February 28, 2007
Metal Roof Seam ConfigurationMetal roofs have steadily gained popularity in the United States. One industry estimate indicates that nearly two-thirds of all new steep-slope nonresidential roofs are constructed of metal. The main reasons for this sharp increase in market share over the last decade are due to both the aesthetics of metal roofs and advances in equipment that have contributed to the ease of application. Aesthetically, metal roofs are available in a wide variety of finishes and profiles, affording the designer a plethora of color choices that can substantially enhance the look of the building while ensuring it blends in with its surroundings.
Application has been enhanced by improved on-site forming equipment. Until recently, most metal panels were pre-formed at the manufacturer’s facility prior to delivery to the project site. Portable roll forming machinery is now available, allowing many types of panels to be formed on the site. Seam forming equipment is also available, which allows seams to be constructed in an efficient manner with less manpower.
Classifications of Metal Roof Systems
There are several classifications of metal roof systems, each of which has a variance on the seam configuration. Metal roof systems are classified by their ability to resist water, the type of structural supports that are required and by the method of attachment to the supports.
Metal roofs are often classified by the way they resist water intrusion. Watershedding roofs act in similar fashion to roof shingles, relying on the steep slope of the roof to shed water. These types of systems are installed on roof slopes in excess of 3:12 pitch and are normally applied over an underlayment or base sheet. Water barrier roofs are designed for low-slope applications and can function under long-term standing water. Most industry experts agree that the minimum roof slope for this type of application is 1:12 pitch. However, some manufacturers allow application at slopes in excess of ¼ :12 pitch.
Regardless of the slope to which these systems are applied, waterproofing at valleys, eaves, rakes, ridges and penetrations is crucial.
Metal roof systems are often labeled as “architectural” or “structural” standing seam systems in reference to the type of structural supports that are required. Architectural metal roof systems must be applied over structural supports, such as decking or closely spaced furring channels. These types of applications are used in water-shedding or steep-slope applications as described above. Structural metal roof panels can span the distance between roof purlins without further assistance. These types of applications are considered water barriers and can be applied (depending on the manufacturer) on roof slopes in excess of ¼ :12 pitch. Structural metal roof systems are considered as a form of roof decking, and they are required to meet all wind uplift, live load and dead load weight requirements. The maximum deflection criteria should also be provided in the design documents.
Metal roof systems can also be classified by the method of attachment of the panels to the structural supports. Throughfastened roofs are attached directly to the structural purlins. Attachment is completed with screws or rivets. Standing-seam roofs are indirectly connected to the structure by concealed clips that are formed into the seams. All of these classifications require a different seam configuration and application procedures.
Types of Seam Configurations
There are four types of seam configurations that have been used for conventional metal roof systems. They are:
1. Flat seam.
2. Batten seam.
3. Lapped seam.
4. Standing seam.
Flat seams are formed by bending the sides of the two adjoining metal panels 180 degrees and hooking them together. These types of seams have become obsolete due to new seam forming technology, and they are rarely used. Batten seams are formed by bending the sides of the two adjoining panels and separating them with a wood batten strip. A snap-on cap is then installed over the batten strip. These types of seams were predominately used when the metal panels were hand formed, and their use has also decreased with the advent of new technology. Modern batten seams have eliminated the use of the wood strip and now use a metal batten design.
Lapped seams are typically formed on through-fastened metal roof systems. They are normally installed on industrial and warehouse facilities because of the ease of application and economics. In these configurations, the edges of the metal panels are overlapped and a bead of sealant is applied over the seam. The seams are then fastened to the structural roof purlins with exposed fasteners. Because the fasteners are exposed to the weather, proper selftapping screws with rubber or neoprene washers are required.
Fastener spacing is critical to achieving proper wind uplift criteria. The U.S. Army Corps of Engineers recommends maximum fastener spacing of 8 inches at end laps and at connections to intermediate supports and 12 inches at all side laps. The selected manufacturer’s specific fastening requirements should be followed. Also, it is important that the fasteners are driven to the proper depth for the selected washers to function properly. All roofing joints should be sealed and laid away from the prevailing winds.
Standing seams are the most technologically advanced of all metal seam configurations. Industry estimates indicate that standing seams are applied on approximately 60 percent of all current metal roof systems. They can be applied on both architectural and structural metal roof systems. The main advantage of standing seams is that once the metal panels are joined they become a monolithic membrane that can move as a unit with temperature changes. In standing seam configurations, the seams are elevated 2 to 3 inches above the metal surface and are formed together with a portable seaming machine. The metal panels are attached to purlins with concealed clips, which allow for movement to accommodate expansion and contraction. The clips consist of two pieces - a rigid base that is attached to the purlin and a moveable insert that is rolled into the seam. It is advantageous to have self-centering clips that allow for an equal amount of movement up and down the roof slope.
Standing seams can be formed on stainless steel and galvanized metal panels in a variety of panel widths. Structural systems usually have wider panels and longer spans than architectural panels, which are typically applied on steep slopes in shorter lengths and spans. In low-slope structural applications, joint sealant is required at the seams for waterproofing capacity. Architectural metal roof systems do not require joint sealant application.
Proper seam application is critical to the success of the metal roof system in both its structural and weatherproofing capacity. Seam forming application has improved with the advent of new technology and innovative equipment. Contractors now have the opportunity to provide building owners with metal systems that are aesthetically pleasing and have the capability to provide structural and weatherproofing integrity. These advancements have led to increased market share for metal systems, which can be profitable applications for contractors looking to bolster the bottom line.