Last reviewed October 31, 2026.
The old shingles are in the dumpster. The front slope is bare plywood for the first time in decades. A crew member rolls a gray sheet across it, fastening as they go, and by afternoon new shingles have buried every inch. The estimate on the kitchen counter may have named that sheet in one line. It may have skipped it.
That sheet is the underlayment. CertainTeed describes its job as an added moisture-resistant layer that backs up the shingles. Once the shingles go on, nobody sees it again.
The underlayment floor in the 2025 state code
The 2025 Residential Code of New York State took over on December 31, 2025, per the NYS Department of State’s amended adoption notice. Section R905.1.1 requires underlayment under asphalt shingles, and under clay and concrete tile and several other coverings too.
Table R905.1.1(1) lists what may go under shingles outside the coastal hurricane column: asphalt-saturated felt made to ASTM D226 Type I or II or ASTM D4869, self-adhering membrane made to ASTM D1970, ASTM D6757, and synthetic sheet made to ASTM D8257. Felt and synthetic both make the list.
The pitch of the roof decides how it goes down, under Table R905.1.1(2):
| Roof slope | What the table calls for |
|---|---|
| 4 in 12 or steeper | One layer, run parallel to the eave, side laps of 2 inches |
| 2 in 12 up to 4 in 12 | Two layers, starting with a half-width strip at the eave, each sheet lapped half its width plus 2 inches |
| Either | End laps of 4 inches, offset by 6 feet |
Anything flatter than 2 in 12 is off limits for asphalt shingles altogether, under Section R905.2.2. On any slope the table does allow, there is a third route: a single layer of self-adhering ASTM D1970 membrane, installed to the makers’ instructions. Outside the coastal column, Table R905.1.1(3) asks only that the sheet be fastened well enough to stay put.
When the coastal wind column applies
The same tables carry a second column for hurricane-prone regions. The code’s Chapter 2 defines those as Atlantic and Gulf coast areas where the ultimate design wind speed is greater than 115 miles per hour.
Inside that column, the list shrinks. Under shingles it allows ASTM D226 Type II felt, ASTM D4869 Type III or IV, ASTM D1970 and ASTM D8257. Lighter Type I felt drops off.
The application rules tighten too. The roofer has three options: two mechanically fastened layers, a 4-inch self-adhering strip over the deck joints under a full layer, or one self-adhered polymer-modified bitumen layer over the whole deck.
So does the fastening. Under Table R905.1.1(3), mechanically fastened sheets in that column go down with cap nails in a 12-inch grid, spaced 6 inches along the side and end laps.
A house’s design wind speed comes from the local version of Table R301.2, which each town or village fills in for itself. The Smithtown page shows one completed table, a town copy written for the 2020 code. For any other address, the building department can say which column its roofs fall under. The departments are listed on the permit hub.
Felt or synthetic on the deck
CertainTeed makes both kinds, and says felt was the standard choice for a long time. Its underlayment page says synthetics cost more but weigh less, lie flatter and resist water better. The top synthetic products, it adds, can even act as a short-term roof cover.
CertainTeed also splits one roof into two jobs. A waterproof ice and water membrane runs along the perimeter, through the valleys and around skylights and plumbing vents. A tear-resistant, water-resistant sheet covers everything else and gives the crew a grippy surface to walk on. The membrane gets its own page, on ice and water shield.
None of the Long Island roofers we looked at break out felt and synthetic prices. The cost usually rides inside the full job, and the ranges contractors do publish are on the roof replacement cost guide.
What GAF, Owens Corning and CertainTeed ask for
The standard shingle warranty is one thing. The enhanced system warranties roofers advertise are where the underlayment brand starts to count.
Owens Corning. The System Protection warranty, published August 2026, requires a code-approved underlayment and installation to Owens Corning’s instructions and local building code. The old roof comes off down to a clean deck, old felt and old membrane included. On top of Owens Corning shingles and hip and ridge, the owner needs any 2 of 4 accessories: VentSure ventilation, synthetic underlayment (Owens Corning brand, RhinoRoof U20 or Titanium), starter shingles, or a self-adhered barrier. Felt is not on the underlayment list.
GAF. A 2021 GAF qualification guide says lifetime coverage on shingles and accessories requires a GAF Lifetime shingle and at least 3 qualifying GAF accessories. Roof deck protection, GAF’s name for underlayment, is one of those categories, with products such as Deck-Armor, FeltBuster and Tiger Paw. The same guide sets 3 accessories for System Plus, 4 for Silver Pledge and 5 for Golden Pledge. Its WindProven wind warranty requires roof deck protection along with starter strips and ridge caps.
CertainTeed. An April 2026 CertainTeed flyer offers a 15-year, 160 mph wind upgrade on Landmark shingles, and unlimited wind coverage when one product from each Integrity Roof System category is installed. Two of those categories are underlayment: waterproofing (WinterGuard or VYCOR) and water-resistant (DiamondDeck or RoofRunner).
Warranty terms change. Ask the roofer for the current document behind the exact warranty on the quote.
Underlayment questions to settle before signing
- Which underlayment, by brand and product name?
- Does it match the shingle brand, if the quote promises an enhanced warranty?
- Two layers on any slope under 4 in 12, such as a porch roof or a shed dormer?
- Which column of the code table does the town put the house in?
- Where does the self-adhering membrane go, and where does the regular sheet take over?
The edge metal and the underlayment lap in a set order, laid out on the drip edge page. The rest of a full quote checklist lives on the roof replacement page, and shingle choices on the asphalt shingle page.
