter box, 4′ x 10′ by 3′ tall out of pressure treated lumber. 2″ x 6″ sides/ends and 4″ x 4″ corner posts and one in the middle on the sides. I bought #10 x 3 1/2″ coated screws. Do I need to pre drill? If so what size drill bit and how deep? My drill bits aren’t sized by number but by the inch? 1/8″, 3/16″, etc.
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There are two different things you might "pre-drill": The top piece that the screw will go THROUGH, and the bottom piiece that the screw will go INTO.
The top hole would be a "clearance" hole, about the diameter of the screw. To size this you pick up a screw and a candidate drill bit, hold the bit behind the screw, and pick a bit where you can barely see the bit around the thick part of the screw.
To size the bottom hole, pick a bit that you can barely see looking THROUGH the threads of the screw, so that it is about the size of the screw if the threads were all ground off.
Generally speaking, with a relatively soft wood like PT you don't absolutely need to pre-drill at all, and you can usually get away with just drilling the smaller hole through both pieces. With a hard wood like oak you'd want to use the two different sizes.
With PT, be sure to use screws rated for PT use, as the wood is corrosive.
Thanks. I will probably drill the first piece as you state and not drill into the 4 x 4. Let the screw get a good binding into that 4 x 4.
Greg
Many of those screws made for the application are self drilling but your idea of pre-drilling the show pic is good. It'll keep drift, alignment and splits from occurring.