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I'm trying to build a planter box that is 2ft cubed (outer dimensions) thats a cube constructed out of 2x4s with 1x4s paneling the sides. I've constructed the top and bottom squares and now i need vertical posts.
My main concern with the vertical posts is how to join them to the top and bottom squares. If I go straight through the top or bottom 2x4 then i need very long screws just to get through the initial 3.5" of 2x4. I could also add screws at an angle but it seems easy to have poor aim and come out the side.
does that make sense to anyone? any advice?
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or

on the outside edges
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lap joints, glue and screws
if it is cheap pine predrill all the holes to avoid splits.
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I would have used 2x2s. However, since you're already there, just go ahead with long screws or toenail the screws by starting them in the vertical posts and screwing from outside towards the inside.
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For a 2 by cube you might be over engineering/reinforcing the frame.
If it were plywood instead of 4 inch boards ( cedar ? ) the frame wouldn't really have to serve any other purpose than a nailing surface. For that purpose you could even cut 2x2 or 2x3 diagonally (triangle sticks for the frame) to further minimize weight and maximize planting volume.
Even your 1x4 will go a long way towards holding the frame work together as a completed assembled package. The frame can be a little wiggly until you gt the 1x4 skin on it.
I highly recommend pre-drilling all the screw holes thoughUnless you're nailing. Even then I might pre-drill the nail / brad holes in the 1x4 ends to stop the ends from splitting.
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not exactly as requested, but: use pressure treated lumber, and if you can, find some smaller dimension stock. if they are going to be outside, PT is a must. plus, it will last a lot longer if they mistakenly get left out in the cold and rain and snow.
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You could also use a "cleat" of 1x1 on the inside of the joint. Nestle the cleat in the inside corner of the joint, and screw through the edge of the cleat into your 2 x 4s. That's a pretty strong joint and you probably won't care that the cleat protrudes inside the box; also, there will be no external fasteners showing.
If you want to overkill, apply epoxy to the cleat before you screw it in.
I agree - use PT lumber and stainless fasteners.
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haikuman, arsenic is no longer used in PT wood commonly available. also, I assumed plants and not veggies to eat, but a lining of some kind of planting membrane - if used - would negate most bad stuff in the soil, I would imagine.