I have been using metal flashing on several buildings. To account for construction tolerances, I use two-piece flashing with a bottom section and counter flashing. I typically splice the bottom section by overlapping the two sections 4 to 6 inches. I use butyl sealant in the overlap to create a watertight seal. There are many cases, however, where the flashing materials do not nest properly and pop open before the brick are laid. How can I prevent this from happening?
One good way to splice the lower section of two-piece sheet-metal flashing is to use a splice cover. The two lengths of flashing that meet at the splice should be installed with a 1/4-inch gap between them. I use a section of self-adhered Ethylene Propylene Diene Monomer (EPDM) or rubberized-asphalt membrane to seal this gap by fully bonding the membrane to the metal on both sides of the joint. Whenever a membrane is used, I recommend forming a 45-degree angle at the rear corner of the metal flashing to soften the transition between horizontal and vertical. It is far easier to seal a membrane to the sheet metal when the transition is from horizontal to a 45-degree angle than when it is from horizontal to vertical. In the latter case, a void usually is created in the back corner, which can allow leakage.After installing the membrane splice, I would use a metal splice cover that is longer than the membrane splice and is set in butyl sealant, where it extends beyond the membrane splice on both ends. The splice cover should nest neatly into the flashing and be crimped at the top and the drip edge to engage the piece with the two sections of flashing at the splice, as shown in the figure below.
This splice cover not only protects the membrane splice, but also provides a second seal, which greatly increases the likelihood of developing a lasting, watertight seal.