Contractor Job Estimator
$29.00
BuyIce & Water Shield vs Synthetic / Felt Underlayment: Where Each Is Required
Self-adhering polymer-modified bitumen at the eave vs #15 / #30 felt or synthetic on the field — IRC R905.1.2 is a location rule, not a whole-roof product swap
Size the eave band with the ice & water shield coverage calculator first. This page is which product spends that width, and where ordinary underlayment is still the field layer. Ice-dam mechanics: guide. Leaks that are really attic heat: troubleshooting.
Field squares for felt rolls: roofing square calculator. Hub: roofing calculators. Field roll takeoff: underlayment calculator. 15# vs 30# vs synthetic: within-category comparison.
Know the eave width before you pick a SKU.A 200 sq ft WeatherWatch roll is not a felt roll, and felt is not an ice barrier.
Open Calculator →Jump to what you need
Where Self-Adhering Ice Barrier Is Required
IRC R905.1.2 (where Table R301.2 designates ice-dam history): at the eaves, from the lowest edges to 24 in inside the exterior wall line, using either two layers of underlayment cemented together or a self-adhering polymer-modified bitumen sheet. That second option is the peel-and-stick ice & water shield in the carton.
It is required at that location when the table says so. It is not, by that section, required over the entire roof. Manufacturers still sell it for valleys, rakes, and penetrations because it seals around fasteners; that is good practice, not the eave-band sentence.
On ≥8/12 roofs, also at least 36 in along the slope from the eave. Detached unconditioned accessory structures are excepted.
Any trade
Checkout opens here — you stay on this page.
Ice Barrier Gear
The calculator returns a slope width and a roll count. A self-adhering membrane and a hook knife are how that band gets on the deck. CSV rows 400–410 were nailers, vents, and gutter gear — not ice barrier — so these two SKUs are new.
Grace Ice & Water Shield 36 in x 36 ft Roll
GCP 36 in x 36 ft roll = 108 sq ft (datasheet). The calculator defaults to a 2.0-square / 200 sq ft roll (GAF WeatherWatch and Grace 36x66.6 ft). Switch roll size to match the carton in your hand.
View on Amazon →
Stanley 10-510 FatMax Hook Knife
Hook blade for cutting membrane and shingles without gouging the deck. Size the ice-barrier band first; this is how you trim the course.
View on Amazon →
Run the Eave-Band Math
Overhang + wall + 24 in, converted along the slope. Net area, 36 in courses, rolls from published coverage.
Open Ice Barrier Calculator →
Attic Heat Feeds Ice Dams
Membrane at the eave does not cool the attic. Size soffit intake and ridge exhaust with IRC R806.2 NFA.
Open Attic Ventilation Calculator →
Need Eave Length?
The square calculator uses plan length × width. Bring the eave (horizontal edge) here for the ice-barrier band — not the sloped squares.
Open Roofing Square Calculator →As an Amazon Associate, TestTalkHQ earns from qualifying purchases.
Where Synthetic or Felt Underlayment Belongs
Upslope of the ice-barrier band, asphalt-shingle roofs still want underlayment per R905.1.1 / the shingle manufacturer: typically ASTM D226 felt or a listed synthetic. That layer is a water-shedding underlayment, not a dam-proofing membrane. It does not replace R905.1.2 at the eave in designated climates.
Do not run self-adhering membrane over the whole deck unless the manufacturer and the venting design allow it. Grace’s own ice-dam notes say the product is an air and vapor barrier — cathedral ceilings still need a vented air space, and a sealed deck can trap moisture. Field coverage is a different takeoff: squares and felt rolls live in the square calculator (synthetic often ~10 squares/roll; #15 felt ~4; #30 ~2).
Published Roll Coverage (Not a Guess)
| Product (published spec) | Width × length | Coverage (excludes laps) |
|---|---|---|
| GAF WeatherWatch 2.0 square | 36 in × 66.7 ft | 200 sq ft |
| GAF WeatherWatch 1.5 square | 36 in × 50 ft | 150 sq ft |
| Grace Ice & Water Shield | 36 in × 66.6 ft | 200 sq ft |
| Grace Ice & Water Shield | 36 in × 75 ft | 225 sq ft |
| Grace Ice & Water Shield | 36 in × 36 ft | 108 sq ft |
Sources: GAF WeatherWatch sell sheet RESUL103; Grace Ice & Water Shield datasheet (36 in side lap minimum 3.5 in). The calculator defaults to 200 sq ft and lets you switch. That table is why this cluster has no separate chart page: three SKU sizes plus a continuous pitch conversion already live in the tool and this comparison.
Felt is cheaper and wrong at the dam.Two layers of felt cemented together can meet the letter of R905.1.2. A single dry layer of #15 under the first course does not. If you came to buy felt squares, use the square calculator for the field and this eave tool for the barrier.
FAQ
Can I use ice and water shield instead of felt on the whole roof?
Only if the manufacturer, the venting design, and the AHJ allow a fully adhered vapor-barrier deck. R905.1.2 does not require that. Most houses want barrier at the eave and felt or synthetic on the field.
Is WeatherWatch the same as Grace?
Both are self-adhering polymer-modified bitumen ice barriers that meet ASTM D1970-style ice-barrier criteria. Coverage per roll differs by SKU — use the published square footage, not the brand name.
Do I still need attic vents if I use peel-and-stick at the eave?
Yes. Membrane manages water that ponds. Vents and insulation keep the deck cold. Attic ventilation calculator.