I'm rolling out #30 felt on a 6:12 shingle roof.

Lap Requirements (IRC R905.1.1 / ASTM D226):

• Horizontal (end) laps: Minimum 6 inches.
• Vertical (side) laps: Minimum 4 inches (6 inches is standard best practice).
• Eave overhang: Extend felt ¼"–¾" past the drip edge into the gutter.
• Rake edges: Extend felt to the outside edge of the drip edge.
• Ridge/hip: Lap minimum 12 inches over the ridge/hip from both sides.

Starting Point & Direction: Yes, it matters critically.

1. Start at the eave (bottom) and work upward toward the ridge.
2. Why: This creates a shingle-style lap where the upper course overlaps the lower course. Water shedding requires this orientation. Starting at the ridge traps water under the laps.
3. First course: Roll out parallel to the eave. Fasten top edge only (caps/nails 12"–24" O.C.) so the next course overlaps the fasteners.

Fastening (IRC R905.2.7):

• Use cap nails (plastic/metal caps) or tin caps with 11ga roofing nails.
• Pattern: 12" O.C. along edges and endlaps; 24" O.C. in the field (two staggered rows).
• Low-slope note: On 6:12, standard fastening is fine. If slope drops to 4:12 or below, double coverage (19" laps) and ice barrier are required (R905.1.1, R905.2.7.1).

Pro Tip: Snap horizontal chalk lines every 34"–36" (standard roll width) to keep courses straight. Keep felt tight and wrinkle-free; buckles telegraph through shingles.

For #30 asphalt-saturated felt underlayment on a 6:12 shingle roof, follow these guidelines:

Lap Requirements:

• Horizontal laps: Minimum 2 inches (per ASTM D226 and IRC R905.1.1). Overlap each course by at least 2" to ensure water shedding.
• Vertical (end) laps: Minimum 6 inches where rolls meet end-to-end. Stagger these joints by at least 24 inches between courses to avoid aligned seams.

Starting Point:

• Begin at the eaves (bottom edge) and work upward toward the ridge.
• This ensures each upper course laps over the lower one, directing water away from the roof deck.
• Do not start at the ridge or mid-slope — starting low and lapping uphill is critical for proper water resistance.

Additional Notes:

• Secure with corrosion-resistant fasteners (e.g., 1" galvanized roofing nails or staples) spaced adequately to resist wind uplift (follow manufacturer specs, typically 6–8" along edges and 12" in the field).
• In high-wind areas or per local code, consider using plastic caps or adhesive strips for better hold.
• #30 felt is acceptable under asphalt shingles per IRC R905.2.7, but synthetic underlayments are increasingly preferred for durability and UV exposure tolerance.

Bottom line: Start at the eaves, lap 2" horizontally and 6" vertically, stagger end joints, and work uphill. This is standard practice and code-compliant for most jurisdictions. If your local amendments differ (e.g., Miami-Dade, Florida), verify with the AHJ. Always prioritize manufacturer instructions if they exceed minimums.