Can shingles be applied directly over existing roof?

sure, as long as the old roof is still flat and not wavy or cupped and doesn't already have 4 or 5 layers of shingles on it. I've seen a lot of roofs with a couple of layers of shingles on them having no problems. Your local roofing contractor will say "hell no" because he'll want to do a tear-off and reroof and make a bit more money. If you can afford it, that is probably the best way to go, but if you want to save some bucks roof right over the old. It's common practice and should serve you well