Heavy Metal
Jul 1, 1998 12:00 PM, W. Lee Shoemaker
Many of today's older schools are wearing layer after layer of patches or plugs to keep occupants dry. While the buildings themselves may be structurally sound, roof leaks and related problems are a common occurrence. Maintenance on these roofs-most of which are flat rather than sloped-has become a huge expense for many school districts, and the significant sums spent patching these roofs repeatedly provide only temporary relief.
Cost and long-term performance are paramount considerations for school officials whenever a replacement roof is to be installed on a school building. One of the roofing options available to districts is the standing-seam metal roof. This system provides a weathertight seal while accommodating expansion and contraction. It also offers the ability to add a slope to flat roof surfaces.
Long-term protection Utilizing formed, interlocking seams, the standing-seam metal roof literally can cover an entire building with minimal penetration of the roof covering by structural fasteners. As a result, the system fulfills the need for durable, long-term protection against virtually any type of weather.
This system can be installed on both new construction and reroofing projects. In most retrofit projects, the standing-seam roof can be installed over an existing roof, eliminating costly and time-consuming tear-offs and disposals. Tear-offs and disposals can add 50 percent or more to the bottom line of a reroofing project. By leaving the old roof in place, the release of potentially dangerous materials into the environment also is avoided. Eliminating the need to dispose of old materials preserves valuable landfill space.
Structural vs.non-structural There are some major differences between the architectural (non-structural) and structural standing-seam metal roofs: *The architectural metal roof generally is specified for slopes of 3:12 or greater. Designed primarily as a water-shedder, it requires the support of a substrate such as wood, plywood or gypsum. Base felts or modified bitumen are applied between the substrate and metal for added protection against moisture penetration.
The panels normally are installed in short lengths. Expansion and contraction of the panels is not accommodated by the concealed fasteners, but thermal movement is provided by the metal roof's short panel lengths.
Clips are made of a metal compatible with the panels to prevent galvanic corrosion. They are attached rigidly to both the roof panel and supporting substrate. The female joint needs no sealants because of the steeper slopes. Like wood shakes, tile and slate, the architectural roof is a good water-shedder when it is designed, specified and installed correctly.
*The structural standing-seam roof most commonly is used on low-slope buildings. The roof panels span from purlin to purlin or joist to joist, normally positioned 5 feet apart. No substrate or additional deck is required. This application features slopes as lowas 1/4:12, and is an alternative to flat, built-up and single-ply systems.
The female ribs have factory-applied sealants to which the male ribs are seamed, providing a weathertight membrane without the need for a base felt for backup protection. Automatic field-seaming machines reinforce the connection between panels, adding to the roof's weathertightness.
The fastening clips inside the seam hold the roof panels to the purlins, and allow the roof panels to expand and contract during temperature changes. A sliding portion of the clip enables the roof to expand and contract, accommodating much wider buildings than non-structural panels. Because the clips are concealed inside the seam, the use of through-roof fasteners that could threaten the weathertightness of the roof is minimized.
With both types of standing-seam roofing systems, outdated skylights and HVAC equipment, complex multilevel slopes, and flashings and penetrations can be covered with little or no interruption of classroom activities.
Adding insulation It is relatively easy to improve the standing-seam roof's thermal performance during a retrofit project. A sloped sub-assembly can be attached to the existing building structural system to create additional pitch and ensure adequate drainage. The extra space created not only makes it easy to add insulation that greatly increases the rooftop's thermal performance, but also provides ventilation to dry out the existing roof membrane.
While metal roofs will accept a variety of insulation systems, the most common option for retrofit applications is a fiberglass blanket. As a general rule, fiberglass insulation is installed over the existing roof surface before installation of the new standing-seam roof. In new construction, insulation blankets can be installed directly under the metal panels and stretched over the supporting structural members.
Vapor retarders such as foils, coated papers or plastic films also are applied to fiberglass-blanket insulation. On most reroofing applications, however, existing barriers are adequate and unfaced insulation is sufficient.
Roof finishes New finishes have improved the performance and appearance of metal roof panels. The base steel is protected by zinc, aluminum and zinc-aluminum alloy. The coating oxidizes at a low rate to protect the base steel, even on uncoated exposed edges. The coating also acts as a barrier, sealing off the base steel from the environment.
Roofing-system manufacturers provide a variety of organically based finishes such as acrylics, alkyds, epoxies and vinyls. In addition, an increasing range of standard and custom colors is available to harmonize the roof with the building exterior and surrounding environment.
The standing-seam metal roof's minimal maintenance requirements contribute significantly to its favorable life-cycle costing. A properly designed and constructed metal roof will not rip, tear, puncture, shrink, creep, slip, blow off or burn. It also will not be sensitive to ultraviolet degradation or chemical contaminants, such as grease and compressor oils.
