Underlayment and Flashing — Roofing | License Contractor School

Underlayment

Underlayment is the secondary water barrier under the roof covering. Felt paper, often called 15-pound or 30-pound felt, is traditional. Synthetic underlayments are lighter and tougher. In ice-prone areas, an ice and water shield goes at eaves and valleys; California uses similar membranes at low slopes and chimneys.

Flashing

Flashing redirects water away from joints. Step flashing wraps around vertical surfaces like chimneys. Drip edge goes at eaves and rakes. Valley flashing goes under or over the shingles depending on the system. Most leaks come from flashing failures, not field shingles.

Penetrations

Pipe boots seal plumbing vents. Skylight curbs must be flashed per the manufacturer. Solar mounts need flashed standoffs that lift the mount above the roof plane. Every penetration is a potential leak; choose details that shed water rather than rely on sealant alone.