The underlayment under your shingles is the layer nobody sees once the roof is closed, and bids often name it only as felt or underlayment with no weight attached. Felt for asphalt shingle roofs comes mainly in two weights. The number on the roll is a nominal weight per 100 square feet, and it tells you roughly how much asphalt-saturated organic material went into each sheet. A crew can install either weight correctly, so the choice is not about workmanship. It is about what the roof spec says, and whether the roll that arrives on the truck is the one you were quoted.
What the two weights are
Fifteen pound felt is the lighter sheet, usually sold as ASTM D226 Type I. Thirty pound felt is the heavier sheet, usually ASTM D226 Type II. Both fall under the same ASTM standard, and both are in common use under asphalt shingles on pitched decks. The heavier sheet has more asphalt and more mass per square, so it is less likely to tear when a crew walks across it and less likely to show a wrinkle once the shingles are nailed down. The lighter sheet is thinner, rolls lighter, and costs less per roll. Neither one is a synthetic underlayment, and neither one replaces the self-adhering ice barrier that belongs at the eave.
The difference is not a question of which weight is good and which is bad. The question is what the shingle manufacturer's installation instructions allow, what the crew can keep dry during the working day, and what the homeowner is paying for on the line that reads underlayment.
Where the felt sits on the deck
On a shingle roof over a plywood or OSB deck, the felt runs over the deck and under the shingles. It is laid in horizontal courses that start at the eave and work up the slope, with each upper course lapping over the one below it. That lap is what sends water that gets past a shingle down and off the lower edge instead of into the seam. Where a valley, a step wall, or a pipe boot needs its own piece, the crew cuts and fits it as a separate part of the layout, and the weight of the felt and the product used at each detail should match what the spec says.
Keys installs Atlas and Malarkey shingles, and the felt named in the bid goes under whichever of those the spec names. The point of writing the felt into the spec is that a homeowner can check it after the work. If the bid says one layer of 30 pound felt with horizontal laps, a delivery slip, a roll wrapper, or a photo of the deck before the shingles go on can confirm it.
Why St. Louis winters change the answer
St. Louis roofs go through freeze and thaw cycles from December into March. Snow melts on a warm afternoon, runs down the slope, and refreezes over the overhang, which is how an ice dam forms at the eave. Water that backs up behind the dam has to go somewhere, and the layers at the eave decide whether it reaches the inside of the house. That is why the self-adhering ice barrier is usually laid first at the eave, with the felt running over the rest of the field. The felt does its job in the field, but it does not hold back water at the eave on its own.
Spring is the other stretch to plan for. Wind-driven rain often arrives in the same weeks a crew is tearing off, and a roof can sit with bare felt for part of a day or longer while the shingles wait for a dry window. The heavier sheet is sturdier during a short open window, but the rain plan in the bid still has to answer what happens if water reaches the felt before the shingles are on. Ask the bid to say that in writing.
What a felt line in the spec should say
A line that reads felt underlayment is not enough to check. A spec line should name the weight, the ASTM type, the number of layers over the deck, the lap direction, and the fastening method. One layer of 30 pound felt, ASTM D226 Type II, laid in horizontal courses, is a line you can compare against the rolls on the truck. Ask whether the felt goes over the whole deck or only the field, and whether the eave and valley pieces are a separate product on the bid. Ask how the felt is fastened, since staples and cap nails behave differently on a sloped deck and in cold weather. Ask whether the shingle manufacturer's installation instructions name a felt weight. Some do. A bid that departs from those instructions should say why, in writing.
What it costs
Felt is a small line in a roof bid. It is priced by the roll or by the square, and the gap between the two weights is usually small next to the tear-off labor, the decking, and the ice barrier. The reason to see it as its own line is not the dollar amount. A bid that lists felt separately has to say which felt it means. If one bid shows a felt line with a weight and the other shows only underlayment inside a general materials figure, the two totals cannot be compared until the second bidder says what the felt is.
If a bid is lower than the others, check whether the difference is in the felt. A lighter sheet in place of a heavier one can lower the total without the homeowner seeing it, and the spec line is the place to catch it. This is the same kind of check that applies to any material line in a roof bid, and it takes a few minutes with the written proposal in hand.
What to ask before you sign
- Which weight is on the roll, and what ASTM type is printed on the packaging?
- How many layers go over the deck, and where does the eave ice barrier sit in relation to the felt?
- How is the felt fastened, and what method does the crew use on this slope?
- How long can the deck stay open once the felt is down, and what happens if rain arrives before the shingles?
- Does the shingle manufacturer's installation instructions name a felt weight, and does the bid match it?
If your current bid says only underlayment, call Keys at (314) 220-2333 and ask for the weight, the ASTM type, and the layer count in writing before you choose a start date.