The lower edge of a roof should extend beyond the fascia board by about 3/4 inch so that runoff drains into the gutters. If this slight overhang is too short, water will drip behind the gutters, leading to rotted roof sheathing and fascia, stained siding, and even soil erosion and a flooded basement. Fix the problem by installing sheet-metal drip edge, also called drip cap, which is available at lumberyards and home centers. Slip the wide, flat flange of the material all the way under the first course of roof shingles, as illustrated. Make sure the outer edge extends over the gutter. If it doesn't, pull the drip edge out slightly from under the shingles. Secure the drip edge with roofing cement. Apply a continuous bead of cement along the top of the drip edge, then press down on the shingle. If necessary, secure the ends of the drip edge with short roofing nails; just be sure they're positioned high up under the shingles.