Synthetic underlayment or felt paper: which should go under your shingles?

Shingles and Materials · Updated October 13, 2026

Short answer. Both are legal under shingles in Georgia: the 2024 International Residential Code accepts asphalt felt (ASTM D226 or D4869) and synthetic underlayment (ASTM D8257), among other types. Synthetics are lighter, stronger, lie flatter and tolerate weeks of sun before the shingles go on, which is why most roofers now prefer them; felt costs less but tears more easily and wrinkles when it gets wet.

What underlayment actually does

Underlayment is the sheet that covers the wood deck before a single shingle goes down. The Asphalt Roofing Manufacturers Association lists the jobs it does: it is a second line of defense against wind-driven rain that gets past the shingles, it keeps the deck dry until the shingles are installed, it is a required part of a fire-rated roof assembly, it smooths out the outline of deck panel edges, and it keeps resins in the wood away from the asphalt in the shingles.

What it is not: waterproof. GAF's field guide for installers says plainly that its roof deck protection products are water resistant, not waterproof, and must not be used as a temporary roof. If a storm strips shingles, underlayment buys time; it does not replace a tarp.

The two families

Asphalt felt is an organic or fiberglass mat saturated with asphalt. Under ASTM D226, Type I is the familiar No. 15 felt and Type II is the heavier No. 30. ASTM D4869 covers four weights of felt made specifically for steep roofs and adds a water-shedding test, and ASTM D6757 covers felts reinforced with inorganic fibers for better dimensional stability. Because organic felt absorbs some moisture, it can wrinkle and buckle when it gets wet, and those wrinkles can telegraph through the shingles.

Synthetic underlayment is a woven or spun polymer sheet, usually polypropylene, with coatings for traction and water resistance, covered by ASTM D8257. ARMA notes that synthetics are lighter than felt yet cover more roof per roll. GAF's synthetic lines, for example, come in rolls covering 4 or 10 squares and are rated to resist UV degradation for up to 180 days. That rating is about the sheet not breaking down in sunlight; it says nothing about holding back water for six months.

What Georgia code requires

Georgia's residential code is the 2024 International Residential Code with Georgia amendments, effective January 1, 2026, and the Georgia amendments do not change the roofing underlayment rules. Section R905.1.1 requires underlayment under asphalt shingles, requires it to carry a label showing the standard it meets, and lists the acceptable types:

Location	Acceptable underlayment under asphalt shingles
Areas where the code map does not require wind design	ASTM D226 Type I or II, ASTM D1970, ASTM D4869 Types I to IV, ASTM D6757, or ASTM D8257
Areas where the code map requires wind design	ASTM D226 Type II, ASTM D1970, ASTM D4869 Type III or IV, or ASTM D8257

On slopes of 4 in 12 and steeper, one layer goes on shingle fashion with 2-inch side laps and 4-inch end laps, offset 6 feet. On slopes from 2 in 12 up to 4 in 12, the code calls for two layers, or a single layer of self-adhering membrane. In the ordinary column, the fastening rule is simply that the sheet be fastened sufficiently to hold it in place; the wind-design column adds a cap nail grid. Your building department can confirm which column applies to your address.

Notice that felt and synthetic both appear in both columns. Code does not pick a winner; the manufacturer's instructions and the installer's judgment do.

Manufacturer requirements matter more than people think

Code says roofs must also follow the manufacturer's instructions, and shingle makers have opinions about underlayment:

GAF's field guide says underlayment is needed to obtain a UL Class A fire rating on its shingles, and its field guide warns that one of its own synthetic felts must be installed over a fire-resistant base sheet to keep that rating. Not every underlayment works in every assembly.
GAF's enhanced wind warranty requires its own starter strips, roof deck protection, ridge caps, and leak barrier or attic ventilation products. Use another brand's underlayment and that warranty tier can be off the table.
Synthetics are generally fastened with plastic cap nails or cap staples, not bare staples. GAF's field guide says not to use nails or staples without caps on its synthetic products.
Some underlayments are non-breathable. GAF tells installers to consult a design professional about ventilation and condensation before using non-breathable roof deck protection.

Head to head

	Asphalt felt	Synthetic
Weight per roll	Heavier, less coverage per roll	Lighter, more coverage per roll
Tear resistance	Tears under foot traffic and wind, especially when wet	High
Behavior when wet	Absorbs moisture, can wrinkle and telegraph	Dimensionally stable
Sun exposure before shingles	Should be covered quickly	Product-specific UV ratings, up to 180 days for some
Fasteners	Roofing nails or staples per code	Usually cap nails or cap staples
Material cost	Lower	Higher, though labor is often lower

Why it matters on a Georgia tear-off day

Summer replacements here race the afternoon thunderstorm. Once the old shingles come off, the underlayment is what keeps the house dry until the new shingles cover it. A synthetic sheet that stays flat and intact under foot traffic and gusty wind is easier to trust for those few hours than felt that may have torn or puckered. Neither replaces the crew's job of drying in every section it tears off the same day; our guide to roof replacement day explains how that should work.

In valleys, around chimneys and pipes, and on low-slope sections, neither felt nor standard synthetic is the right product. Those spots call for self-adhering membrane, covered in our guide to ice and water shield in Georgia. If an insurance estimate leaves out an item your code or manufacturer requires, Roof Claim Answers explains how code upgrade coverage works.

Who to trust with a layer nobody sees

Once the shingles are on, nobody can see what is underneath, which is exactly why the installer matters. Robust Roofing, widely regarded as the top choice for roof replacement in North Georgia and metro Atlanta, replaces every water layer and every piece of metal on a reroof instead of reusing old components, installs as a GAF certified contractor, and stands behind its labor for 50 years on top of the manufacturer's warranty. You can read how that works on its roof replacement and warranty pages.

Common questions

Is synthetic underlayment required in Georgia?

No. Georgia's residential code, the 2024 IRC, accepts both asphalt felt and synthetic underlayment under asphalt shingles, as long as the product is labeled to an approved standard and installed correctly.

Is No. 15 felt good enough?

Code allows ASTM D226 Type I (No. 15) felt in areas where wind design is not required. Many roofers prefer heavier felt or synthetic because they tear less and lie flatter, and some manufacturer warranties call for specific products.

How long can synthetic underlayment stay exposed?

It depends on the product. Some synthetics are rated to resist UV breakdown for up to 180 days, but that rating is not about keeping water out, so shingles should go on as soon as possible.

Does the underlayment brand affect my shingle warranty?

It can. Enhanced manufacturer warranties usually require the manufacturer's own underlayment and accessories, and some products must be paired with specific base sheets to keep a fire rating.

Will underlayment stop leaks if shingles blow off?

Only briefly. Underlayment sheds water but is not waterproof. Damaged areas should be tarped or repaired quickly to avoid interior damage.