Service Life Assessment

SUMMARY REPORT: Service Life Assessment of Low-Slope Unpainted 55%Al-Zn Alloy-Coated Steel Standing Seam Metal Roof Systems Ron Dutton, Ron Dutton Consulting Services LLC Rob Haddock, Metal Roof Advisory Group Chuck Howard, Metal Roof Consultants Scott Kriner, Metal Construction Association Executive Summary: This report documents and summarizes the work conducted to determine with reasonable certainty the roof service life that can be expected of a “like-in-kind”, low-slope 55%AlZn alloy-coated steel Standing Seam Roof (SSR) system when installed today in a like environment using best practices. It incorporates the results of multiple field inspections, independent laboratory analyses of metallic corrosion of the roof panels, components and sealants, and includes assessment of all integral ancillary components that impact the end of roof service life.

 

Background and Introduction: The desire to be able to accurately predict low-slope roof service life has been an important objective of the roofing industry for years. The benefits of achieving this objective include more accurate Life Cycle Cost (LCC) or whole building Life Cycle Assessment (LCA) analyses, as well as better preventive maintenance/repair cost estimating and scheduling. One method used previously to estimate roof service life relies on opinion surveys of roofing professionals [1, 2]. Another method uses tabulations of actual roof replacements at the end of their service lives [3]. J.L. Hoff has discussed the merits and limitations of these methods, as well as the use of manufacturers’ warranty service records [4] and warranty periods [5] to develop a meaningful number for roof service life of low-slope membrane roof coverings. One of the shortcomings of using manufacturers’ warranty periods is that they can change as more experience is gained and actual field performance is documented. For example, Bethlehem Steel Corporation developed a highly corrosion-resistant 55% Al-Zn alloy-coated steel product in the 1960’s and began to market it in 1972 under the trade name of GALVALUME® sheet. Shortly thereafter, a 20- year warranty against through-penetration corrosion was offered, based on 9-year atmospheric corrosion data measured on pilot-line produced specimens [6].

 

As the product gained more widespread use through worldwide licensing agreements and additional corrosion data were developed [7-10], the warranty period was extended to 25 years. More recently, field inspections of 12 low-slope standing seam roofs in place in the U.S. for 30-36 years [11] have shown that the product continues to perform well in a wide range of environments, and that the current 25-year warranty period clearly underestimates the actual service life of a 55% Al-Zn alloy-coated steel standing seam roof. The objective of this investigation was to determine with reasonable certainty the service life that can be expected of a “like-in-kind” 55% Al-Zn alloy-coated steel SSR system when installed today in the Continental United States. To arrive at such a determination, numerous elements require consideration. A “roof system” is comprised of many components, each having a different service life. Thus, in order to accurately assess the system service life, it is necessary to evaluate the service life of each individual component that comes to bear on the life of the roof system in total. GALVALUME® is an internationally registered trademark of BIEC International, Inc. or one of its licensed producers.

 

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