Roofing Underlayment Calculator
Calculate felt and synthetic underlayment rolls needed for your roof project.
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Does not include ice & water shield for eaves, valleys, and penetrations. Add ice & water shield separately if required by code.
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Types of Roofing Underlayment Compared
Each underlayment type has different coverage, durability, and cost characteristics.
| Type | Roll Size | Effective Coverage |
|---|---|---|
| Felt #15 | 36" × 144' (432 sq. ft.) | ~4 squares (400 sq. ft.) |
| Felt #30 | 36" × 72' (216 sq. ft.) | ~2 squares (200 sq. ft.) |
| Synthetic | 48" × 250' (1,000 sq. ft.) | ~10 squares (1,000 sq. ft.) |
Understanding Underlayment Overlap Rules
Proper overlap is critical to prevent water infiltration beneath your roofing underlayment. Each underlayment type has specific overlap requirements that affect how many rolls you need.
Horizontal Overlaps (Side Laps)
When two courses of underlayment run parallel up the roof, the upper course must overlap the lower course. Felt #15 requires a minimum 2-inch horizontal overlap. Felt #30 and synthetic require at least 3 inches. On low-slope roofs (2/12 to 4/12), increase horizontal overlaps to 4-6 inches or use a double layer of felt.
Vertical Overlaps (End Laps)
Where two rolls meet end-to-end along the same course, vertical overlaps of 4-6 inches are standard. Some synthetic products require 6 inches. Always stagger vertical seams so they don't align between adjacent courses.
Valley and Hip Lines
At valleys and hips, extend the underlayment at least 36 inches beyond the centerline on each side. Many contractors install a full-width roll of ice and water shield in valleys before laying the field underlayment.
Ice & Water Shield: When You Need It
Ice and water shield is a self-adhering rubberized asphalt membrane that seals around nail penetrations and prevents water backup from ice dams. It is not a substitute for standard underlayment but works in conjunction with it in critical areas.
Where Code Requires It
- Eaves: From the roof edge to at least 24 inches past the interior wall line (typically 3-6 feet up the roof)
- Valleys: 36 inches on each side of the valley centerline
- Around penetrations: Chimneys, skylights, plumbing vents, and other roof penetrations
- Low-slope transitions: Where steep-slope roofing meets low-slope areas
Cost
Ice and water shield typically costs $1.00-$2.50 per square foot installed, or $75-$150 per roll (a roll covers approximately 65-75 sq. ft.). For a typical home, ice and water shield adds $500-$1,500 to the underlayment cost.
Underlayment Installation Tips
Proper installation of roofing underlayment is just as important as choosing the right type. Even the best felt underlayment or synthetic product will fail if installed incorrectly. Follow these guidelines to ensure a watertight barrier beneath your shingles.
Always start from the eave and work upward. Begin by rolling your first course of underlayment along the eave (bottom edge) of the roof, making sure it overhangs the drip edge by about 1/4 inch. Each subsequent course overlaps the one below it, so water flows over the lap rather than under it. This shingle-style layering is critical for preventing water infiltration.
Overlap minimums vary by material. For felt underlayment (#15 or #30), maintain a minimum 2-inch horizontal overlap (side lap) between courses. Increase this to 4 inches on low-slope roofs (2/12 to 4/12). Synthetic underlayment typically requires 3-inch horizontal overlaps. At vertical seams where two rolls meet end-to-end along the same course, overlap by at least 4 inches for felt and 6 inches for synthetic. Our underlayment calculator already factors in these overlaps when calculating the number of rolls you need.
Fasten properly and avoid wrinkles. Use plastic-cap nails or staples spaced 12–18 inches apart along the top edge and at overlaps. Felt underlayment is especially prone to wrinkling in hot weather — lay it flat and work out wrinkles before fastening. Synthetic products are more dimensionally stable but should still be pulled taut during installation. Wrinkles can telegraph through shingles and create an uneven roof surface.
Protect against wind before shingling. If your underlayment will be exposed for more than a few days before shingles are installed, secure all edges with additional fasteners. Felt underlayment can degrade in UV light within 30–60 days. Synthetic products are more UV-resistant (typically rated for 90–180 days of exposure) but should not be left uncovered indefinitely.
When to Use Ice and Water Shield
Ice and water shield is a self-adhering rubberized membrane that seals around nail holes and prevents water from backing up under shingles. While it costs more than standard roofing underlayment, it provides an essential layer of protection in vulnerable areas.
Cold-climate eaves: Building codes in cold climates require ice and water shield along eaves from the roof edge to at least 24 inches past the interior wall line. This protects against ice dams — ridges of ice that form at the eave and force meltwater to back up under the shingles. Without ice and water shield, this backed-up water can leak into your attic and walls, causing serious damage.
Roof valleys: Valleys channel large volumes of water and are one of the most leak-prone areas on any roof. Install ice and water shield at least 36 inches on each side of the valley centerline before laying felt underlayment or synthetic over the top. This double-layer protection guards against leaks even if debris or ice accumulates in the valley.
Around penetrations: Chimneys, skylights, plumbing vents, and any other objects that pierce the roof surface need ice and water shield worked into the flashing details. The self-sealing property of the membrane around nail penetrations provides an extra layer of defense that standard underlayment cannot match. Our underlayment calculator covers field underlayment; add ice and water shield for these critical areas separately.
Underlayment Cost Comparison
Understanding the true cost of roofing underlayment requires looking beyond the price tag per roll. Different products cover different amounts of area, so comparing cost per square foot gives you a more accurate picture.
Felt #15: At $10–$20 per roll covering approximately 400 sq. ft. (4 squares), felt underlayment costs roughly $0.03–$0.05 per square foot. It is the most affordable option but also the least durable. Felt tears easily during installation, absorbs moisture, and degrades quickly in UV exposure. For budget-conscious projects or homes in mild climates, it remains a viable choice.
Synthetic underlayment: Priced at $50–$100 per roll covering approximately 1,000 sq. ft. (10 squares), synthetic underlayment costs $0.05–$0.10 per square foot. Despite the higher roll price, the cost per square foot is comparable to felt because each roll covers 2.5 times more area. Synthetic is lighter, stronger, and far more resistant to tearing and UV degradation.
Ice and water shield: At $60–$150 per roll covering approximately 65–75 sq. ft., this premium membrane runs $0.90–$2.00 per square foot — significantly more than field underlayment. However, it is used only in critical areas (eaves, valleys, penetrations), not across the entire roof. For a typical 2,000 sq. ft. roof, ice and water shield adds $500–$1,500 to the total underlayment budget.
Frequently Asked Questions
How many rolls of roofing underlayment do I need?
What is the difference between felt and synthetic underlayment?
Do I need ice and water shield?
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