Perimeter metal attachment should be determined through the completion of a perimeter edge calculation. This calculation determines the proper securement spacing required to meet the wind design pressure of an existing building. In high-velocity wind zones, the base perimeter materials should be secured to the structural components - not the deck - with properly approved fasteners. The perimeter metal materials must comply with ANSI ES-1 regulations, which require testing and certification of the materials.
An important roof component at the perimeter area that is often overlooked is blocking material. Wood blocking has long been used as an anchoring substrate for metal edge and flashing components, such as expansion joints, curbs and accessories. Traditionally, blocking has been manufactured from pressure-treated lumber. Recently, there have been changes to wood blocking due to environmental concerns relating to the treatment of the materials. These changes could have an impact on wood blocking material used at perimeter edges, particularly in high-velocity wind zones.
In January of 2004 federal regulators halted the production of lumber that was treated with arsenic as a preservative to protect against wood rot, insects and deterioration from moisture. The EPA classified the chemical used in the pressure treatment process - chromated copper arsenate (CCA) - as a potential cancer risk. EPA studies concluded that CCA was linked to bladder, liver and lung cancer. It has been estimated that CCA was used on approximately 90 percent of all pressure-treated lumber in the United States.
Alternative Roof BlockingSince most roof damage from high-velocity winds occurs at the perimeter edge, a metal blocking system has been developed to enhance the roof’s perimeter for wind uplift resistance. The Alternative Roof Blocking System (ARBS) is an engineered metal fabrication system that was developed and patented by Henry Gembala. The materials used for the manufacturing of the ARBS System are constructed from materials meeting the standards, quality and thickness recommended by SMACNA, NRCA, IBC and ASCE 7. The blocking assembly includes one or more pre-formed component structures fabricated from a non-corrosive, durable material. The blocking can be fabricated from galvanized steel, aluminum or stainless steel, depending on project requirements.
The products are fully recyclable, which provides a significant environmental advantage in LEED-engineered buildings. ARBS also has excellent fire resistance and provides a Class A Fire Rating. The products are noncombustible and there is no threat of accelerated fire spread, which is a concern with traditional wood blocking. Torch-applied products can be applied directly to this system.
The blocking system is prefabricated and formed to fit any thickness of lightweight concrete or insulation - including tapered insulation. The blocking is properly formed to the slope of the lightweight or tapered insulation. On lightweight concrete systems, the blocking is secured directly to the lightweight concrete and prefabricated vent holes are set at the intermediate portion of the blocking to provide for venting of gases and moisture that may accumulate in the roof system, thereby reducing the likelihood of premature roof failure.
Finally, metal blocking is less expensive per lineal foot than lumber, and since the cleat is built into the blocking system, application of edge metal and coping is seamless and is less labor intensive. The blocking system can be formed on-site, allowing the contractor to have the material shipped in coils to the jobsite. This eliminates contractors from having to turn their jobsites or shops into lumberyards for each project.
ARBS provides a Quality Assurance Certificate (upon request) that includes the data from the specific lot of material referenced and an assurance to quality in the materials and manufacturing process. ARBS also offers a warranty on all of its manufactured components that is consistent with the terms and materials offered by the roof membrane manufacturer used on the project.