Whether or not you NEED ice and water shield (don't know wtf fire and ice is) on the entire roof is dependent on many factors. It's far too expensive to just assume you need it on the entire roof instead of JUST a 3 foot strip around all roof edges (maybe drip edge as well), under valleys and around problem areas like chimneys/skylights.
Stripping more than one layer is a pita but it MUST be done. Code only allows for 2 layers max. I always advise customers to strip the roof every time because you're not really saving money by not stripping it. You're only deferring payment for 20 years because it'll cost you more than twice as much next time. Plus you want to get a look at the wood underneath every 20 years and replace any questionable areas. There's also cosmetics. The new roof will only look as good as the surface it's put on. In addition, after stripping the roof and replacing any questionable patches, you'll want to remove all old roofing nails etc. and go around and tighten all other nails holding the roof surface (plywood or whatever) on. They loosen over time and can damage your new shingles if poking up.
Sweep often while stripping the roof. The pebbles from the old shingles can be slippery.
Work from the farthest area from the ladders/dumpster to the closest. No need reshingling an area only to travel across it over and over while doing another area. When an area is done try to stay off of it as much as possible.
Don't forget to clean and tighten your eaves troughs after stripping an area. Get all that debris out of there while the overhang of the shingles is out of the way.
My last piece of advice...ignore the people that give advice even though they only have less than a basic understanding of roofing.
Yeah Player99, I mean you. Wasn't it you who, in a previous thread suggested SCREWING down shingles, removing the screws, sealing the holes and then rescrewing? I realize you're trying to help but bad/uninformed advice can often cause more problems than it solves.
