How do I calculate the roof area for underlayment?
Calculate the base area (length × width) and adjust for roof pitch. Higher pitch roofs have more actual surface area than their base footprint. See also our calculate Metal Roof.
How much overlap should I use for roof underlayment?
Standard practice is 4-6 inches of side overlap and 6 inches of end overlap. Check your local building codes as requirements may vary.
What waste factor should I include when ordering underlayment?
Include 10-15% waste factor to account for cuts around roof features, mistakes, and material defects. Complex roofs may need higher waste factors.
Does roof pitch affect how much underlayment I need?
Yes, steeper roof pitches have more actual surface area than the base footprint. A 6/12 pitch roof has about 18% more area than its base dimensions.
What is the standard width of underlayment rolls?
Most synthetic underlayment comes in 3-foot or 4-foot wide rolls. Traditional felt underlayment is typically 3 feet wide.
How long are typical underlayment rolls?
Roll lengths vary by manufacturer and type. Synthetic underlayment typically comes in 250-foot rolls, while felt may be 144 or 216 feet. You might also find our Roof Shingle Calculator useful.