Roof Underlayment Basics: The Layer You Never See

You are comparing roof quotes and one line says synthetic underlayment, another says felt, and a third says ice and water shield at the eaves. None of it will be visible once the roof is finished, so it is tempting to skip past it. That line is worth a minute, because underlayment is the layer that decides what happens when water gets under the metal or the shingles.

Every roof covering lets a little water through at some point: wind lifts a shingle, a seam sealant ages, snow melts and backs up at the eave. Underlayment catches that water and lets it run down to the drip edge instead of soaking the deck. It also protects the deck while the roof is being built, between tear-off and the day the panels or shingles are finished.

This guide explains the common types, what the adopted code in South Carolina asks for, and what to ask before you sign. If you are deciding between a roof-over and a full tear-off, read roof-over vs tear-off too, because a roof-over changes how underlayment is used.

Underlayment comes in three broad families. Each has a standard it is tested to, and the standard name is what you should see on the product label and, ideally, on your estimate.

• Asphalt-saturated felt. The traditional paper-based sheet. ASTM D226 covers asphalt-saturated organic felt used in roofing, and ASTM D4869 covers organic felt made for steep-slope underlayment. It is inexpensive and familiar, but it wrinkles when wet and tears more easily than the newer sheets.
• Synthetic underlayment. A woven or spun plastic sheet that is lighter, lies flatter and resists tearing. Synthetics are approved through evaluation reports rather than one single ASTM standard, so ask for the report number or the manufacturer's data sheet. IBHS lists synthetic underlayment as one acceptable cover over taped deck seams in its FORTIFIED program.
• Self-adhered membrane (often called ice and water shield). A rubberized asphalt sheet with a peel-off backing that sticks to the deck and seals around nails. ASTM D1970 is the standard for self-adhering polymer-modified bituminous sheets used at eaves and valleys to stop leaks from ice-dam backup on shingle, tile or metal roofs. Installers also use it under valleys, around penetrations and, in some cases, over the whole deck.

South Carolina enforces the 2021 International Residential Code with state modifications, in force for local jurisdictions since January 1, 2023, according to the SC Building Codes Council. The Council adopted the 2024 editions in August 2025 with an implementation date of January 1, 2027. The sections below are from the 2021 edition, and the state's published modification list does not change them.

Section R905.1.1 requires underlayment for asphalt shingles, metal roof panels and the other listed coverings to be labeled and to meet the standards in Table R905.1.1(1), applied per Table R905.1.1(2) and fastened per Table R905.1.1(3). For asphalt shingles, the 2021 IRC calls for two layers of underlayment on slopes from 2:12 up to 4:12 and one layer above 4:12 (Section R905.2.2 also sets 2:12 as the minimum slope for shingles). For metal panels, the code table defers in large part to the panel manufacturer's installation instructions, so ask your installer which product the manufacturer specifies.

Section R905.1.2 requires an ice barrier only where the local jurisdiction has recorded a history of ice forming along the eaves in Table R301.2. Whether that applies to your address is a question for the Anderson County or City of Anderson building department, not something to assume either way. South Carolina also modified Section R905.2.8.5 to require a drip edge at the eaves and rake edges of shingle roofs, and the underlayment laps relate to that drip edge.

Metal panels run hotter than shingles in direct sun, and the air space under them can be hot and humid. Some underlayment manufacturers publish a service-temperature rating for use under metal and some do not. Ask for the data sheet of the exact product on your estimate and check that the manufacturer allows it under metal panels. The panel maker's instructions may name acceptable underlayments, and those instructions are what a building inspector will look at.

Slope matters for metal too. Under 2021 IRC Section R905.10.2, lapped metal panels with unsealed seams need a slope of at least 3:12, lapped panels with lap sealant applied per the manufacturer can go to 1/2:12, and standing-seam systems can go to 1/4:12. The lower the slope, the longer water sits on the roof and the more the underlayment and seam sealant matter.

If metal is going over existing shingles, the old shingles are not underlayment. The 2021 IRC allows a roof recover when it follows the manufacturer's approved instructions (Section R908.3.1), and those instructions typically call for a new underlayment or slip sheet over the old roof. Condensation under metal is a related topic; see roof condensation and attic ventilation basics.

IBHS, the insurance industry's research institute, builds its FORTIFIED Roof standard around a sealed roof deck. The reasoning on its FORTIFIED Roof page is simple: when wind strips the roof covering, water pours through the gaps between sheathing panels, and IBHS says sealing those seams can reduce that water intrusion by up to 95 percent.

The FORTIFIED technical bulletin on taping deck seams describes two approaches. One tapes every horizontal and vertical joint in the sheathing with an approved self-adhering flashing tape and then covers the whole deck with felt or synthetic underlayment fastened with capped nails. The other covers the entire deck with a self-adhering membrane meeting ASTM D1970. Either way, the tape or membrane has to be fully adhered without wrinkles, and the tape is never nailed or stapled.

You do not need to pursue a FORTIFIED designation to borrow the idea. Asking a roofer whether they tape deck seams or use a full self-adhered deck, and what it adds to the estimate, is a reasonable question for a home in the Upstate that sees thunderstorms and the inland side of tropical systems.

Underlayment is a backup, not a cure. If your roof leaks today at a chimney, a pipe boot, a wall or a skylight, the problem is almost always the flashing, not the sheet under the shingles. The post on types of roof flashing explains the pieces, and common causes of roof leaks covers the usual suspects. A leak in one spot may be a repair, not a new roof, and we will tell you so after an inspection.

Underlayment also cannot fix a low-slope roof that was given a covering meant for a steeper pitch, or a deck that is soft and rotten. Those are decisions about the roof system and the structure, and they belong in a conversation with a roofer who has looked at the attic and the deck. Stay off the roof while you check any of this; look from the ground or a window and let a crew with fall protection do the rest.

If you are planning a new roof, ask for the underlayment product name and standard in writing on the estimate. Then read about our residential metal roofing or residential shingle roofing, or request a free quote. We need to verify all measurements on site, which could adjust the price.