The following is taken from SMACNA's Architectural Sheet Metal Manual, fifth edition.

Careful consideration must be given to flashing systems where a roof joins a wall. The base flashing system must keep water from entering the building and must be designed to provide for building movement. Counter flashing serves to turn water from a wall onto the roof or base flashing.

Metal counter flashing should be used in conjunction with composition base flashings. Composition base flashing should be applied to according to the roofing manufacturer's specifications.

It is recommended that base flashings be applied over a cant and be extended up the wall a minimum of 10 inches (254 millimeters) above the roof line. Metal counter flashing is installed so that a minimum of four inches (102 millimeters) of the base flashing is covered. Metal base flashing is used with shingle or metal roofs. Metal base flashing is not recommended for use with membrane roofing systems. A metal base flashing may be used over a composition flashing as a protective cover in locations where the base flashing may be abused by traffic. Joints in flashing should be lapped four inches.

Counter flashing that is removable is cost-effective for work installation sequence and for repair of roofing systems. All membrane roofing should have removable counter flashing.

All counter-flashing receivers should be elevated 10 inches above the finished roof. The lower edge of metal counter flashing should be 1-inch minimum above a cant. All reglets must be capable of supporting flashing. In high-wind areas, clips can be specified for the lower edge of the counter flashing. These would be visible on the edge.

Figure 4-3A illustrates the installation of a complete metal counter-flashing system using a metal-flashing receiver. The counter flashing is notched and lapped at inside corners and joints, and seamed at outside corners. The flashing receiver is notched and lapped 4 inches at corners and joints.

After the counter flashing is installed, the receiver is bent at a 45-degree angle to provide a drip edge. This type of counter flashing may be removed with comparative ease when roofing is replaced.

Figure 4-3B shows an alternative receiver that is set as the wall is built. The counter flashing is easily inserted into a spring lock condition as shown in Detail 1.

Figures 4-3C and D illustrate other alternatives for using two-piece counter flashings on new or existing construction. Figure 4-3C shows a snap-lock receiver. Figure 4-3D shows a pocket receiver, through which fasteners are installed at 24-inch (610 millimeter) maximum spacing after the counter flashing is inserted.

Figure 4-3E shows a method of installing a counter flashing in an existing masonry wall. A reglet is cut in the masonry joint to a depth of at least 1 1/2-inches (38 millimeters). The counter flashing is inserted into the reglet and held in place by spring action. See Detail 1. The reglet is then filled with a sealant. The counter flashing is notched and lapped at corners and joints.

The recommended gauge for counter flashing is 16-ounce (.55 millimeter) copper, 26-gauge (.5512 millimeter) galvanized steel or 26-gauge (.477 millimeter) stainless steel. Flashing receivers should be (made) of 16-ounce copper, 26-gauge galvanized steel or 28-gauge (.396 millimeter) stainless steel.

(For information on ordering SMACNA's Architectural Sheet Metal Manual, write 4201 Lafayette Center Drive, Chantilly, VA 20151-1209; call (703) 803-2980; fax (703) 803-3732; see www.smacna.org on the Internet.)