Step flashing is a thin sheet of metal, generally copper, aluminum or galvanized steel, that is sometimes available in pre-cut pieces and other times seen in sheets or rolls. Metal step flashing is generally purchased at a specific size or cut so that it is about 8 to 10 inches wide. Step flashing is bent at a right angle along the horizontal center. The length of the step flashing should be about 3 inches more than the length of exposed shingle. For example, if shingles have 5 inches exposed to the weather, each piece of flashing should be about 8 inches long.
Step flashing is installed on roofs where the roof gable meets a sidewall or where a roof gable meets another structure such as a chimney or skylight. These crevices are virtually impossible to cover with roofing materials like shingles or siding. So, these pieces of folded metal are inserted between layers of shingles on the roof and under housewrap and siding on the sidewall. They overlap with each other, with fasteners used only under the overlapping portion, so that water directed downward will have no opportunity to penetrate through this vulnerable portion of a roof.
Installing Step Flashing
Step flashing is installed while a roof is being shingled or after shingling is complete. Begin fastening flashing at the bottom of the roof edge, making the bottom edge of the first piece of flashing flush with the drip edge. Secure each piece of flashing using two fasteners, one in each upper corner. The first course of shingles is laid on top of the first piece of step flashing. The next step flashing overlaps so that the bottom edge of the second flashing is about one-half inch above the bottom of the next shingle. After the step flashing has been fastened along the entire length of the sidewall-roof intersection, housewrap and siding materials are placed on top of the flashing. So, once the entire roof is completed, very little, if any, of the step flashing is visible.
Potential Problems with Flashing
Step flashing can fail to keep water out of a structure as a result of poor installation techniques or other factors. Step flashing can become damaged, loose or rusted. Looseness generally results from physical damage or roof or building settling. If dissimilar metals are touching, corrosion will result. Sometimes, tar is used to repair a leak in flashing. However, the tar can actually trap water against the metal and speed up corrosion. Snow drifts, ice dams and collected melt may occur along the roof-wall intersection and may lead to leaks through even properly installed flashing and shingles.