Basics of a Metal Roof Design
by Jonathan McGaha | October 1, 2020 12:00 am
Specifying a metal roof requires an understanding of all the parts
A metal roof is a building component that requires proper design, specification and detailing. To do this, the designer must understand the individual parts, not only on their own, but- more importantly-how they all work together to give the building all the benefits of a metal roof system. Therefore, the specifiers of such a roof must be familiar with all characteristics of a metal roofing system to provide a professional design for this important building component.
Many designs today attempt to use a generic specification or one-size-fits-all approach, leaving many critical details left with a limited or no specific description. While often a limited specification lacking proper description will still yield a properly performing metal roof system, the lack of a properly specified and installed metal roof is the No. 1 cause of metal roof problems and/or failures.
Using the following specification suggestions will cover the most critical items needed to provide practical and necessary direction for a fully functioning metal roof.
Structural System
First, it must be stated that a metal roof system is a structural system. The panels themselves certainly are required to protect the interior of the building from weather, but they also have to collect, distribute and resist natural forces such as wind, rain, snow, thermal variations, etc. Therefore, the first item that must be considered when selecting and specifying a metal roof is to make sure the system selected can resist all of nature’s forces, while at the same time providing weather protection to the interior. The following are two specific design publications required to be used when designing a non-residential metal roofing system in compliance with the current requirements of the International Building Code (IBC):
- ASCE 7-current publication: The American Society of Civil Engineers (ASCE) has developed a publication labeled “Minimum Design Loads for Buildings and Other Structures” and is designated as ASCE 7. This manual provides wind, snow, seismic and other live loads that a structure is required to accommodate without failing. With respect to metal roofs, it provides design parameters for determining the different wind zones and the design loads associated with these specific areas. These loads, by code, are the ones to be used when determining the structural requirements of a metal roofing system.
- ASTM E-1592: The current version of the IBC requires that a metal roof system must be designed to meet UL 90 or ASTM E-1592 loading conditions. While UL 90 testing was appropriate for roofs prior to the development of ASTM E-1592 standards in the early 2000s, only the procedures associated with an ASTM E-1592 test allow the designer to determine the wind design loads that can be resisted in all the various wind-loading areas of a particular metal roof as determined by an ASCE 7 evaluation. Since these evaluations involve structural calculations, the specifications should require them to be performed by a professional engineer licensed in the state where the roof is to be installed.
Roof Panel Type
There are numerous panel configurations, thicknesses, coatings and detailing that are available for metal roofs. The most common for a non-residential project is a standing seam system, which, in general, involves a series of flat-surface metal sheets with vertical seams (standing seams) that allow the individual panels to be field connected to each other. The configuration of these seams can vary, but the end result should always be a contiguous steel surface that is impervious to water intrusion.
In addition, the fastening of these panels at the seams needs to be designed with continuous or individual clips that connect the metal panels to the structure below in a manner that satisfies the structural requirements discussed above. The attachment of these clips should be performed under the surface of the metal roof and allow for necessary thermal movement of the roof panels. Only a panel system that has been tested in compliance with an ASTM E-1592 procedure, and one which can structurally resist the design loads developed using the ASCE 7 wind load production, should be specified.
Since the termination of the roof panels is the main location of roof leaks, they need to be specifically identified during the roof design process. There are two different basic designs, which are hydrokinetic and hydrostatic.
- Hydrokinetic: A hydrokinetic roof directs the majority of the water over its surface due to kinetic (moving) forces of gravity. This type of roof does not require the seams or ends of the panels to be completely watertight and, therefore, requires the use of a waterproof underlayment to completely resist all water intrusion. The metal panel merely provides a cover over the underlayment, much like shingles do.
Even though a minimum roof slope of between 3:12 and 4:12 is suggested by many manufacturers and associations, critical areas such as valleys, curbs and eaves are vulnerable and susceptible to water intrusion. This type of metal roof should only be specified when all of the specific conditions have been considered and allowing water under the steel surface in certain areas can be completely handled by underlayment and, therefore, is acceptable.
- Hydrostatic: A hydrostatic roof is designed to keep all of the water on the outside surface and, therefore, does not require the use of a waterproof underlayment. It is designed to handle standing and/or blowing water from entering the interior space for as long as necessary and until the moisture is no longer present.
The basic principle for the seams, valleys and eaves associated with this type of system is to seal these areas permanently. Use either a sealant between metal members, secured by a weatherseal fastener or crimping the seam to permanently prevent any differential movement between metal surfaces involved.
Only specifying a concealed fastened metal roof eliminates the problems of fastener back-out and/or fastener hole elongation. All thru-fasteners should be at non-moving locations and flashing locations where both flashing member components are allowed to move together.
Finally, the introduction of solid zinc or stainless steel fastener heads, powdercoated with paint to match the metal roof surface, plus weatherseal washers, which can be totally encapsulated within a concave washer attached to the fastener head, provides a practically non-visible attachment system.
Warranties
Most architectural non-residential metal roofing systems will include a factory-applied paint. Use only a system which is comparable to a Kynar system, which will carry a minimum of a 20-year total paint warranty.
With respect to weathertightness warranties, make sure that the duration of the warranty and the total manufacturer liability is identified in the specifications. Also, ensure that manufacturer inspections are required and the final warranty will not then exclude contractor workmanship. Manufacturer warranties should be necessary for a product to be included in a specification, but they should be inspected by the specifier to ensure that they are specific to a particular job and meet the requirements of the specifications.
A metal roof system is a structural building element that, when designed and installed properly, will last as long as or longer than the other building elements. However, it cannot design or install itself without a competent specification and professional installation. The designer’s responsibility is to ensure that these two critical items are thoroughly covered in the specifications, as well as observed for general compliance during installation.
A successful metal roofing project always starts with a professional and complete specification and ends with a metal roof that conforms to those specifications. Make sure that this happens on all metal roof projects in order that a long-lasting, aesthetically pleasing and leak-free roof will be the result.
Chuck Howard, PE, Metal Roof Consultants[1], Cary, N.C., is a member of the Metal Construction Hall of Fame, a frequent contributor and speaker on metal roofing issues, and has been a leader in the metal roofing industry for more than 30 years.
- Metal Roof Consultants: http://www.metalroofconsultants.net
Source URL: https://www.metalarchitecture.com/articles/basics-of-a-metal-roof-design/