BuildMeter guide · Structures & Carpentry
Roof Underlayment Selection and Coverage Guide
Convert roof plan area to sloped coverage, use effective roll coverage and separate general underlayment from eave, valley and penetration membranes.
Direct answer
What to know first
Estimate underlayment from sloped roof area plus dormers, overhangs and detail zones, then divide by effective installed coverage after laps. Product choice and lap rules depend on roof covering, slope, deck, climate and manufacturer requirements.
Open Roof Underlayment CalculatorStart with sloped roof area and effective coverage
Horizontal plan area must be converted to roof-surface area using pitch, then adjusted for overhangs, dormers and separately measured roof sections. Divide by effective installed roll coverage after laps—not nominal roll width multiplied by roll length.
| Layer | What to include |
|---|---|
| Sloped field area | Plan area × pitch multiplier |
| Geometry additions | Overhangs, dormers, crickets and small roofs |
| Lap allowance | Side and end overlaps required by product |
| Waste | Cuts, tears and detail transitions |
| Separate membranes | Eaves, valleys and penetrations when specified |
Worked example
A 2,200 ft² plan area at 6:12 pitch has about 2,460 ft² of sloped area. Add 120 ft² for overhang and dormer details: 2,580 ft². Applying 8% lap allowance and 7% waste gives about 2,981 ft². At 400 ft² effective coverage per roll, purchase eight rolls.
Selection questions are separate from area
- What roof covering will be installed?
- What minimum slope and lap pattern does the manufacturer require?
- Will the underlayment be exposed before roofing?
- Are high-temperature or self-adhered products required?
- How are eaves, rakes, valleys, walls and penetrations integrated?
Common mistakes
- Using horizontal area without a pitch multiplier.
- Using nominal roll area instead of effective coverage.
- Combining general underlayment and ice-barrier quantities.
- Ignoring incompatible sealants or membranes.
- Leaving underlayment exposed beyond its rating.
Sources and professional limits
DOE Building Science Education resources emphasize integrating roof underlayment, flashing and cladding as water-control layers. Product instructions and local requirements control the final assembly.