Roof Underlayment: The Hidden Layer
Roof underlayment is installed directly on the roof deck (plywood or OSB) before shingles. It provides a secondary weather barrier, protecting the roof deck from wind-driven rain that penetrates beneath the shingles. Building codes require underlayment on virtually all sloped roofs.
Synthetic Underlayment
Synthetic underlayment has largely replaced traditional felt in new construction. Made from woven or spun polypropylene, synthetic underlayment is lighter, stronger, and more resistant to tearing during installation. It lies flatter on the roof deck, does not wrinkle when wet, and provides better traction for workers.
Standard synthetic underlayment rolls cover approximately 1,000 square feet (10 squares) with typical dimensions of 4 feet wide by 250 feet long. Coverage varies by brand and roll size, so always check the manufacturer specification.
Asphalt Felt (Tar Paper)
Traditional #15 and #30 felt underlayment is still used on some projects, particularly re-roofs where code allows it. A roll of #15 felt covers about 400 square feet (4 squares) of roof area. #30 felt is heavier and more durable, covering about 200 square feet per roll. Felt is less expensive per square foot but tears more easily and wrinkles when wet.
Calculating Overlap
Underlayment rows must overlap to prevent water infiltration. Standard overlap is 4-6 inches horizontally (along the eave line) and 6 inches vertically (at roll end joints). This overlap reduces the effective coverage of each roll by approximately 10-15% compared to the total roll area.
The effective coverage formula is: nominal roll coverage × overlap factor. With a typical overlap factor of 0.85-0.90, a 1,000 sq ft roll effectively covers about 850-900 square feet.
Ridge Cap Shingles
Ridge cap shingles cover the peak (ridge) of the roof and any hip ridges. They are installed after all field shingles are in place, overlapping the top course on each side. Ridge cap shingles are different from field shingles — they are either pre-cut ridge cap pieces or regular 3-tab shingles cut into thirds.
Exposure Settings
Ridge cap exposure is the amount of each piece that is visible after the next piece overlaps it. Standard exposure is 5 inches for most asphalt ridge caps. Some premium products use 5-5/8 inch or 6-inch exposure. The exposure setting determines how many pieces you need per linear foot of ridge.
- 5-inch exposure: 2.4 pieces per linear foot of ridge
- 6-inch exposure: 2.0 pieces per linear foot of ridge
- 4-inch exposure: 3.0 pieces per linear foot of ridge (for extra coverage in high-wind areas)
Calculating Ridge Cap Quantities
Measure all ridge lines (the peak of the roof) and hip lines (diagonal ridges on hip roofs). Add these measurements together for total linear footage. Divide by the coverage per bundle (typically 33 linear feet per bundle at 5-inch exposure) and round up.
For a hip roof with 40 feet of main ridge and 2 hip ridges at 20 feet each (80 total linear feet): 80 / 33 = 2.4, rounded up to 3 bundles of ridge cap.
Ridge Vents
If your roof has a ridge vent (for attic ventilation), the ridge cap shingles install over the vent strip. Ridge vent length equals the main ridge length only — it does not extend along hip ridges. The ridge cap quantity calculation remains the same whether a ridge vent is present or not.
Ordering Checklist
- Underlayment rolls: roof area / effective roll coverage, rounded up
- Ridge cap bundles: total ridge + hip length / coverage per bundle, rounded up
- Starter strip: full eave perimeter, sold in rolls or bundles
- Ice and water shield: for eaves (first 3 feet) and valleys, where required by code
- Drip edge: full eave and rake perimeter in 10-foot sections
Calculate Your Materials
Use our underlayment calculator to determine the number of rolls needed for your roof area with overlap factor applied. For ridge cap quantities, use the ridge cap calculator. And for the field shingles themselves, start with the shingles calculator.