Drip edge installation — eaves (under underlayment) vs rakes (over underlayment), IRC R905.2.8.5 requirements

Drip edge is the metal flashing at the roof's eaves and rakes (cross-reference KB-D2-003 for the component identification, profiles, and budget-bid omission signal). Proper drip edge installation is governed by IRC R905.2.8.5 and is one of the most-commonly-mistaken install details on residential asphalt roofs — this entry covers the directional install discipline that determines whether it works. The direction of underlayment lap relative to drip edge differs between eaves and rakes, and the reason matters for water management. At eaves: the drip edge is installed FIRST, directly on the roof deck along the eave edge. The underlayment is then laid OVER the drip edge, extending past the drip edge onto the deck. This orientation means water flowing DOWN the underlayment (the dominant water path on a sloped roof) reaches the lower edge of the underlayment, drains onto the metal drip edge surface, and is carried off the roof — never reaching the fascia or sheathing edge underneath. If the drip edge is installed OVER the underlayment at the eaves (the common installer mistake), water flowing down the underlayment reaches the metal drip edge, runs UNDER the drip edge, and can soak the deck edge or rot the fascia from inside. At rakes: the drip edge is installed LAST, OVER the underlayment along the rake edge. This orientation matters because at rakes, water flows ACROSS the roof surface (perpendicular to the dominant slope-driven flow) and the dominant water-management concern is sealing the horizontal path so wind-driven rain doesn't penetrate under the rake shingles. Installing rake drip edge under the underlayment would create a gap at the underlayment edge where wind-driven water could ingress. The directional rule is consistent across the 2021 and 2024 IRC cycles (cross-reference KB-D4-003 for the IRC code-cycle context). Common installer mistakes include: installing drip edge over the underlayment at both eaves and rakes (the most frequent error, driven by installer convenience — both orientations get the same metal applied last); omitting drip edge entirely (some older homes were built without drip edge at eaves, but current IRC requires it on new installs); using interior-grade trim metal as drip edge instead of corrosion-resistant aluminum, painted steel, or copper. Manufacturer warranty interaction: most major-brand enhanced warranties (cross-reference D9-004, KB-D4-001) require drip edge installation per IRC; deviations void the enhanced-warranty enforcement even when the base limited warranty technically remains. Texas / Houston relevance: drip edge at eaves is particularly important in the Houston market because wind-driven rain from coastal squalls and hurricanes drives water up the slope as well as across, increasing the likelihood that water reaches the eave edge underneath the underlayment if the drip-edge direction is wrong. Visual inspection from the ground can usually verify the drip edge IS installed; verifying the direction-of-lap relative to underlayment typically requires either inspection at install time or an attic inspection. [Source: International Residential Code R905.2.8.5 drip edge requirements (2021 and 2024 cycles); NRCA Roofing Manual asphalt shingle drip edge installation chapter; ASTM A653 standard for coated steel sheet (drip edge material); manufacturer install instructions specifying drip-edge accessory requirements for system warranty]