How To Repair a Leaky Roof

Metal flashing is used to seal out water around the chimney, at vent pipes, along the valleys where two roof pitches meet, and sometimes over exposed windows. To prevent leaks at the flashing, inspect it every spring. If you see thin spots or gaps along a flashing joint, spread roof cement over the entire joint, applying it generously with a trowel. The flashing edge should be covered completely.

At the chimney, examine the flashing carefully. Chimney flashing is installed in two parts: the base, which covers the bottom of the chimney and extends onto the roof; and the cap, which is mortared into the chimney bricks. If the mortar holding the cap flashing is crumbling or if the flashing has pulled loose, you'll have to resecure the flashing.

To resecure the flashing around a chimney:

Step 1: Pull lip of cap flashing out of mortar joint, only as far as it comes easily. Do not yank entire flashing out or pull it completely away from chimney. The less you have to separate it, the easier it will be to fix.

­Step 2: With flashing out of mortar joint, clean out old mortar with hammer and chisel, wearing safety goggles to protect your eyes. Then, being careful not to damage flashing, use wire brush on joint to clean out debris.

Step 3: Wet joint with paintbrush dipped in water. With small trowel, fill joint firmly with cement mortar.

Step 4: When joint is full, press lip of flashing into mortar, in same position it was in before. Press flashing in firmly, but don't push too far or it may pop back out and you'll have to start all over again. Let mortar dry as directed.

Step 5: When joint is completely cured, caulk around joint and over lip of cap flashing with butyl rubber caulk.

At vent pipes or metal chimneys, make sure the joint at the base of the pipe or chimney is sealed. If you can see gaps at the roof line, caulk around the base of the pipe or chimney with roof cement in a caulking gun. Vent pipes on pitched roofs usually have a protective collar; if the collar is loose, tap it back into place, and then caulk the collar base joint with roof caulk.

Learn even more about flashing -- specifically the type used around a roof's vent pipes -- in the next section.