Figuring out where to place your flashing is the last step in a roof installation. A roof's design is meant to waterproof the home, and before you get to the flashing material, you will lay down several layers of material which keep the roof sealed and dry. The first layer is underlayment of 'felt', installed from the bottom of the roof to the top, the pieces layered over each other for maximum protection. The next layer is the shingling, installed in the same manner. The flashing is the last step, covering all the intersecting areas of the roof and the corners where the roof meets other sections of the house.
Places for Flashing
Several types of flashing are needed for an effective roof. The first is valley flashing. This is a flashing used at sections where a part of the roof meets another section at an ascending angle. The flashing is installed to direct water flow from collecting in the 'valley' between the two slopes. Step flashing is installed around the base of a chimney. Vent pipe flashing is installed over pipes or flutes which crawl along the roof. This flashing is usually covered with shingles for the best waterproofing protection. Drip edges run along eaves and rakes on the roof to draw water off the roof's edge, keeping it from seeping under the shingles. In all cases, the top of the flashing attaches to the roof under the underlayment, and the bottom overlaps the roofing material, so water is directed away from the underlying structure of the roof. Intermediate pieces of flashing overlap in the same manner. In two-part flashing, a second piece of flashing overlaps the base flashing but is not fastened to it, so the pieces can expand and contract independently under changing weather conditions. With good flashing you won't have to worry about water getting under the shingles and leaking through the roof.
Corner flashing is used when you have a tight corner which must be protected from the elements. There are two ways to deal with this particular need. You could take two pieces of metal, bend them to the shape of the corner, and caulk the two pieces together. This is not the best solution, as caulk can decay over time. The second method is to measure the corner that requires flashing, and get a custom made piece that is bent and soldered according to your specifications. Attempting to do a piece yourself can prove difficult, as aluminum flashing cannot be soldered, and soldering galvanized steel is time consuming. Buying a custom piece from a hardware store will normally cost less than $50.