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Caulk the plywood roof seams Fasten shingles on top of the plywood roof elements Reinforce the joints with metal plates. .
. To keep the elements out, it's important to cover the roof with tar paper or a synthetic paper and create as waterproof a surface as possible before shingling over it. There's no sense in creating a carport to keep your car out of the rain if it leaks. Would it be smarter to insulate the structure? Maybe, but probably not cost-effective. Remember, you're not building an addition to your house, you're building a simple structure to keep junk off your car. . Visit the hardware store and buy enough three-tab shingles to lay over the plywood and finish the surface of the carport. It might be a good idea to nail some weather sheeting over the plywood before you lay the shingles for an extra layer of protection. Alternatively, if you don't want to shingle the roof, you could skip the plywood roof step altogether and install a metal roof over the shingles. A slanted aluminum roof is common in outbuildings and you'll be that much closer to being finished. It might be a good idea, if you can stand the look of it and the loud sound of rain on metal. For extra stability in the places where the structure meets, it's a good idea to reinforce with metal bracing. Your hardware store sells a variety of metal plates that can be nailed onto the various joints in the structural composition, especially where the posts meet the beams, where the beams meet the rafters, and at other places. tain the wooden elements. Since you've gone to all the work, it's a good idea to treat the exposed wood with a coat of protecting stain. This will increase the life of the wood and keep you from having to repeat the project in a  few years. It's a good idea to restain regularly to keep the life of the structure up. Revisit your work periodically to catch up with repairs before they become a major problem.