1/2" plywood. It doesn't deteriorate with humidity and other factors like MDF and Fiberboard do. I built my kayak outside under a tarp and then later moved it to the porch and never had any issues with the plywood. I did go the extra step of using an outdoor plywood since I knew it might get rained on now and then (which it did). I did not use staples, so I don't know which is better for staple holding, but I've never had a problem stapling things to plywood.