Drip edges are used for watershedding at the eaves and rakes and for preventing wood materials from rotting. It is important that the drip edge is “made of a corrosive-resistant material that extends approximately three inches back from the roof edges and bends downward over them.” (ARMA Residential Asphalt Roofing Manual)
The drip edge should be applied beneath the underlayment or eave protection along the eaves and over the underlayment on the rakes.