One of the common places for a roof leak to occur is at the point a building element, such as a chimney, extends through the shingle roof. A waterproof seal must occur between the chimney and the shingles to prevent moisture from entering the attic and ultimately the living space. Water leaking through the shingles at the chimney can cause damage to the ceiling drywall in the home or lead to mold problems in the attic. A properly installed metal flashing helps provide a waterproof fit for the shingles.
Things You'll Need
- Galvanized metal flashing, 12 inches wide 26 gauge
- Roofing tar
- Caulking gun
- Roofing nails
Order or make metal flashing. The flashing is a galvanized metal bent in an L-shape. One side of the L extends up the chimney while the other side lies along the roof under the shingles. Bend the metal using a wood board as form if you're forming the flashing yourself.
Fasten the upper portion of the L-shaped flashing to the chimney using roofing cement. Nail the bottom pieces of the flashing to the roof with roofing nails. Use four pieces of flashing to go all the way around the chimney.
Place shingles up to the corner of the flashing proceeding up the roof in the normal manner. Cut the shingles so the edges are about 1/2 inch from the corner of the flashing. Do not nail the shingles through the flashing but instead fasten in place using roofing tar.
Seal all the open edges of the flashing with roofing tar. Roofing tar is commonly available in tubes similar to caulk. Use the caulking gun to squeeze out beads of roofing tar to fill any gaps between the flashing and the chimney and between pieces of flashing at the corners of the chimney. Seal the top edge of the shingles as they lay on top of the flashing.
Tips & Warnings
- Remember, water flows downhill. Seal any areas where flowing water could get behind or under the flashing or under the shingle. Check the quality of the roofing tar every few years to maintain a perfect seal between the shingles and the flashing and the flashing and the chimney.
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