its half inch sanded plywood with no knots, sealed with Spar and top coated with 4 layers of spray bed liner both sides, on top of 1.5" s of pink Owen Corning closed cell foam board. seems bullet proof so far an is rock hard, fingers crossed. really seems to make the boat feel more solid while underway. no battle wounds on first trip, not even a scratch and is not slippery in the least. I used aluminum insert nuts in the ribs, kinda like a pop rivet but with a nut inside to hold everything down. this way if it does rot i can take out the SS 10/32 srews and use the deck as a template and cut me a new one in ten minutes. Hopefully that will be a very long time. My friend who saw it thought i had found a plastic board to use haha.