Repairing Leaks in Chimney and Dormer Flashing

The watertight material used around chimneys, dormers, and other angled joints in a roof is known as flashing. The flashing is made up of two parts referred to as the base piece and the cap piece. The bottom edge of the cap flashing overlaps the top edge of the base flashing to prevent water leakage. The top edge is grouted into the mortar joints of the chimney to keep rainwater or melting snow from seeping into this area. Dormers have siding that actually performs the work of the cap flashing and the base flashing is placed beneath the siding.

In the case of shingled roofs, the base flashing is actually made up of many sections of step flashing, one section placed under each course of shingles. This method enables any leakage to be routed directly onto the section of flashing below it, and so on, until the water reaches the gutter or falls from the roof to the ground.

There are however, two particular areas where flashing has a tendency to leak. The first is the area where the cap flashing is placed into the chimney mortar, and the other is the joints between the sections of step flashing under the shingled roof. When gaps appear in these areas, fasten the cap flashing with small metal wedges into the mortar joint. Folded sheet metal or lead works well for this purpose. Once the cap flashing has been secured, re-mortar the joint or caulk the mortar joint with quality outdoor-rated caulk. Any vertical gaps located in the step flashing should be caulked with a quality roofing cement product.

There may be areas of the roof where the flashing was incorrectly installed. Rather than having step sections of flashing, one large section of metal flashing may have been installed instead. This may have been done for expedience, but even one corroded hole can quickly compromise the entire section. When working with these situations, tarring the gaps in the flashing is a quick fix to keep water out, but the best course is to remove the old material and replace it with properly installed step sections.

Keep in mind when working with flashing that many metals have an adverse effect on each other. Therefore, when nailing aluminum flashing, be sure to use aluminum nails, copper with copper nails, and so on.

When properly installed, flashing will protect the roof and the home from the nuisance of damaging water leaks.

Remember, always work safely.