How thick should a plywood benchtop be?
I have recently found myself wanting for some work surfaces that aren't walmart folding tables. I've concocted a basic design in my head, which calls for a plywood top screwed down to a 2x4 frame underneath it. It'll be 2' deep, and 6' or 8' long depending on what I'm doing with it. In either case, it'll be supported every 2' with "joists" (for lack of a better term) made from 2x4s, with notches cut similar to what you see here. There will also be a lower shelf of the same design as the top sitting about a foot off the ground.
First bench I'm planning is going to be a glorified table in my sunroom that's just going to hold seedlings/potted plants before they're moved outside. At most, it'll have to support some tools and bags of soil. But I'm not planning to stack an entire raised bed's worth of dirt on this thing. Will 1/4" plywood be sufficient, or should I step it up a bit?
Second bench, which will come much later, is going to go in my basement as a reloading bench. I reckon I'll want a thicker top for that, but I'm not sure how much. In any case, I'm going to want to bolt things down to the joists, or add extra supports underneath just so I'm not relying entirely on the plywood. I'm thinking 1/2" or 3/4", but do you think I'll need more?
Edit: And while I have your attention, would you suggest screwing it down from the top, or using pocket screws from underneath?