According to the National Roofing Contractors Association Metal Roof Systems for underlayment applications:

Standard Slopes: 3:12 to 7:12-Minimum Code Requirements

Fontana VulcaSeal #30 is ICBO ES ER-5434 certified and UL approved. Other underlayments may be used as approved by local building officials. The underlayment shall completely cover the entire deck while lapping over hips, ridges and through valleys with a 2″ head lap (horizontal) and 6″ side lap (vertical). For metal panel roof systems, metal shingles and metal shingle panels, NRCA recommends a minimum of one layer of No. 30 asphalt-saturated felt applied horizontally in shingle fashion on roof decks having a slope of 4:12 (18 degrees) or more. For roof decks having slopes of 3:12 (14 degrees) up to 4:12 (18 degrees), a minimum of two layers of No. 30 asphalt- saturated underlayment should be applied horizontally in shingle fashion.

Fontana’s Optional Upgrades

Begin at the low point of the roof; install one layer of VulcaSeal coated membrane ASTM D2626-97b or D4601 to cover the entire wood deck at right angles with a minimum of a 4″ head lap (horizontal) and a 6″ side lap (vertical). Secure membrane with plastic cap nails at head lap 18″ on center, and 24″ throughout the field. Over 7:12 pitches, nail the head lap 12″ on center and 18″ throughout the field. Fasteners shall be of sufficient length to hold underlayment securely with a minimum No.12 gauge, 3/8″ diameter head (9.5mm) corrosion-resistant nail that is in compliance with code requirements. Adequate additional fastening should be considered in high wind areas. Hammer tacker staples should never be used under tile. Staples are not recommended as fasteners.