Steep-Slope Roof Systems, Part 2
February 4, 2020
In addition to maintaining proper ventilation, proper application of flashings is integral to the installation of water-tight, steep-slope roof systems. There are a variety of flashing conditions to consider, and requirements vary among roof systems. Here are just a few examples.
Where the roof application involves flashings at chimneys, a metal “cricket” is typically required on the down-slope side of the chimney to properly drain water around the chimney. And each side of a masonry chimney should be flashed with sidewall flashing, followed by step flashing. The step flashing should be installed in specially fabricated pieces, with each piece of step flashing firmly set in stepped, “sawed-in” grooves. A lead wedge is then hammered in to hold the step flashing firmly in place. The top edge of the step flashing is sealed with elastomeric caulk or similar sealant.
The up-slope side of a masonry chimney is typically flashed with “apron flashing”. Apron flashing matches the roof slope with the flashing’s top edge being etched into the brick. The apron flashing extends down from the etch in the brick and drops vertically to the surface of the new roof system. Typically, the vertical extension of the apron flashing is 6 to 9 inches. The apron flashing is also fabricated to extend out approximately 6 inches onto the sloped roof system. And it is crimped at the leading edge so that the flashing will sit firmly on the roof surface. On chimneys with metal siding, stucco or similar material, the need for step flashing is eliminated. However, cricket, sidewall and apron flashings must be installed ahead of the siding material to establish a water-tight condition.
Other flashing conditions include prefabricated flashings for soil vent pipes and “valley flashing”, which addresses the conditions created when a gable condition intersects with another gable. The valley flashing is typically 24 inches wide, crimped in the middle of the flashing and 10 feet long per piece. Each side of the valley is crimped back 1/8” – 1/4” to prevent water from entering the roof system. The valley flashing is secured by “cleats”, which are nailed into the substrate. Never penetrate the valley with nails.
Another primary flashing condition is “eave flashing”. Eave flashing occurs, literally, at the eave, or leading edge of the roof. Installed after the underlayment, but before the roofing material, eave flashing is fabricated to match the roof slope. The up-slope “flange” of the eave flashing should extend a minimum of four inches under the roofing system. The bottom “leg” of the flashing extends vertically down to cover the edge of the underlayment substrate. If a gutter system is going to be installed, the bottom leg of the flashing should extend down the fascia so that it will be halfway down the back-side of the gutter. The eave flashing is given a 1/16” to 1/8” crimp to create a drip edge away from the fascia.
As to the flashing material, 26 oz. or 28 oz. copper is the preferable material and prefinished steel or stainless steel are also good choices. Aluminum is used on occasion. It is best to avoid mill finish galvanized steel.