Underlayment and Drip Edge Installation

Why Sequencing Matters

Underlayment and drip edge are the first true water-management layers on any shingle roof. Install them in the wrong order and water driven by wind or ice can migrate under the shingles and into the structure — even if every shingle above looks perfect. This lesson covers the correct sequence, the products involved, and the fastening standards that protect the building for decades.

Drip Edge: Two Locations, Two Rules

Drip edge is a pre-formed metal flashing — typically aluminum — installed along roof edges to direct water into the gutter and away from the fascia. It appears in two locations, and each follows a different sequencing rule.

Eave drip edge runs along the low edge of the roof. Install it directly on the deck before any underlayment. The underlayment then laps over the top flange of the drip edge. This way, water shedding off the underlayment falls onto the drip edge and into the gutter rather than being trapped behind the metal.

Rake drip edge runs along the sloped side edges (gables). Install it after the underlayment is in place. The rake drip edge laps over the top edge of the underlayment. This keeps wind-driven rain from lifting the underlayment edge and channels water away from the rake board.

A simple memory rule: eave drip edge goes under; rake drip edge goes over.

Fasten drip edge with corrosion-resistant roofing nails spaced per the manufacturer's instructions and local code. Overlap drip edge end joints by at least 2 inches, keeping the overlap pointing downhill so water sheds cleanly.

Ice-and-Water Shield

Ice-and-water shield (sometimes called self-adhering underlayment or peel-and-stick membrane) is a rubberized asphalt sheet with a factory-applied adhesive backing. It self-seals around fasteners and bonds tightly to the deck, making it the only underlayment that resists water intrusion from ice dams and wind-driven rain at vulnerable spots.

Where it goes:

• Eave zone: Starting at the eave drip edge, apply ice-and-water shield up the slope to a point that extends at least the minimum distance required by the manufacturer and applicable building code past the interior wall line (the heated space below). In cold climates this zone is commonly 24 inches inside the exterior wall, but always verify local code requirements.
• Valleys: Center ice-and-water shield in the valley, running it the full length. Apply it before field underlayment so the field sheets lap onto the shield.

Application steps:

1. Confirm the deck is clean, dry, and free of protruding fasteners.

2. Peel back only a short section of release film at a time — the adhesive grabs immediately and repositioning is difficult.

3. Press firmly with a hand roller or the flat of your gloved hand, working from the center outward to eliminate air pockets.

4. Trim edges flush with the drip edge using a utility knife and straightedge.

5. At seams, overlap per the manufacturer's specification (commonly 6 inches for end laps) and press firmly.

Avoid fishmouths — bubbles or lifted edges at laps. A fishmouth creates a path for water to travel under the membrane. If one forms, slit it, apply roofing cement under both flaps, and press flat.

Synthetic Underlayment: Layout and Fastening

Synthetic underlayment (woven or non-woven polypropylene) has largely replaced traditional felt on new construction because it is lighter, stronger, and more resistant to tearing. However, the same principles of overlapping and fastening apply.

Snapping chalk lines: Begin by measuring up from the eave and snapping horizontal chalk lines across the deck. These lines guide each course so laps stay consistent. Laps vary by product, but a typical synthetic underlayment requires a side lap (horizontal overlap between courses) of at least 4 inches and an end lap (vertical overlap where rolls meet end-to-end) of at least 6 inches. Always follow the specific manufacturer's instructions — they override generic rules.

Fastening: Use roofing staples with plastic caps, or roofing nails with large-head caps. Cap fasteners spread the load and prevent the underlayment from tearing through in wind. Space fasteners per the manufacturer's schedule, typically along the top edge of each course and through the lap zone. On steep slopes, add fasteners in the field of the sheet to prevent slippage before shingles are applied.

Work uphill: Always start at the eave and work toward the ridge so each upper course laps over the course below, just like shingles. Never lap a lower course over an upper course.

Safety on the Roof Deck

Underlayment is slippery — especially synthetic — and becomes extremely hazardous when wet or frost-covered. Follow these non-negotiable rules:

• Wear non-slip footwear with clean, rubber-soled soles at all times on the deck.
• Fall protection is required on any roof where you could fall 6 feet or more. Use a personal fall arrest system (PFAS) or ensure guardrails or safety nets are in place per OSHA fall protection standards.
• Inspect the deck before stepping onto it — soft spots, protruding nails, or wet wood create slip and fall hazards.
• Never stand on ice-and-water shield in direct sun immediately after application; the surface can be slick until the material fully sets.
• Keep the work area clear of roll ends, packaging, and tools that could cause trips.
• Hard hat, safety glasses, and gloves are required on the mock-roof section during all activities.

Checking Your Work

Before calling the instructor for final inspection, run through these self-checks:

1. Eave drip edge is under the underlayment; rake drip edge is over it.

2. All lap dimensions meet manufacturer specs — measure with a tape, not by eye.

3. Ice-and-water shield covers the full eave zone and valley with no lifted edges or fishmouths.

4. All cap fasteners are fully seated with no torn underlayment around them.

Passing inspection on all four criteria is the standard for advancing in this module.