Like any other elements that make up a building exterior envelope, a metal roof has several basic elements that, when designed and installed properly, will provide a building with a roof surface that will yield decades of satisfactory service. These basics hold true for both new metal roofs and retrofit metal roofs. On the other hand, if the basics are not adhered to, the finished metal roof system will provide only marginal service. An architect, engineer, or consultant, knowing these basics and making sure they are incorporated into metal roofing projects, insures that the building owner is getting a roof system that will last well beyond its warranty terms.
A metal roof is a functional element of a building that has certain wind load resistance responsibilities. It is certainly responsible for protecting the interior contents from the exterior atmospheric elements, but it has to have the capacity to remain in place to provide this protection. As has been evidenced by the effects of hurricanes, tornadoes, and the like, their capacity to withstand such devastating forces is unsurpassed in the roofing industry. The reality is that a metal roof’s capacity to withstand these extreme loads is dependent upon the metal roof being properly designed as well as installed per the contract documents. Let’s discuss how the basic elements can have a positive effect on a metal roof system.
First, it must be noted that a locally licensed professional engineer must be retained to produce the actual structural design work associated with the roof. This work must be compatible with the building’s main str4uctural design. That does not mean that if the consultant is not a licensed engineer he or she does not need to understand the design basics. A metal roof consultant needs to be able to understand the professional design results in order to be effective in his or her capacity on a metal roof project. The main design elements for a metal roof are, 1) using the proper documents to determine the expected wind loads throughout the roof area and, 2) choosing the proper metal roof products to resist these loads.
In most areas, the effects of uplift wind pressure on different portions of the installed roof surface are the most severe roof loads to consider. The American Society of Civil Engineers (ASCE) has a publication that is accepted in the International Building Code (IBC) as the standard for determining these loads. The IBC is almost uniformly accepted by all states. The publication, ASCE – 7- Minimum Design Loads for Buildings and Other Structures, clearly defines how to determine the various zones on the roof that have different design loads, along with the actual loads for those respective areas. Chapter 6, Wind Loads, goes into great depth to show what wind speeds should be used, depending on the geographic location of the building. It also provides information necessary to determine the dimensions and locations of the various wind zone areas within the plane of the roof. In addition, there are several variables that impose increasing factors and decreasing factors to the design load based on parameters such as surrounding topography, occupancy of the building, building height, etc. All of these factors need to be understood and considered when determining the required wind loads for a metal roof.
After the proper loads are determined for all zones of the roof, a metal roof system must be analyzed to determine how it can resist these loads. The American Standard for Testing and Materials (ASTM) has developed a pressure testing procedure commonly referred to as an “air bag test”. This procedure applies uniform pressure from the bottom of the panel surface by introducing air into an airtight chamber and allowing pressure to form on the bottom of the panel. This pressure produces forces on the panel that simulate actual wind pressure forces that would be introduced onto the panel system when wind blows over the roof. The actual procedure is labeled as an ASTM E-1592 test and yields ultimate load forces that the panel and its attachment components can withstand. Needless to say, the spacing of the attachment components greatly determines the capacity of the engineered metal roof system. The spacing of these attachment points, and the associated allowable load capacity, are shown within the report.
The use of the ASCE – 7 design loads and the ASTM E-1592 metal panel system capacity will properly determine under what conditions a panel system can be used to resist the design loads for a particular roof. These methods should always be incorporated in any metal roof design to insure that the proper metal roof system, along with the proper attachment method, is used.
In addition to the structural evaluation required to determine what materials and associated panel assemblies can be used for the metal roof, there are other, non-structural, basics to evaluate. Some of the most important are as follows:
Drainage
Every metal roof must have positive drainage to eliminate the possibility of ponding water on the roof surface. This slope (as little as ½” in 12” for structural standing seam panel systems) directs water to the building’s perimeter or internal gutter. The size of the guttering system, location of outlets, size of outlets, distribution of the water when it is introduced to the surrounding areas, etc. need to be taken into consideration when determining the proper performance of a metal roof system. Most metal roof panel manufacturers have design information about their gutter and downspout system’s capacity, and all building codes identify the amount of rain in a specific time-frame that is required to be accommodated. Make sure that these resources are utilized when determining how to accommodate run-off drainage.
Flashings and curbs
For the metal roof system to be useful for the building owner, it must not only remain structurally sound during design loading conditions, but also remain watertight. After all, that is why we need roofs! For that to be accomplished, the flashings that seal the roof to the eaves, ridges, walls, etc. must be designed to withstand water intrusion. They must also be designed to accommodate the forces of thermal movement. Additionally, curbs must be integrated into the roof system in such a manner that they, too, will resist water penetration as the panel is transferring collected water to a lower location such as a gutter or exposed eave. Like the roof flashings, curbs must take panel expansion and contraction into account. As with the drainage design, the metal panel manufacturer will have standard details showing how their total roof system should be flashed and curbed. While this standard information is available, the consultant/designer is responsible for determining the particular requirements of each roof, and modifying the standard details to suit.
Insulation and condensation control
With the design and installation of every metal roof there is possibility of incorporating thermal insulation under the metal roof panel. The main purpose for this material is to separate the exterior temperatures from the building interior temperatures. Some basic design points are as follows:
• When a vapor barrier is required (and it is in all but a few cases), place it on the “warm” side of the insulation. Make sure the vapor barrier is sealed properly in the field application to prohibit interior air from coming into contact with the exterior metal skin, forming condensation.
• Fill all voids between the top of the insulation and the bottom of the metal panel with insulation. That includes the voids created at the ridge and hips. Left un-insulated, these areas are prone to condensation formation. If the voids cannot be completely filled, the cavity must be vented on the “cold” side of the insulation with outside air. Air exchange in any given cavity of 3 to 5 times per hour is required to control condensation in most conditions.
• More insulation is not necessarily better. Usually after an R value of approximately 20 is attained, the return on investment of more insulation decreases quickly. Do not think that “If 6” of insulation is good, 12” is twice as good”. Do the thermal and associated expected energy consumption calculations, using the most current ASHRAE procedures, and use the most efficient amount of insulation.
• Insure that the proper insulation and vapor barrier are used for the actual conditions. Consult with the metal panel manufacturer to insure that either product will not negatively affect the panel or its components.
In addition to reviewing the basics discussed in this article for every metal roof project, it is imperative that documentation of the final design determinations be properly prepared. That requires that a scaled shop drawing, complete with large scale details for every flashing condition incorporated in the project, be prepared by the contractor and/or manufacturer and reviewed by the consultant/engineer to insure that the proposed metal roof system meets the expectations of the design documents. During this review process the consultant/engineer and contractor need to be extremely candid in expressing their expectations, thus eliminating costly arguments after installation has begun.
Last but certainly not least to consider is how the metal roof is installed. Installation errors are by far the leading source of law suits for metal roofs. While this phase of the project is often times relegated to minimal attention by “junior” staff, it is the most important phase of any metal roof project. Making sure that the contractor knows that the installation of the roof will be closely observed to insure that the finished product meets or exceeds the level of detail indicated in the approved shop drawings sets the stage for a very successful metal roof project. As a consultant, establish this understanding early in the schedule and maintain its diligence throughout the entire project. It will yield positive dividends for all parties involved.
The bare basics of metal roofing have been introduced in this article. Each of the listed basics deserves further explanation and research by the consultant serious about providing related services to a building owner. As with all areas of building envelope study, the basics of metal roofing need to be explored exhaustively in order to provide excellent advice on a metal roofing project. Take the basics described in this article as a foundation from which to build a greater understanding of the proper design and installation for a metal roof system that will perform properly.
