How to Flash a Chimney on a Ribbed Roof

Installing the Base Flashing

• 1

  Cut a piece of flashing 4 inches wide and 8 inches in length with tin snips.

• 2

  Measure 4 inches from either end of the flashing to the opposite end of the flashing with a measuring tape, then draw a line at the 4-inch mark across the width of the flashing.

• 3

  Position the 4-inch mark over the edge of a block of wood and bend the flashing piece to a 90-degree angle.

• 4

  Apply roofing cement to the back and to the underside of the flashing.

• 5

  Press the flashing into the roof and one end of the chimney.

• 6

  Install the adjoining flashing piece in a similar manner, but overlap this piece onto the previously installed piece by 2 inches.

• 7

  Install the remaining flashing pieces with a 2-inch overlap until flashing covers the length of the chimney.

Installing the Counterflashing

• 8

  Measure 4 inches up the chimney from the point at which the roof meets the chimney, using a measuring tape.

• 9

  Note the mortar seam at or just above the 4 inch point.

• 10

  Cut a 1 1/2-inch groove into the entire length of the mortar seam with a circular saw equipped with a masonry blade.

• 11

  Cut a piece of counterflashing 5 inches wide and 5 1/2-inches in length with tin snips. Although the step flashing is 4 inches wide and the counterflashing is 5 inches wide, the extra inch will ensure that the edge of the step flashing is covered by the counterflashing.

• 12

  Measure 1 1/2-inches from either end of the counterflashing to the opposite end of the counterflashing, then draw a line at the 1 1/2-inch mark across the width of the counterflashing.

• 13

  Position the 1 1/2-inch mark over the edge of a block of wood and bend the counterflashing piece to a 90-degree angle.

• 14

  Insert the counterflashing into the groove within the chimney.

• 15

  Apply premixed mortar across the groove within the chimney, using a caulking gun to set the counterflashing in place.

• 16

  Install the remaining counterflashing pieces, using the same method, until the counterflashing extends across the entire width of the chimney.