A standard detail on a steep-slope residential roof application involves flashing at masonry chimneys. Proper detail design is required at this critical component to provide waterproofing protection. 

Masonry chimney backer flashing for a chimney or other penetration 24 inches or less in width. Detail courtesy of the Western States Roofing Contractors Association.


Residential Chimney Flashing

A standard detail on a steep-slope residential roof application involves flashing at masonry chimneys. Proper detail design is required at this critical component to provide waterproofing protection. The detail shown here depicts a chimney on a steep-slope shingled roof. There are three different types of flashing details that can be completed to accommodate this design:

1. Apron flashing: A metal apron flashing should be secured around the perimeter of the chimney and over the roof deck. The Apron flashing shall be fabricated from a minimum 26-gauge pre-fabricated galvanized steel, 16-ounce copper or an equivalent non-corrosive metal. The dimensions of the metal shall be based on the size of the chimney and the slope of the roof. The apron flashing shall extend approximately 6 inches up the chimney and a minimum of eighteen 18 inches onto the roof deck.

2. Step flashing: Apply step flashings to the vertical masonry chimney. The step flashings shall only be secured to the vertical masonry and not at the roof deck to allow for expansion and contraction.

3. Counterflashing: In the scenario where a roof flashing material is applied at the perimeter of the chimney, a metal counterflashing shall be secured over the completed flashings to serve as a proper termination.

Cricket Flashing

This detail shows a chimney less than 24 inches in width without saddle or cricket flashing at the back end (up slope) of the chimney to promote positive drainage. Saddle or cricket flashing is optional with chimneys or penetrations less than 24 inches in width, but many experts recommend including it with chimneys of any size. Saddle or cricket flashing is definitely recommended with chimneys and other penetrations equal or greater than 24 inches or more in width. Crickets shall be fabricated at the back end (up slope) of the apron flashing to divert water and prevent it from ponding at the back end of the chimney area. The cricket flashing shall be fabricated from the same metal material used for the apron flashing.