Underlayment, Drip Edge, and Ice-and-Water Barrier

Why Sequencing Matters

Roofing layers work together as a drainage plane: each layer sheds water onto the layer below and away from the structure. Installing them in the wrong order—or skipping one—creates a path for water to travel backward under a layer and into the sheathing or framing. The correct sequence at the eave is: drip edge first, then ice-and-water barrier, then underlayment. At the rake, the sequence reverses: underlayment goes down first, then rake drip edge on top. Understanding why each step occurs when it does is just as important as performing it correctly.

Drip Edge

Drip edge is a pre-formed metal flashing—typically galvanized steel or aluminum—installed along the eaves and rakes of a roof. Its job is to direct water away from the fascia and into the gutter or away from the wall below. Two common profiles exist: the L-shaped eave drip edge and the T-shaped or hemmed rake drip edge; always follow the manufacturer's specification for the profile required at each location.

At the eave, drip edge is nailed to the decking before any membrane or underlayment is installed. This allows the ice-and-water barrier to lap over the top of the drip edge, so any water that works under the shingles drains over the metal and into the gutter rather than behind it.

At the rake, drip edge is installed after the underlayment. The underlayment tucks under the rake drip edge, so wind-driven rain that gets under the edge drip is caught by the underlayment rather than reaching bare sheathing.

When cutting drip edge at corners, use tin snips to miter the return so pieces nest tightly together. Any gap wider than 1/4 inch at a joint can admit water and debris—keep joints tight. Fasten drip edge with roofing nails or staples per the manufacturer's instructions, typically near the top edge at regular intervals.

Ice-and-Water Barrier

Ice-and-water barrier (also called self-adhering membrane or peel-and-stick) is a rubberized asphalt sheet with a self-sealing surface that bonds around fasteners. It protects against two specific hazards: ice damming at the eave and wind-driven rain at valleys and other vulnerable areas.

An ice dam forms when heat from the conditioned space melts snow on the roof; the meltwater runs down to the cold eave, refreezes, and backs up under shingles. Standard felt or synthetic underlayment is not waterproof—it will shed bulk water but will allow slow-moving backed-up water to wick through. Ice-and-water barrier is fully waterproof and seals around nails, so even if water backs up beneath it, it cannot penetrate to the sheathing.

Local building codes specify the minimum coverage distance up from the eave—commonly to a point at least 24 inches inside the interior wall line, but always verify the requirement in your jurisdiction's adopted code. Valleys are also a required location in most codes. Install barrier by removing the release liner in sections so the membrane does not curl or wrinkle. Press firmly from the center outward to eliminate air pockets.

Underlayment

Underlayment is the secondary weather barrier that covers the rest of the deck above the ice-and-water barrier. Two types are in common use:

• Felt underlayment (also called roofing felt or tar paper) is asphalt-saturated felt, available in weights commonly called #15 and #30. Heavier felt is more durable during installation.
• Synthetic underlayment is woven or laminated polypropylene or polyester. It is lighter, stronger, and more slip-resistant than felt, and it does not wrinkle when wet.

Underlayment is installed in horizontal courses, starting at the eave and working up toward the ridge. Each course overlaps the one below it—the horizontal (side) lap sheds bulk water downward. Courses also overlap each other at the ends—the vertical (end) lap—to prevent a straight path for wind-driven water. Mark lap lines with a chalk line before unrolling so you can verify correct overlap before fastening. After fastening, check every lap with a tape measure. Any short lap must be corrected before laying the next course.

Cap staples or roofing nails through plastic caps hold the underlayment flat and resist wind uplift. Space fasteners per the manufacturer's instructions.

Safety

Work on any slope above ground requires fall protection. Before stepping onto any roof section, confirm your personal fall-arrest system (PFAS)—harness, lanyard, and anchor—is rigged to a certified anchor point rated for fall arrest. Inspect the harness for cuts, worn stitching, and damaged hardware before each use; never use a harness that has arrested a fall.

Keep the deck clear of scrap membrane, paper backing, and cut metal to prevent slipping. Tin snips and utility knife blades are sharp—cut away from your body, and replace dull blades before they require extra force. When using a roofing nailer or cap stapler, keep hands clear of the muzzle, and engage the safety mechanism whenever you set the tool down. Wear safety glasses and gloves when cutting metal drip edge, as cut edges are razor-sharp.

The Installation Sequence at a Glance

1. Eave drip edge → nailed to bare decking

2. Ice-and-water barrier → laps over eave drip edge

3. Underlayment → laps over ice-and-water barrier, courses run up the slope

4. Rake drip edge → installed on top of underlayment at rake edges