How to Flash a Fireplace Chimney

Pry the roof shingles that adjoin the chimney away from the roof. Each shingle is secured to the roof with nails along the top edge. The shingle will most likely also have a bead of roof caulking on the underside of the top edge. Slide a pry bar underneath the shingle, and lift up to detach it from the roof. Remove the shingle at the base of the chimney, and work upward until the entire edge of the chimney is exposed.

Grasp the bottom edge of the counterflashing, and bend it upward to expose the top of the base flashing.

Pry the vertical side of the base flashing away from the chimney, then pry the base flashing off the roof. Save the base flashing pieces to use as a guide when you cut the new flashing pieces.

Remove the mortar that secures the top of the counterflashing to the chimney seams. Chisel the mortar away from the flashing with a chisel and hammer, then slide the counterflashing out of the chimney. As with the base flashing, save the counterflashing pieces to use as a guide when you cut the new flashing pieces.

Cut new flashing pieces with aviation snips, using the old flashing pieces as templates.

Bend the new pieces to match the shapes of the old pieces. Place the flashing pieces on a flat surface with a straight edge, such as a sawhorse or a workbench, then bend the flashing over the edge of the work surface.

Apply roofing cement with a caulking gun along the base of the chimney and along the roof, where the roof joins the chimney, then press the base flashing pieces into the cement. Position the piece at the bottom of the chimney first, then work up the roofline. Overlap the base flashing pieces by about 1/2 inch. Drive rubber-gasket nails through the base flashing and into the roof deck to secure the base flashing in place.

Apply a bead of roofing cement along the top edges of the base flashing pieces.

Slide the counterflashing into the chimney seam, and press the vertical portion of the counterflashing into the roofing cement on the base flashing.

Fill the gaps in the chimney mortar by applying a bead of premixed mortar across the gaps with a caulking gun.

Apply a bead of roofing cement across the underside of the top edge of a new roof shingle. Position the shingle onto the roof and over the base flashing at the lowest point of the chimney, then nail the shingle into the roof deck with roofing nails. Install the remaining shingles, working up the roofline. You will not be able to drive nails into the uppermost shingle of the repair area due to the shingle located just above the repair area. Apply roofing cement to the back of the final shingle, and slide it between the roof and the shingle above it.