General Flashing Designs
Flashings around roof penetrations, as well as around the roof perimeter, are the most common source of roof troubles. Since the property owners have chosen to invest in metal to solve their roofing troubles, those areas must be carefully addressed.
Because, realistically, sealants will not last the life of a metal roof, they should not be depended upon for watertightness when it comes to roof flashings. This is not the case with standard shingles where installers might apply a 20-year sealant, knowing that it will likely last at least the life of the shingles themselves.
Many metal roof systems come with available pre-formed flashings to simplify and safeguard installation. Whenever appropriate, these flashings should be used. Just the same, there will be occasions when custom flashings must be made for individual applications. The number one rule to flashing design is to always lap uphill flashings on top of downhill flashings, and to use a lap of at least 6”. Next, always think about water and how it will travel. Make certain that you do not trap water. Keep in mind that, particularly on low slope roofs, roofs with long rafters, and roofs in areas subject to heavy rains or wind-driven rains, there might be very deep “rivers” of water running down the roof before the rainwater actually exits the roof. You must allow for this volume of water. Also, keep in mind that debris will settle on a roof so do not use “J”, “C”, or “U” channels to receive dimensional panels as those channels will collect roof debris such as pine needles, tree leaves, ice and snow and, once that debris collects, it will stop the flashing’s ability to carry water. If any flashing designs do allow for the chance that some small amounts of water (such as water that is blown sideways by wind) might be carried in channels beneath the metal roofing, make sure that that water is channeled back on top of the roof and is not forced beneath the roof panels or into the starter strip detail. Also, make sure that such flashings are held in place by clips and are not penetrated with fasteners.
Refer to the manufacturer and their detailed installation instructions as necessary.