Most chimneys have a two-part flashing system to ride out minor structural shifting. Base flashing fits under shingles along the sides of the chimney (called step flashing there) and "up roof" from the chimney; it lays on top of the shingles below the chimney. Counterflashing is applied over the base flashing and serves as a cap to keep water out.

It's top edge, bent into an L shape, is motared into the chimney's motar joints to hold it securely in place. Nowadays, there are chimney kits available at the local builder's supply store which only require a bead of roofer's cement at the top lip of the counterflashing.

Replacing chimney flashing is a task that calls for time and patience.

You can use copper, aluminum, or galvanized steel flashing (use copper for the counterflashing for a more durable seal). Flashing is available in sheet roll form that you can cut with tin snips; often step flashing is available in pre-bent forms.

You should build an A-frame cricket on the up-roof side of the chimney so water will not settle there and deteriorate the flashing.

Procedure for flashing a chimney.

1. Apply asphalt primer to bricks and install base flashing to the front, overlapping roof shingles 4 inches.
2. Nail step flashing over plastic cement. Bed overlapping roof shingles in addition to plastic cement.
3. Install a plywood cricket on the up-roof side and shingle to it's edge. Bed the rear corner flashings in cement.
4. Bed the rear base flashing that covers the cricket in plastic cement. Nail the flashing to the deck only.
5. Set the counterflashing on the front and sides into raked-out motar joints. Refill joints with new motar.
6. Make and install counterflashing suitable to the situation on the up-roof corners and side of the chimney.