Roofing Underlayment Calculator
Estimate roofing underlayment rolls, waste-adjusted coverage and optional material cost.
Use this roofing underlayment calculator to estimate whole rolls for a measured roof area. Choose Imperial or Metric units and a planning product type, confirm or customize effective coverage after overlaps, add waste and optionally enter price per roll. For final ordering, replace the planning coverage with the exact coverage printed on the selected product.
Your Estimate
Roof Area
2000.00 sq ft
Waste Allowance (10%)
200.00 sq ft
Waste-Adjusted Area
2200.00 sq ft
Effective Coverage per Roll
900 sq ft
Rolls Required
3 rolls
Uses editable effective coverage for Synthetic after ordinary overlaps. Roll dimensions, laps, roof slope, fastening and exposure limits vary by product; use the coverage printed by the manufacturer for final ordering.
Results Actions
The future import option is a placeholder only. Until calculator-to-calculator transfer is available, copy the pitch-adjusted surface area from the Roofing Calculator into the roof area field.
Roofing Underlayment Type and Coverage Comparison
Coverage values are conservative planning defaults after ordinary overlaps, not manufacturer guarantees. Roll size, effective coverage, lap requirements and permitted applications vary substantially by product.
How to Use the Roofing Underlayment Calculator
- 1Choose Imperial or Metric units.
- 2Enter the actual sloped roof area in square feet or square meters.
- 3Select #15 felt, #30 felt, synthetic underlayment or ice and water shield.
- 4Review the loaded planning coverage and replace it with the effective coverage printed on the product when available.
- 5Choose a 5%, 10% or 15% waste allowance for laps, cuts, valleys and penetrations.
- 6Optionally enter price per roll.
- 7Review roof area, waste, adjusted area, effective roll coverage, whole rolls and optional cost.
- 8Until area import is implemented, manually copy surface area from the Roofing Calculator when using both tools.
Roofing Underlayment Roll Formulas
- 1Roof area = entered actual sloped surface area in sq ft or m².
- 2Waste area = roof area × waste percentage.
- 3Waste-adjusted area = roof area × (1 + waste percentage).
- 4Effective coverage per roll is the usable area after required side and end laps, not necessarily the roll’s gross area.
- 5Rolls required = waste-adjusted roof area ÷ effective coverage per roll, rounded up to a whole roll.
- 6Estimated material cost = rolls required × price per roll when a price is entered.
Example Calculation
Imperial example: a 2,000 sq ft roof using synthetic underlayment with 900 sq ft effective coverage and 10% waste has 200 sq ft of waste area and 2,200 sq ft of adjusted area. Divide 2,200 sq ft by 900 sq ft per roll to get 2.44, then round up to 3 rolls. At $95 per roll, estimated material cost is $285. Metric example: a 185 m² roof using synthetic underlayment with 84 m² effective coverage and 10% waste has 203.5 m² of adjusted area. Divide by 84 m² per roll to get 2.42, then round up to 3 rolls.
Accuracy & Assumptions
- Entered roof area represents actual sloped surface area rather than the building footprint.
- Effective coverage already accounts for ordinary side and end laps.
- The loaded coverage values are planning defaults and should be replaced with the selected manufacturer’s published effective coverage.
- Waste is applied to roof area before dividing by roll coverage.
- Roll quantities always round up to whole rolls.
- The estimate does not separately calculate eave ice barriers, valleys, ridges, hips, penetrations or wall transitions.
- Ice and water shield may supplement rather than replace general underlayment, depending on the roof system and code.
- Optional cost covers rolls only and excludes fasteners, cap nails, primers, flashing, delivery, labor, tax and equipment.
- Required layers, laps, fastening, exposure time and approved roof-covering compatibility vary by product, slope, climate and code.
Frequently Asked Questions
How do I calculate roofing underlayment rolls?
Add waste to actual sloped roof area, divide by the product’s effective coverage per roll and round up. Use effective coverage after overlaps rather than gross roll dimensions.
What is effective roll coverage?
Effective coverage is the usable roof area covered after required side and end laps. It is usually lower than roll width multiplied by roll length.
How much does a roll of #15 felt cover?
This calculator uses about 400 sq ft or 37 m² as a conservative planning default after overlaps. Confirm the exact roll size and required laps on the product label.
How much does a roll of #30 felt cover?
The planning default is about 200 sq ft or 18.5 m² after overlaps. Products and application requirements vary, so use manufacturer coverage for ordering.
Is synthetic underlayment better than felt?
Synthetic underlayment often offers greater tear resistance, lighter rolls and more coverage, while felt may cost less and suit familiar applications. Compatibility, fastening, exposure and code requirements should guide selection.
Is ice and water shield the same as underlayment?
It is a self-adhered underlayment membrane commonly used in vulnerable locations. It may be installed alongside a general underlayment rather than replacing it across the entire roof.
How much waste should I add?
Five percent may suit simple planes with verified coverage, 10% is a practical planning allowance and 15% may suit complex roofs with valleys, hips or many penetrations.
Can I use Metric measurements?
Yes. Metric mode accepts roof area and effective coverage in m² and uses native Metric planning defaults for each underlayment type.
Can I import area from the Roofing Calculator?
Not yet. The interface includes a future-ready placeholder. For now, copy the Roofing Calculator’s actual surface area into this calculator manually.
Does the calculator determine code-required underlayment?
No. Underlayment type, layers, ice-barrier locations and low-slope requirements depend on roof covering, slope, climate, manufacturer instructions and local code.
This calculator provides a preliminary roofing-underlayment quantity and roll-cost estimate only. It does not determine the required material, number of layers, lap dimensions, fastening, exposure duration, ice-barrier locations, substrate preparation, primer, temperature limits or compatibility with the roof covering. Roofing work involves serious fall hazards. Follow local code, approved plans, fall-protection requirements and the roof-covering and underlayment manufacturers’ installation instructions, and confirm effective roll coverage and final quantities with the supplier or roofing contractor before ordering.