🛡️ Roof Underlayment & Ice & Water Shield Calculator
Estimate underlayment OR ice & water shield rolls for your roof
Smart Roofing Calculator — Roof Underlayment & Ice & Water Shield Calculator — smartroofingcalculator.com/underlayment/ — printed
💡 Synthetic underlayment is lighter and more tear-resistant than felt.
🛠️ How This Calculator Works
Enter roof area, coverage per roll, waste, and pitch factor. The calculator gives you rolls needed.
📐 Formula
Rolls = ⌈(Area × Pitch Factor × Sections × (1 + Waste%)) ÷ Coverage per Roll⌉📝 Example Calculation
1,500 sq ft with 10% waste: • 1,650 sq ft → 5 rolls at 400 sq ft/roll
💡 Pro Tips from Roofers
- Overlap rows by 4" minimum; 6" at hips and valleys.
- Start at the eave and work upward.
- Use cap nails every 6" in high-wind areas.
- Never shingle over wet underlayment.
❓ Frequently Asked Questions
How many rolls of underlayment do I need?
Divide total roof area by coverage per roll (400 sq ft typical), add 10-15% waste.
Felt vs synthetic — which is better?
Synthetic is lighter, stronger, more water-resistant. Felt is cheaper but tears easier.
Do I need ice and water shield?
Yes in cold climates. Required along eaves 24" past exterior wall, in valleys, and around penetrations.
How much does underlayment cost?
Synthetic: $50-$100/roll. #30 felt: $25-$45/roll. Ice & water shield: $90-$150/roll.
How many rolls of ice and water shield do I need?
Add up the linear feet of eave courses, valleys and penetration patches, multiply by the 3 ft roll width, add 10% and divide by 200 sq ft per roll. A 50 ft gable house with a 2 ft overhang at 6/12 and two valleys needs about 4 rolls.
How far up the roof does ice and water shield go?
Where required, from the eave edge to a point at least 24 inches inside the exterior wall line, measured horizontally. With a 2 ft overhang on a 6/12 roof that is about 4.5 ft up the slope, so two courses.
How much does a roll of ice and water shield cover?
A standard roll is 36 inches by about 66.7 feet, which covers 200 square feet — two roofing squares.
Is #30 felt the same coverage as #15?
No. A roll of #30 felt usually covers about 200 sq ft because it is heavier and shorter, while #15 covers about 400 sq ft. Check the roll label.
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📝 Related Articles
About the Smart Calculator Team
We build free, accurate construction calculators for homeowners and contractors. This article is part of our roofing knowledge base — we also publish guides and tools for other home improvement projects:
Shopping for someone? See our gift guide for roofers & DIY homeowners.
Worked examples updated . Every figure below is computed with the same formulas as the calculator.
Where ice and water shield goes
- Eaves: from the edge to at least 24 inches inside the exterior wall, where the code requires an ice barrier. A deep overhang or a low pitch means two or three courses.
- Valleys: a full 36-inch width centred on the valley, before the valley metal or shingles go on.
- Around chimneys, skylights, walls and large pipe penetrations: a patch lapped up the vertical surface.
- Low slopes from 2/12 to 4/12: some installers cover the whole deck; the code minimum there is a doubled underlayment layer.
Ice and water shield calculator: a worked example
Worked example: A 50 ft gable house, 2 ft overhang, 6/12, two 15 ft valleys
- Distance to cover at each eave, horizontally: 2 ft of overhang + 2 ft inside the wall = 4 ft.
- On a 6/12 slope that is 4 x 1.1180 = 4.47 ft up the roof. One 3 ft course is not enough; 2 courses (lapped) are.
- Eaves: 50 ft x 2 eaves x 2 courses = 200 linear ft of 3 ft membrane.
- Valleys: two 15 ft valleys in plan are about 34 ft on the slope. Penetrations: three 3 x 3 ft patches = 27 sq ft.
- Total: (200 + 34) x 3 ft + 27 = 728 sq ft; with 10% waste, 800 sq ft.
5 rolls of ice and water shield at 200 sq ft per roll (3 ft x 66.7 ft). Enter 728 in the calculator's area field with 200 as the coverage to get the same answer.
The field of the same roof — about 1,600 sq ft of footprint at 6/12 — needs 2 rolls of synthetic underlayment at 1,000 sq ft per roll. Ice and water shield is additional to that, not instead of it, wherever it is used.
Roll coverage to enter in the calculator
| Product | Typical roll | Coverage to enter |
|---|---|---|
| Ice and water shield (self-adhered) | 36 in x 66.7 ft | 200 sq ft |
| Synthetic underlayment | 48 in x 250 ft (common size) | 1,000 sq ft |
| #15 felt | 36 in x 144 ft | 400 sq ft |
| #30 felt | 36 in x 72 ft | 200 sq ft |
Roll sizes vary by brand: the coverage printed on the roll is the number to use. The ice dam prevention guide explains why the eave detail matters, and how to install underlayment covers laps and fastening.
Ice & Water Shield Calculator
This same calculator doubles as an ice & water shield calculator. To estimate ice and water shield, just change the Coverage per Roll field above:
- Standard ice & water shield roll: 200 sq ft (3 ft × 66.7 ft)
- Synthetic underlayment roll: 1,000 sq ft (10 squares)
- #15 / #30 felt paper: 400 sq ft
For ice & water shield, only enter the area you actually need to protect — typically a 3-foot strip along all eaves, valleys, around penetrations, and 24 inches past the interior wall line. Most homes need 1–4 rolls of ice & water shield, even though they need 4–6 rolls of full underlayment.
How much ice & water shield do I need?
Multiply your eave length by 3 feet, then add valley length × 3 feet. For a 50 ft wide × 30 ft deep house with two 30 ft valleys, that's roughly (50 × 3) + (2 × 30 × 3) = 330 sq ft — so about 2 rolls of standard ice & water shield (200 sq ft each).
Between 2/12 and 4/12 shingles need a double layer — see the minimum roof pitch for shingles and what else your slope allows.
Choosing between synthetic, felt and self-adhered? See our roof underlayment buying guide — what each type is for and which specs to check.
Need temporary cover after a storm? The tarps and underlayment roundup covers both.
Not sure of the roof area? The roof square footage calculator gives it from the footprint and pitch.
Everything else on the order? See the roofing material list for a worked example.
🔗 Related Roofing Calculators
- Plywood / OSB Sheathing Calculator — Decking sheets that go under your underlayment
- Shingles Calculator — Bundle count for the layer above
- Roof Pitch Calculator — Get your pitch factor for accurate area
- Drip Edge Calculator — Required at every eave & rake
- Ice Dam Prevention Guide — Where ice & water shield matters most
· Reviewed by the Smart Roofing Calculator editorial team