Question 1

How much underlayment do I need for my roof?

Accepted Answer

Match underlayment coverage to your total roof surface area. A standard roll of 15-lb asphalt felt covers 4 squares (400 sq ft), 30-lb felt covers 2 squares (200 sq ft), and synthetic underlayment covers up to 10 squares (1,000 sq ft) per roll. Add 10% to 15% for overlap and trim waste. For a 2,500 sq ft roof, you would need about 7 rolls of 15-lb felt or 3 rolls of synthetic.

Question 2

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

Accepted Answer

15-lb felt is lighter (about 8 lbs per square) and faster to install, suitable for most asphalt shingle roofs in moderate climates. 30-lb felt is heavier (16 lbs per square), tougher, and provides more weather protection while shingles are being installed. Use 30-lb under heavy shingles, in high-wind areas, or where the roof may be exposed for several days before shingles are installed.

Question 3

Should I use synthetic underlayment instead of felt?

Accepted Answer

Synthetic underlayment offers significant advantages: lighter weight, larger roll size (faster installation), water resistance up to 6 months of UV exposure, and superior tear strength. It costs more upfront ($150-$250 per roll vs $20-$30 for felt) but covers 2 to 5 times more area per roll. For new construction or a complete tear-off, synthetic is the modern choice. For small repairs, felt is still cost-effective.

Question 4

Where do I need to install ice and water shield?

Accepted Answer

IRC R905.1.2 requires ice and water shield in cold climates (where winter design temp is 20F / -7C or lower) along the eaves, extending from the edge to at least 24 inches inside the exterior wall line. Most roofers install a 3-foot strip along all eaves regardless of climate, plus around skylights, valleys, chimneys, and any roof penetration. In severe ice dam zones, install up to 6 feet from the eave.

Question 5

How many ice and water shield rolls do I need?

Accepted Answer

Standard ice and water shield rolls cover 200 sq ft, typically 3 ft wide x 67 ft long. Multiply your eave linear footage by the strip width (in feet) to get the total area, then divide by 200 to get rolls. For a home with 100 linear feet of eave and a 3 ft strip, you need 300 sq ft of coverage, or 2 rolls. Add 10% overlap.

Question 6

Can I install shingles directly without underlayment?

Accepted Answer

No. Building codes (IRC R905.2.7) and all shingle manufacturer warranties require underlayment beneath asphalt shingles. Underlayment provides a secondary water barrier if shingles fail, protects the wood deck from moisture, helps shingles lay flat, and serves as a temporary roof during installation. Skipping it voids your shingle warranty and likely fails inspection.

Type	Coverage / Roll	Cost / Roll	Best Use
15-lb Felt	400 sq ft (4 sq)	$20 - $30	Standard residential
30-lb Felt	200 sq ft (2 sq)	$30 - $45	Heavy shingles, low slope
Synthetic	1000 sq ft (10 sq)	$150 - $250	Modern reroofs, fast install
Peel & Stick	200 sq ft (2 sq)	$80 - $120	Low slope, high wind

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