# Roof Underlayment Basics: Synthetic, Felt, and Ice Barrier Underlayment and ice barrier explained, with laps, low-slope rules, and a worked roll count for a gable roof. Guide page: (published 2026-10-05, updated 2026-10-05). This Markdown version has the guide's key points, outline, FAQs and sources; the full article, with its worked examples and diagrams, is on the page. ## Key takeaways - Underlayment is the water-shedding layer under the shingles; an ice barrier is a self-adhered membrane at the eaves and other leak-prone spots. - Laps eat coverage: IKO Stormtite is 1,000 sq ft per roll, but its 4 in side laps and 6 in end laps leave about 915 sq ft of roof per roll. - Under the 2018 IRC, where an ice barrier is required it runs from the eave to at least 24 in inside the exterior wall line; IKO measures from the inside face of the wall. - Asphalt shingles need a slope of at least 2:12, and the 2018 IRC requires double underlayment from 2:12 up to 4:12. - Count rolls from net coverage and round up; gross coverage can leave you a roll short. ## In this guide 1. [What underlayment does](https://www.howmanyyardsdoineed.com/guides/roof-underlayment-basics/#what-underlayment-does) 2. [Ice barrier at the eaves](https://www.howmanyyardsdoineed.com/guides/roof-underlayment-basics/#ice-barrier) 3. [Low-slope roofs](https://www.howmanyyardsdoineed.com/guides/roof-underlayment-basics/#low-slope-roofs) 4. [Laps and net coverage](https://www.howmanyyardsdoineed.com/guides/roof-underlayment-basics/#laps-and-net-coverage) 5. [Installing and fastening](https://www.howmanyyardsdoineed.com/guides/roof-underlayment-basics/#installing-and-fastening) 6. [Worked example: a 44 × 32 ft gable roof](https://www.howmanyyardsdoineed.com/guides/roof-underlayment-basics/#worked-example) 7. [Before you order](https://www.howmanyyardsdoineed.com/guides/roof-underlayment-basics/#before-you-order) ## Frequently asked questions ### How much does one roll of synthetic underlayment cover? It depends on the product. IKO Stormtite is 250 ft by 48 in, or 1,000 sq ft gross. After its required 4 in side laps and 6 in end laps, each roll covers about 915 sq ft of roof. ### Do I need ice and water shield? Where your local code requires an ice barrier, yes: the Town of Firestone, Colorado, for example, requires one under the 2018 IRC. IKO also recommends covering the whole roof with its ice and water protector on slopes from 2:12 to 4:12. ### How far up the roof does an ice barrier go? The 2018 IRC says at least 24 in inside the exterior wall line. IKO's instructions measure 24 in from the inside face of the outside wall and also say to go above the highest expected ice dams, so with an overhang it often takes two courses. ### What is the minimum roof pitch for asphalt shingles? Under the 2018 IRC, as summarized by the Town of Firestone, asphalt shingles may be used only on slopes of 2:12 or steeper, and slopes from 2:12 up to 4:12 need double underlayment. ### Is synthetic underlayment better than felt? IKO describes its Stormtite synthetic as an alternative to #15 asphalt-saturated felt that resists tearing when walked on. It is also a vapor retarder, so the attic needs good ventilation. Follow your shingle maker's underlayment requirements. ## Related calculators and guides ### Calculators - [Roofing Calculator](https://www.howmanyyardsdoineed.com/roofing-calculator/) — Roof area, roofing squares, and shingle bundles from a footprint and pitch, using the coverage on your shingle wrapper. - [Roof Pitch Calculator](https://www.howmanyyardsdoineed.com/roof-pitch-calculator/) — Pitch as x/12, roof angle in degrees, slope percentage, and the sloped length for a given horizontal run. ### Guides - [How to Measure Roof Pitch](https://www.howmanyyardsdoineed.com/guides/how-to-measure-roof-pitch/) — Measure rise over 12 in of run from the attic or roof, read an angle, or work it out from plans, then convert. - [How to Measure an Irregular Area](https://www.howmanyyardsdoineed.com/guides/measuring-irregular-areas/) — Split odd shapes into rectangles, triangles, circles, and trapezoids, then average width and depth. ## Sources - [IKO Stormtite Synthetic Underlayment technical data sheet (10/2025)](https://mediaresourcecenter.iko.com/m/74bbf770bb84bde4/original/IKO_TDS_STMT_EN.pdf) — IKO Industries. Used for: Roll 250 ft × 48 in; 1,000 ft² per roll (gross); 4 in lap line; minimum 4 in side laps and 6 in end laps. Checked 2026-10-05. - [IKO Stormtite Synthetic Underlayment application instructions](https://mediaresourcecenter.iko.com/m/2e6ae55be550f6f2/original/IKO_STMT_APP_INST_EN.pdf) — IKO Industries. Used for: Lay parallel to the eave, lapping eave protection ≥ 4 in; side laps ≥ 4 in, end laps ≥ 6 in; capped fasteners about 8 in on center at laps (4 in in high-wind zones), 24 in down the middle. Checked 2026-10-05. - [IKO StormShield Ice and Water Protector technical data sheet (12/2025)](https://mediaresourcecenter.iko.com/m/3ff39f461acf5468/original/IKO_TDS_SSHD_EN.pdf) — IKO Industries. Used for: Roll 65 ft × 36 in; 195 ft² per roll (gross); 3 in selvage; end laps ≥ 6 in, staggered ≥ 24 in; full coverage recommended on 2:12–4:12 roofs. Checked 2026-10-05. - [IKO ArmourGard and StormShield Ice and Water Protector application instructions](https://mediaresourcecenter.iko.com/asset/d52daa3c-8c3b-4dd2-ba36-9bac444e08e3/IKO_AMGD_SSHD_PCTO_APPI_EN.pdf) — IKO Industries. Used for: Overhang eaves 1/4–3/4 in; extend ≥ 24 in up-roof from the vertical projection of the interior surface of the outside wall; cut into 10–15 ft lengths; upper course laps the lower course's selvage. Checked 2026-10-05. - [Residential Asphalt Shingle Re-Roofing Requirements (2018 IRC)](https://www.firestoneco.gov/DocumentCenter/View/9461/Firestone-Reroof-Requirements-2018-IRC-updated-81123) — Town of Firestone, Colorado. Used for: Ice barrier ≥ 24 in inside the exterior wall line (required in Firestone); double underlayment for 2:12 up to 4:12; asphalt shingles only at 2:12 or steeper. Checked 2026-10-05. - [TruDefinition Duration Shingles product page (Owens Corning)](https://www.owenscorning.com/en-us/roofing/shingles/trudefinition-duration) — Owens Corning. Used for: 3 bundles per square; 98.4 sq ft coverage per square (32.8 sq ft per bundle). Checked 2026-10-04. **Estimate only.** Results are planning estimates computed from the numbers you enter. They are not engineering, structural, or code advice. Confirm quantities, product yields, and installation requirements with your supplier, the product label, and your local building authority before you order or build.