I have a 23 year old home with the original asphalt shingles and it needs to be replaced. Can I re-roof over the old shingles? What are the pros and cons? Obviously it's easier. Are there any other things such as drip edges and the like that should be replaced at the same time?
You can definitely do an overlay re-roof if there is actually only one previous layer of shingles. Some cities let you have up to three layers but I wouldn't recommend it. Many layers equal too much load on your rafters and sheathing. My house had 4 layers of asphalt comp over 1 layer of ancient cedar shakes w/skip sheathing. You could hear the house take a deep breath once all of that garbage was removed. My method given only one current layer of asphalt comp. shingles would be to put down a fresh layer of 15# felt, use button-cap fasteners. Install new drip edge; pry out the old drip edge from under the existing shinles (unless you don't mind raised edges from 2 layers of edging). You shouldn't have to worry about replacing the valley flashing unless you have woven valleys (which (editorial comment) I think look like hell). Follow the directions on your shingles of choice closely. I am not a roofer but have done my share so other opinions wouldn't hurt. I always consult a professional or plenty of literature before I attempt any job I haven't done before that may cost me more money later if I do it wrong. If you don't know any roofers to talk to just hit your local tavern about noon on a rainy day.