Metal flashing helps seal out water from many critical spots on a home's roof, including valleys, around chimneys and vent pipes, beside dormer walls, and along the rakes and eaves. The roof illustrated here has an open valley made from a continuous piece of metal flashing. Note how the roof shingles are cut back from the valley centerline, leaving the flashing visible.

Valleys require particularly sturdy flashing because they carry more water than any individual roof plane. On most roofs, metal valley flashing is installed after the roofing felt liner and before the primary underlayment and finish roofing, as shown below.

On some asphalt-shingle roofs, shingles are woven across the valleys, eliminating the need for metal flashing. This is generally the best way to handle valleys between roof planes of different pitches. Another fairly common technique for asphalt-shingle roofs is to flash valleys with roll roofing that's the same color as the shingles. To do this, first nail an 18" wide strip along the valley, with the finished surface down; set nails 1" from the edges and 12" apart. Then roll out a 36" wide strip, finished side up. Center it over the first strip and nail it down.

Toinstall metal valley flashing for an asphalt-shingle roof, first roll out a length of 15-pound roofing felt cut to the length of the valley. Push it snugly into the valley, then nail it every 2-4' along the outside edges. If you need more than one length of flashing to cover the valley, start at the bottom and overlap the first piece with the second one by at least 6".

Drip edge flashing helps keep water from wicking back under the shingles (see below). In climates where ice dams occur, flash the eaves with a special rubber ice-shield membrane or roll roofing. On a cedar shingle or shake roof, use 30-pound felt for this task.

At eaves, nail preformed metal drip edge in place before applying the roofing felt. After the felt is down, nail drip edge along the rakes.

Chimney flashing consists of several parts as shown in the illustrations below.   If the roof is particularly steep or if the chimney is more than 2' wide, build a "cricket" to prevent water and snow from collecting behind the chimney. Cut and install two plywood triangles to form the cricket.Then cover the plywood with a custom-bent saddle flashing. Protect the saddle with cap flashing.
Cap flashing extends down from above to prevent water from seeping behind flashing installed below it. It's typically used all around a chimney to protect base flashing, step flashing, and cricket. Cap flashing is usually installed in narrow slots cut into the mortar joints, but it can also be caulked to the chimney.

Dormer walls are flashed with step flashing placed between each course of roof shingles.Ideally, the vertical flange of step flashing should be slipped under the siding. If that's not possible, caulk the flashing to the siding.