How many rolls of roof underlayment do I need?

Divide your sloped roof area by the coverage per roll: 15-lb felt 400 sq ft, 30-lb felt 200 sq ft, synthetic 1000 sq ft. Always round up. A 2000 sq ft roof needs 5 rolls of 15-lb, 10 rolls of 30-lb, or 2 rolls of synthetic.

How much ice and water shield do I need?

Multiply your eave linear footage by the coverage depth (3 ft IRC minimum, 6 ft recommended, 9 ft for severe cold climates). Divide by 100 for roll count. Example: 100-ft eave x 6 ft = 600 sq ft = 6 rolls.

📋 Roof Underlayment Calculator

Roof Underlayment Calculator15-lb Felt, 30-lb Felt, and Synthetic Compared Side by Side

Enter your roof area directly or calculate it from house footprint and pitch. Get rolls needed for all three US underlayment types in a side-by-side comparison, plus ice and water shield rolls based on eave linear footage and your climate zone depth requirement. Includes a cost comparison when you enter price per roll.

📋 Roof Area, Underlayment Type, and Ice Shield

■ Roof Area Input Method

Roof Area (actual sloped surface, not floor area)

sq ft

■ Underlayment Cost Comparison (optional)

Price / 15-lb Felt Roll

$/roll

Price / 30-lb Felt Roll

$/roll

Price / Synthetic Roll

$/roll

Price / Ice Shield Roll

$/roll

❄️ Ice and Water Shield

Required at eaves by IRC code in most US climate zones. Enter your eave linear footage and select your climate zone coverage depth.

Eave Length (total lin ft)

lin ft

Coverage Depth

📋

Your underlayment estimate appears here

Enter your roof area (or footprint dimensions and pitch), and optionally add eave length for ice and water shield. Results show rolls needed for all three US underlayment types in a side-by-side comparison, plus ice and water shield rolls by climate zone coverage depth. Add optional prices to see a cost comparison across all types.

Roof Underlayment Summary

0 sq ft roof area

0 roofing squares

0 ice shield rolls

Area Details

Roof area used —

Roofing squares (100 sq ft each) —

Footprint x pitch multiplier —

📊 Three-Type Side-by-Side Comparison

15-lb Felt

Coverage/roll400 sq ft

Rolls needed—

Roll size3 ft x 144 ft

Est. total cost—

Best forBudget projects, dry climates

30-lb Felt

Coverage/roll200 sq ft

Rolls needed—

Roll size3 ft x 72 ft

Est. total cost—

Best forSteep roofs, heavy rain

Synthetic

Coverage/roll1,000 sq ft

Rolls needed—

Roll size4 ft x 250 ft

Est. total cost—

Best forAll climates, preferred by contractors

❄️ Ice and Water Shield

Eave length entered —

Coverage depth selected —

Ice shield area needed —

Rolls to buy (1 roll = 100 sq ft, 3 ft x 33 ft) —

Ice shield cost —

💰 Estimated Total Underlayment Cost (synthetic + ice shield) $0.00

Rolls Needed by Underlayment Type

Three Types of US Roof Underlayment: Felt Paper, Synthetic, and Ice and Water Shield

Roof underlayment is the water-resistant layer installed on the roof decking (OSB or plywood sheathing) before the final roofing material goes on. It serves as a secondary water barrier: if shingles or metal panels are damaged, lifted by wind, or improperly installed, the underlayment is the last line of defense against water reaching the sheathing and the structure below. In the US, the International Residential Code (IRC) requires underlayment on all sloped roofs, and most shingle manufacturers require it to validate the product warranty.

Three distinct underlayment product categories are used in US residential construction. Each has a different roll size, coverage area, cost, and performance profile. The comparison calculator above outputs rolls needed for all three simultaneously so you can evaluate the trade-offs before purchasing.

US Underlayment Roll Sizes and Coverage

Type	Roll Dimensions	Coverage Per Roll	Weight Per Roll	UV Exposure Limit
15-lb Felt	3 ft x 144 ft	4 squares (400 sq ft)	~25 lbs	None (must be covered immediately)
30-lb Felt	3 ft x 72 ft	2 squares (200 sq ft)	~60 lbs	None (must be covered immediately)
Synthetic (standard)	4 ft x 250 ft	10 squares (1,000 sq ft)	~20-25 lbs	30 to 90 days (product dependent)
Ice and Water Shield	3 ft x 33.3 ft	1 square (100 sq ft)	~25-30 lbs	180 days (most products)

How the Roof Underlayment Calculator Works

If entering roof area directly: use the sloped surface area in square feet (the same number used to order shingles, not the floor footprint). If calculating from footprint: adjusted area = (L + 2*overhang) x (W + 2*overhang) x pitch_multiplier x (1 + waste). 15-lb felt rolls = ceil(roof_area / 400). 30-lb felt rolls = ceil(roof_area / 200). Synthetic rolls = ceil(roof_area / 1000). Ice and water shield: area = eave_lin_ft x coverage_depth_ft. Ice shield rolls = ceil(ice_area / 100). Each roll of standard US ice and water shield is 3 ft wide x 33 ft long = approximately 99 sq ft, rated as 1 square of coverage. When prices are entered, total cost for each type = rolls x price_per_roll, and total combined cost (synthetic + ice shield) is shown in the summary card.

3 Real Underlayment Estimates

Example 1 — Ranch Home, 5/12 Pitch, 1,800 sq ft Roof, Charlotte, NC

Roof area: 1,800 sq ft (20 squares). Eave length: 108 lin ft. Climate zone: moderate (6-ft ice shield coverage depth).

15-lb felt: ceil(1,800/400) = 5 rolls. 30-lb felt: ceil(1,800/200) = 9 rolls. Synthetic: ceil(1,800/1000) = 2 rolls.

Ice and water shield: 108 lin ft x 6 ft = 648 sq ft. ceil(648/100) = 7 rolls.

Recommendation: 2 rolls synthetic underlayment + 7 rolls ice shield = complete coverage. Total materials at typical 2026 prices ($85/syn roll, $95/ice roll): 2 x $85 + 7 x $95 = $835.

Example 2 — Two-Story Colonial, 7/12 Pitch, Footprint 32 x 60 ft, Denver, CO (Cold Climate)

Footprint with 12-in overhang: (32+2) x (60+2) = 34 x 62 = 2,108 sq ft flat. Sloped: 2,108 x 1.158 = 2,441 sq ft. With 10% waste: 2,685 sq ft. 27 squares.

Synthetic rolls: ceil(2,685/1000) = 3 rolls. Alternative 15-lb felt: ceil(2,685/400) = 7 rolls.

Eave lin ft (gable, 2 long sides): 2 x 62 = 124 lin ft. Colorado is zone 6 — use 9-ft ice shield coverage. Area: 124 x 9 = 1,116 sq ft. Rolls: ceil(1,116/100) = 12 rolls ice shield.

Note: In Denver and other mountain-adjacent US zip codes, ice dam risk is elevated. The 9-ft coverage depth ensures ice shield extends from the eave past the typical interior wall line by a substantial margin, meeting both IRC code and manufacturer warranty requirements.

Example 3 — Garage Addition, 3/12 Pitch, Direct Area Entry, Phoenix, AZ

Roof area: 480 sq ft (4.8 squares). Eave length: 50 lin ft. Phoenix is climate zone 2B — minimal ice dam risk, but code still requires underlayment at eaves. Use 3-ft minimum ice shield depth.

15-lb felt: ceil(480/400) = 2 rolls. Synthetic: ceil(480/1000) = 1 roll (overkill for this size, but 1 roll is the minimum). 1 roll covers 1,000 sq ft, leaving 520 sq ft as leftover for future use.

Ice shield (3 ft depth): 50 x 3 = 150 sq ft. Rolls: ceil(150/100) = 2 rolls.

In Phoenix: 15-lb felt is the standard choice for low-slope desert roofs — the low UV exposure time is not a concern because dry desert conditions allow fast shingle installation after underlayment. For a 480 sq ft garage, 2 rolls of 15-lb felt at approximately $30 per roll = $60 total vs. 1 synthetic roll at $85. The 15-lb felt is the more economical choice for this scale.

3 Expert Tips for Roof Underlayment Installation

❄️ Tip 1: Understand Your IRC Climate Zone Before Ordering Ice and Water Shield

The International Residential Code (IRC) requires ice and water shield at all eaves in climate zones where the January mean temperature is 25 degrees Fahrenheit or lower. In practice, this covers virtually all of the northern United States, the mountain states, and higher elevations in the mid-Atlantic and Pacific Northwest. The IRC minimum is that ice and water shield must extend from the eave edge to a point at least 24 inches inside the interior wall line. For a typical US house with wall plates 8 inches inside the exterior sheathing, this means the ice and water shield must run at least 32 inches up the slope from the eave edge measured horizontally. On a 5/12 pitch roof, that horizontal distance translates to approximately 32 inches of sloped surface or roughly 3 feet of coverage. Most US roofing contractors and shingle manufacturers, however, recommend a minimum of 6 feet of ice and water shield coverage measured up the slope from the eave, even in climate zones where 3 feet is the code minimum. In climate zones 6, 7, and 8 (northern Minnesota, Montana, Maine, Alaska, and high elevations in the Rockies), 9 feet is the professional standard. The coverage depth selector in this calculator reflects these three tiers, and the results show rolls needed for your specific selection so you can match the requirement for your exact US location.

📋 Tip 2: Overlap Felt Underlayment Correctly or It Will Leak at Every Seam

Felt paper underlayment is not a membrane — it is paper saturated with asphalt, and it relies on correct overlap at all horizontal and vertical seams to create an effective water barrier. The required overlaps are: horizontal courses (where the upper course laps over the lower course): 2-inch minimum for 15-lb felt, 4-inch minimum for 30-lb felt. End laps (where two rolls meet end-to-end on the same course): 6 inches minimum for all felt types. At hips and ridges: 6-inch overlap. At valleys: 18-inch overlap (or use full valley flashing instead). These overlaps are built into the coverage ratings on this calculator. The 15-lb felt roll’s 400-square-foot coverage already accounts for standard 2-inch horizontal overlap, so you do not need to manually adjust the roll count for overlaps when using this calculator’s default. Where felt underlayment commonly fails in US residential roofing is when installers do not maintain the required overlap consistently, especially around protrusions (chimneys, pipes, skylights) and at the rake edges where the felt should extend at least 3 inches past the roof edge under the drip edge. Felt that is left exposed to UV for more than a day or two becomes brittle and begins to crack, which is why IRC and most manufacturers require shingles to be installed the same day as the felt in most US climates. In contrast, synthetic underlayment can typically remain exposed for 30 to 90 days without UV degradation, which is why it is preferred when construction schedules are uncertain.

🏠 Tip 3: Install Ice and Water Shield Before Felt Underlayment, and Always Adhere the Full Surface

Ice and water shield is a self-adhering product: it has a peel-and-stick backing that bonds to the roof decking when the release film is removed and the membrane is pressed down. The installation sequence on any US roof where both ice and water shield and felt or synthetic underlayment are specified: first install ice and water shield at the eave from the drip edge up to the required height (3, 6, or 9 feet as applicable). Then install felt or synthetic underlayment starting at the eave over the top of the ice shield, lapping each course from bottom to top so that water running down the roof flows over each lap rather than under it. If the ice and water shield is installed after the underlayment, it cannot adhere properly to the decking in the most critical ice dam zone. Roller-press the entire surface of each ice shield course with a felt roller (approximately $15 at any US home improvement store) immediately after removing the release film. Cold temperatures reduce the adhesive bond, so in winter installations, warm the adhesive surface with a heat gun or install ice shield during the warmest part of the day. The most common failure mode for ice and water shield in the US is incomplete adhesion along the edges and at laps, which allows water to migrate under the membrane at exactly the spots most vulnerable to ice dam backup. Full-surface adhesion, especially at edges and seams, is what makes self-adhering ice shield effective.

Frequently Asked Questions About Roof Underlayment

What is the difference between 15-lb and 30-lb felt underlayment?▼

The 15-lb and 30-lb designations originally referred to the weight per 100 square feet of finished felt paper, though modern manufacturing has made the actual weights variable. In practical terms for US roofing: 15-lb felt is a single-ply asphalt-saturated paper that is lighter, thinner, and less expensive. It is the standard minimum underlayment for most US residential roofing applications on pitches of 4/12 and steeper. It tears more easily when wet and saturated, making it vulnerable in wet weather. 30-lb felt is a heavier, thicker product that resists tearing better and provides better water resistance during the installation window before shingles go on. It is recommended for steep pitches (7/12 and above), where roofers walk on the installed underlayment and the extra weight and durability reduces tear risk. 30-lb felt covers only 2 squares per roll vs. 4 squares for 15-lb, which means you need twice as many rolls and the cost per square is typically 50 to 70 percent higher. In the current US market, both felt types are being replaced by synthetic underlayment for most new residential applications. Synthetic products at similar or slightly higher cost per roll provide 10 squares of coverage per roll, superior UV resistance, lighter weight, better footing for installers, and resistance to wrinkling in wet conditions.

Is synthetic underlayment better than felt for a US roof?▼

In virtually all US residential roofing applications in 2025 and 2026, synthetic underlayment outperforms felt paper on every measurable dimension except initial material cost per square foot. The advantages of synthetic over felt for US roofers and homeowners: synthetic covers 10 squares per roll vs. 4 squares (15-lb) or 2 squares (30-lb) for felt, which means fewer rolls, fewer trips up and down the ladder, and faster installation. Synthetic weighs approximately 20 to 25 pounds per roll vs. 25 pounds for 15-lb felt and 60 pounds for 30-lb felt, making it easier to handle on steep roofs. Synthetic can be left exposed for 30 to 90 days without UV degradation, compared to felt paper which becomes brittle within a day or two of sun exposure. Synthetic does not wrinkle, buckle, or absorb water when it gets wet during installation, which is a significant issue with felt in regions with unpredictable weather. Synthetic provides better slip resistance for workers installing shingles over it. The one area where felt has historically been preferred is breathability: some builders and roofing consultants believe that felt paper allows better vapor movement from the decking, which can be beneficial in certain building assembly types. This is a nuanced point, and in most US residential construction the difference in vapor management between felt and synthetic is not significant. The majority of US roofing contractors now specify synthetic underlayment as their standard product for all residential applications.

Is ice and water shield required on my US roof?▼

Ice and water shield (self-adhering polymer-modified bitumen membrane) is required by the IRC at all eave edges in climate zones where ice dams are a risk, which the code defines as areas where the January mean temperature is 25 degrees Fahrenheit or lower. This covers the majority of the northern United States from roughly the latitude of Kansas City and northward, all of the mountain states above 4,000 feet elevation, and parts of the mid-Atlantic and Pacific Northwest. In southern US states (Florida, lower Texas, coastal Louisiana, southern California) where January temperatures are consistently above 32 degrees Fahrenheit, ice and water shield at the eave is typically not required by code, but many local building departments in those states may still require it in valley and hip areas where water concentration is high. Even where not required by code, most US shingle manufacturers include ice and water shield coverage requirements in their warranty terms. Specific areas where ice and water shield is required even in warmer climates include: all valleys (where two roof planes meet and water concentrates), all roof-to-wall flashing areas, around all penetrations (chimneys, skylights, pipes), and at any area where the slope is less than 2/12. When in doubt, check with your local building department for the specific ice and water shield requirements in your jurisdiction, since local amendments to the IRC may impose different requirements than the base code.

How do I calculate how many rolls of underlayment I need?▼

Divide your total sloped roof area by the coverage per roll for your chosen product. Important: use the sloped roof area (actual surface you are covering with shingles or metal panels), not the floor footprint of your house. A 2,000-square-foot house at a 5/12 pitch does not have a 2,000-square-foot roof — the actual sloped surface is larger by the pitch multiplier (5/12 = 1.083x), so the roof is approximately 2,166 square feet. For 15-lb felt (400 sq ft per roll): 2,166 divided by 400 = 5.4 rolls, round up to 6 rolls. For synthetic (1,000 sq ft per roll): 2,166 divided by 1,000 = 2.2 rolls, round up to 3 rolls. Always round up to the next whole roll, never down. Running out of underlayment mid-installation and making a material run while the decking is exposed is a significant risk, especially in US markets where weather can change quickly. This calculator handles both the footprint-to-sloped-area conversion and the roll count rounding automatically.

How much does roof underlayment cost in the US?▼

Roof underlayment costs in US home improvement stores and roofing supply houses in 2026: 15-lb felt rolls (4 squares each) typically cost $28 to $40 per roll, or approximately $7 to $10 per square. 30-lb felt rolls (2 squares each): $45 to $70 per roll, or $22 to $35 per square. Synthetic underlayment rolls (10 squares each): $70 to $120 per roll at major US retailers (Home Depot, Lowe’s) or $80 to $150 at roofing supply houses for premium products with longer UV exposure ratings, equating to $7 to $15 per square. Ice and water shield rolls (1 square each): $80 to $130 per roll at retail, or $7 to $14 per square for the eave protection area. In a cost-per-square comparison, synthetic underlayment is typically price-competitive with 30-lb felt and close to 15-lb felt, while offering superior performance. Ice and water shield is significantly more expensive per square, which is why it is typically used only in the required eave zone and other specific locations rather than as full-roof coverage. Total underlayment material cost for a typical 20-square US gable roof: $200 to $400 using synthetic underlayment plus ice and water shield at the eaves (approximately 10 squares of ice shield coverage for a 100-linear-foot eave zone).

Can I use one type of underlayment for the whole roof, or do I need multiple types?▼

For most US residential roof replacement projects in areas where ice dams can occur, the standard approach uses two types: ice and water shield at the eaves (and valleys) where ice dam risk is highest, and felt or synthetic underlayment for the remaining field of the roof. The eave ice shield is installed first, then the synthetic or felt is installed over it and up the roof to the ridge. On the main field of the roof (above the ice shield zone), using a single type of product for the rest is standard practice. Many contractors specify synthetic for the field because it covers 10 times the area per roll compared to felt, reducing installation time significantly. In warm climates where code does not require ice shield, a single product (typically 15-lb felt or synthetic) can be used for the entire roof surface. In cold climates, using ice and water shield for the entire roof instead of just the eave zone is an option for maximum protection and is sometimes specified on high-end projects or in very cold climate zones, though the material cost is approximately 5 to 10 times higher than synthetic for the field area. This is not a common approach for standard US residential roofing projects.

Do I need underlayment under metal roofing?▼

Yes, underlayment is required under metal roofing panels in most US jurisdictions, and most metal roofing manufacturers require it for warranty compliance. However, the type of underlayment matters significantly for metal roofing applications. Standard asphalt felt paper (15-lb or 30-lb) is not recommended under standing seam or other metal roofing systems because the petroleum-based asphalt in felt can react with certain metal coatings over time, and the felt can retain moisture against the back of the metal panels, accelerating corrosion at the panel edges. The recommended underlayment for US metal roofing applications is a synthetic underlayment specifically rated for use under metal roofing, which avoids the asphalt-metal compatibility issue. Some synthetic products are specifically marketed as metal roofing underlayment. For corrugated or R-panel screw-down systems installed over purlins (not over solid decking), underlayment is sometimes omitted because the panel ribs create an air gap, but local building codes vary on this requirement. For standing seam over solid OSB or plywood decking, underlayment is universally required. Where vapor condensation is a concern (common in the Southeast and humid climates), an additional vapor-permeable underlayment or a peel-and-stick product with perforations allows condensation to escape from the underside of the metal panels rather than accumulating on the decking surface.

Related Roofing and Exterior Calculators

Editorial Standards and Legal Disclaimer

Direct mode: roof area entered directly in sq ft. Footprint mode: adjusted area = (L + 2*overhang) x (W + 2*overhang) x pitch_multiplier x (1 + waste). Pitch multipliers: 3/12=1.031 through 12/12=1.414. 15-lb felt: 400 sq ft per roll (4 squares). 30-lb felt: 200 sq ft per roll (2 squares). Synthetic: 1,000 sq ft per roll (10 squares). Ice and water shield: 100 sq ft per roll (3 ft x 33.3 ft). Ice shield area = eave_lin_ft x coverage_depth_ft. All roll counts round up (ceil). Roll coverage ratings reflect standard US manufacturer specifications with typical overlap factored in. Actual coverage may vary by product brand. IRC climate zone ice shield requirements vary by jurisdiction; confirm requirements with your local building department. Last reviewed: August 2026.