Underlayment is the layer between the deck and the shingles. Roll coverage varies by product from 200 to 1,000 square feet, and the stated coverage is before overlap — which is why the waste allowance here is higher than most materials.
Felt or synthetic
Fifteen-pound felt is the traditional cheap option. Thirty-pound is heavier and tears less but covers only half as much per roll.
Synthetic underlayment costs more per square but is far lighter to carry, grips better underfoot, and can stay exposed for weeks without degrading. On a job that might sit open between weather windows it is the safer choice.
Overlap eats coverage
Courses lap two inches on a standard pitch and four on a low slope, plus four to six inches at every end lap. That is where the 15 percent goes.
Below a 4/12 pitch most manufacturers require doubled underlayment — effectively halving your coverage — so check the pitch before ordering.
Ice barrier is separate
In cold climates code requires a self-adhering ice-and-water membrane along the eaves, usually extending from the edge to at least two feet inside the exterior wall line.
It also belongs in every valley and around every penetration. It is a different product at several times the price, and it is not included in this count.
It is temporary weather protection too
Underlayment is designed to be the secondary barrier under the shingles for the life of the roof, but it also has a short first job — keeping the house dry between tear-off and the finished roof.
Synthetic underlayments generally carry an exposure rating, often measured in months. Felt has effectively none; it wrinkles in the sun and tears in wind within days.
If the job might stall — a delivery, a weather window, a crew that moves — that exposure rating stops being a detail and becomes the reason the ceiling below is still intact.
Fastening and wind
Underlayment blows off. Cap nails with a plastic or metal washer hold it where staples and smooth roofing nails pull straight through, and most synthetic products require cap fasteners for their warranty.
The fastening pattern is usually tighter at the edges and in the overlap than in the field, and it is printed on the roll or in the instructions.
A roll that lifts overnight can take the whole course with it and leave the deck bare in the rain. Cap nails cost a little more than staples, and the comparison is not really between the two.