Prime spots for water seepage on roofs for exterior walls are where surfaces intersect. These include where dormer walls and the roof join, around skylights and chimneys, and in roof valleys. Also vulnerable are surfaces where water runoff can be heavy, such as roof valleys and eaves. Flashing provides the extra protection that these spots need.
Most flashing is made of galvanized metal; if you install flashing yourself, you may want to work with aluminum flashing because it is easy to bend.
Step flashing protects the joints between the roof and dormer walls and chimneys. It fits to each course of shingles and looks as if it is stepping up a wall or other surface. Though skylights sometimes have step flashing up the sides, they generally come with their own, integral flashing.
Valley flashing protects the valleys where two roof planes meet. This W-shaped channel is placed over the top of building felt before the roof's finishing material is installed.
Vent pipe flashing fits over flues and pipes. It is cone-shaped with a flange at the base, which is worked into the shingles as the roofing is applied.
Drip edges are strips that run along the roof's eaves and rakes to prevent water from seeping under the roofing. Two similar metal strips used for flashing walls are Z-flashing and drip caps. Z-flashing seals the horizontal seams between plywood or hardboard siding panels; drip caps are L- or Z-shaped flashing strips that seal frames above doors and windows.
Flashing materials
You can find flashing made of plastic, roofing felt, and rubber, but your best bet is rust-resistant metal, either galvanized steel, aluminum, or copper.
