The problem with plywood is getting in from here to there. It's heavy and getting a good grip is difficult...especially if you have to carry it for any length of time. An easy solution is to take a couple of C-clamps and tighten them on to each end. This will give you the handle you need to get a good, solid grip.
You should inspect your roof at least once a year. Nothing beats getting up close, but a good pair of binoculars can give you the next best thing. Look for curling or buckling shingles, shingle granules in your gutters, and other signs of wear.