In all roofing systems, installation of flashings and penetrations are among the most critical points of application. These areas typically account for nearly 80 percent of all reported roof leaks.
Flashings and penetrations are particularly critical on low-slope, standing-seam metal-roof systems because the metal serves primarily as a watershed. The water-shedding capacity is functionally similar to roof shingles - it relies on a steep slope to rapidly release the water. In contrast to waterproof roofs, which are intended to function under occasional standing water, metal roofs are not designed to be completely leak-free under long-term water immersion. Therefore, waterproofing must be provided at critical junctures, such as flashings and penetrations.
Proper roof design on low-slope, standing-seam metal roofs should limit the amount of pipes, ducts and openings for rooftop-mounted equipment. Every roof penetration or opening results in cuts in the field of the metal panels. These cuts can restrict temperature expansion, expose the metal to corrosion and lead to leaks. When eliminating these types of penetrations is not feasible, the penetrations and openings should be carefully detailed to allow for both panel movement and elimination of moisture intrusion.
Architectural panelsIn architectural panels with underlayments, it is critical that all penetrations through the felt are detailed as extensively as penetrations through the metal. These are additional points where moisture can enter into the facility. In addition, the manufacturer's criteria should always be reviewed prior to any application procedures.
A typical pipe penetration of a metal panel in the field is completed with a flexible bellow-type boot. The boot should be set over the penetration and installed onto the metal surface. Sealant should be applied between the metal-panel surface and the metal flange of the pipe boot before fastening the flange to the surface. This application detail is acceptable on all penetrations that are located within the flat part of the panel and can be completed on new and remedial roofing applications.
In situations where the existing penetration is in place and the metal panels cannot be set over it, the new panels may lap around the penetration and a specially designed retrofit pipe boot should be installed over the existing penetration. The pipe-boot installation procedures are similar to the other details: Sealant is applied between the boot flange and the metal surface prior to fastener attachment.
In remedial construction installations, penetrations may occur at or near metal ribs. At these locations, factory-fabricated, double-wide metal panels are seamed into the adjacent metal panels. The new seamed metal panel is field notched at the location of the penetration and end lapped. The lower part of the seamed panel must also be field notched to secure around the rib of the adjacent lower panel. Additional structural supports are required to span between the purlins on each side of the penetration. Pipe-boot installation is similar to typical penetration details.
Penetrations larger than 18 inches in diameter require special installation procedures. These penetrations are sealed with a two-piece collar. The sides of the collar are attached to a flashing that is seamed into the panels. Additional structural supports are required to span the purlins at each side of the penetration. In all applications where new panels are fastened over the existing panels, proper waterproofing of the seams is required. This can be accomplished with specially formulated adhesives and membranes, a self-adhered membrane, or any other method deemed acceptable by the metal-system manufacturer.
Curb installationsCurb-installation details are similar to large penetrations. Factory-fabricated panels are seamed into the adjacent panels, field notched at the curb penetration and end lapped. The lower part of the panel is field notched to secure around the ribs of the lower adjacent panel and is sealed over the backup flashing. Additional structural supports are required to span between each side of the curb penetrations.
Most metal-roof systems are installed on rectangular stand-alone buildings. Typical flashings occur at the ridge vent, gable and eaves. The manufacturer's specific requirements should be followed at these locations. Flashing installation on metal roofs are extremely important because of the amount of movement that occurs in these areas. Most flashing details include spacer blocks or continuous cleats to accommodate the movement. Butyl tape or sealant is typically applied in these areas.
If properly completed, the typical flashing areas can provide long-term waterproofing protection and trouble at these points will be minimal. Atypical flashing applications at adjoining walls create more concern. Problems at these types of transitions occur because of the differential movement created by the joining of different types of materials and because they are rarely completed by most contractors.
Roof to wallRoof-to-wall flashings can be designed and applied to accommodate movement. Roof-to-wall movement joints are designed to allow for thermal movement that is independent of the structural wall. Metal for flashings is made independent of the metal-roof surface and the wall panel. The flashing is applied over the metal - extending approximately 4 inches over the surface - and is secured in accordance with the manufacturer's required fastening pattern.
The flashing is fabricated with a continuous cleat at the midpoint to allow for thermal movement and the subsequent expansion and contraction. The flashing metal is set behind the wall panel and secured to the wall substrate with an appropriate fastener in accordance with the manufacturer's instructions.
Waterproofing of the flashing-to-metal transition can be accomplished with specially formulated adhesives and membranes, self-adhered membranes or any other method deemed acceptable by the manufacturer.
When metal roofs are properly completed, they can become profitable projects. A number of roofing contractors with full sheet metal departments are increasing their presence in this market. The most critical detail and application procedures are at flashings and penetrations. Proper procedures must be followed for the success of the system and an opportunity to add to the bottom line.
(John A. D'Annunzio is president of Paragon Roofing Technology Inc., a consulting firm in Shelby Township, Mich. For more information, call (800) 829-3269; or visit www.paragonroofingtech.com on the Internet. This article originally appeared in Roofing Contractor, a sister publication of Snips.)