I would wait to build it until you have boat in hand. You might think of something else like where to move the gauges or how wide to make the deck when the boat is in front of you. I've made about ten decks on different boats and haven't waterproofed the wood. I haven't had trouble with rotting wood. The carpet adhesive does a good job of waterproofing. That being said, it sure can only help to waterproof the deck. I can run over to my parents and measure their Tuffy. They have a Magnum.