Drip edge

Follow a single drop of rain that lands near the ridge, the uppermost line of a sloped roof. It runs down the shingles, lap over lap, toward the eave, the horizontal lower edge where the roof stops. Unless it lands in a valley, the internal angle where two roof planes meet and drainage gathers, its route to the edge is a straight run down one plane.

At the eave the drop has to leave the roof, and a drip edge is the piece set at that edge to send it clear. ARMA's Residential Asphalt Roofing Manual defines it as “A corrosion-resistant, non-staining material used along the eaves and rakes to allow water run-off to drip clear of underlying construction.” The rake is the inclined edge of the roof over a wall, so the same strip can run up the sloped sides of a roof as well as along the bottom.

The bottom lip of the strip is where the design does its work. ARMA's drip edge bulletin says a drip edge should have “an outward bent flange at the lowest edge”, occasionally called a kickout, so that water is directed away from running directly onto the supporting wood. The drop leaves the kickout and falls clear, rather than tracking back along the underside of the edge.

Tile roofs have the same piece under another manual. The FRSA-TRI Florida High Wind Concrete and Clay Tile Installation Manual defines drip edge as a non-corrosive material, usually galvanized steel sheet metal, stainless steel, copper or aluminum, “used along the roof perimeter to allow water run-off to drip clear of the underlying construction.” The two definitions differ in where they place the strip, eaves and rakes in one and the perimeter in the other, and agree on its job.

ARMA's bulletin, Installation of Drip Edge at Eaves and Rakes, gives the reason in a short chain. Drip edge at the eaves, gables and rakes is a common way to “help mitigate the potential rotting of wood materials that can result from water getting underneath the edge of the shingles.” It then names how that water gets there: “This can happen if water flows back onto a roof deck or fascia.”

Two wood parts are named in that sentence: the roof deck the shingles lie on, and the fascia, the board that covers the rafter or truss ends at the eave. ARMA's manual describes the drip edge bending downward over the fascia.

The result ARMA's bulletin names here is rotting wood. Which paths to a leak the roofing sources do name, and whether a cleaning opens any of them, is taken up in will roof cleaning make my roof leak, and it is not argued here.

ARMA's recommendation is plain, and it is written for one covering: “A drip edge is strongly recommended on all shingled roofs and may be required by local building codes.” Strongly recommended is the association speaking. May be required points somewhere else, to the local building code, as the place an actual requirement would come from.

That leaves two questions no glossary answers for a particular house: whether a roof has a drip edge with a kickout at all, and whether the local building code calls for one on it. Both belong to a roofing contractor and the local building code. The Florida tile manual defines the piece for tile roofs, but ARMA's strong recommendation is written for shingled roofs and is not a statement about tile.

A cleaner is not the person to settle either question, and what a clean roof does and does not make easier to see is the subject of whether a clean roof makes damage easier to see.