[dad911]

There should be decking/plywood/osb sheathing under the shingles. Sometimes, like on an 'A' or a rake, they will nail the trim boards against the sheathing, and run the roofing out over it. But otherwise, there should be sheathing under shingles.

Ice and water shield is sticky on one side, and adheres to the roof sheathing. We use it along the bottom of the roof, and sometimes in valleys. It is also a good product to apply against the side of the house where concrete/masonry meets a house, (like front steps) to prevent leaks & rotten wood behind the masonry.

Per National Building code, there should be 1 Square Foot of ventilation for every 150 sq. ft. of attic space. Less for power ventilators.

And there should be a gap where the attic insulation meets the roof, so there is cross ventilation from the eaves.

The above is relatively recent, past 10 years or so. Not common 20 years ago.

If there is no decking, what is holding up the roofing?