How to Install Flash Around a Roof

Match the style of flashing to the location. Use wide metal flashing bent at 90-degree angles where a roof meets a wall or another roof, over the peak of a roof or in any roof valleys. Put one side of the flashing under the wall siding or shingles on the adjoining roof, and put the other side down the slope of the roof. Set peak flashing across the peak, with one side down each slope. Install these flashings before any roofing paper is put down.

Install valley flashing along the sloping junctions of two roof lines. Use 90-degree flashing, but install it upside down, with the bend down into the valley of the roof lines. Overlap flashing pieces from the top if more than one is required for the valley length. Put this down before roofing paper is placed.

Use drip edge or drip cap flashing on the eaves, where water drains off a roof slope, and along the rake edges, the angled sides of a gable roof. Put drip edge on eaves before roofing paper is laid down, so that underlayment will cover it, but over roofing paper on rake edges, to hold the paper in place if wind blows onto the shingled edge. Overlap pieces from the top down or in the direction of the prevailing wind on eaves.

Overlap rake and eave flashing at the corners and at a gable peak. Put the eave portion down first. Let the rake side overlap the end of the eave across the width of the flashing. Cut out the top of the rake side with tin snips, and bend the bottom section around the eave flashing to seal the corner. Put one side of a peak on first, and add the other side so it goes across the first piece. Cut it with tin snips to fit.

Nail flashings about every foot, using shingle nails driven with a hammer. Use galvanized steel or aluminum nails to match the flashing. Add a bead of roofing cement under the upper edges of flashings as extra protection. Cover nail heads with roofing cement to further seal the flashing.