Like all these guys said...1/2 inch...the only other thing I did was the bases of the seats I used 3/4...and the other thing I did was buy 7 ply besides 5, but that was just me...probably a little more over kill, but sometimes when you cut ply you will have cut through a knot in the wood, these knots would be in a ply in the middle of the wood...i would fill them with resin...
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