A. Well, I don't have any firsthand experience with Ice and Water Shield (by the way, I think that's a brand name so there should probably be a TM), but I do understand the concept. This material was introduced in areas where there were problems with ice dams building up on roof edges that caused water and ice to back up under the roofing material and ultimately cause rot. If you are building in that sort of climate, I would definitely consider using this type of material. In dryer or warmer climates where ice dams on roof edges are not generally a problem, I might spend the money elsewhere.

I have torn old roofing off of many houses, some close to 100 years old, and have found that the underlying structure is generally sound except in places where the roofing material had worn out and not been replaced (or roofed over). In the early days of house building here in the Seattle area, roofs were almost always built with skip-sheathing and shingled with cedar shingles (not shakes). When these began to wear thin, roll roofing or some variety of asphalt shingles was nailed down on top of the existing cedar shingles. I've torn off roofs that were 3 or 4 layers thick...not a recommended reroofing scheme.

My point is that what is under the roofing material is not as important as installing a good sound roof and maintaining it. Proper ventilation in the attic or truss space and a good vapor barrier in the living space below will keep moisture from migrating up to the roof sheathing. Asphalt impregnated felt (30# is recommended) under the roofing material provides a bit of a backup layer and helps protect the roof sheathing from condensation that may build up on the bottom of the roofing material.

If you feel the need to spend the money, just put the Ice and Water Shield along the bottom and edges of the roof, and use felt over the whole thing. Make sure that your selected roofing material is installed properly with all appropriate flashing. Don't forget to put drip edge metal all around. That will do more to protect your roof structure than anything. Just remember that the structure can stand for hundreds of years, but the roof will have to be replaced sooner or later no matter how you build it.