Volume 8, Issue 8 (September 2011)
Service Life Assessment of a Low-Slope 55 % Al–Zn Alloy-Coated Steel Standing Seam Roof System
Published surveys concerning roof service life incorporate estimates from roofing professionals based on their observations and opinions. In cases where roofing products have been in service long enough for the end of life to be observed, these data have some merit, but when end-of-life empirical data are not available, accurate projections are difficult to verify. Other attempts at defining the service life relate to product warranties or lab testing. In the case of standing seam metal roofing, the variability of the materials and installation methods strongly colors the survey opinions. In steel roofing, this variability includes that of the steel thickness, the metallic coating type and thickness, the presence or absence of paint films, the types of sealants, the seam-folding details, and other construction or installation details. Related accessories and flashings of different materials and designs further confuse survey criteria and results. In the case of 55 % Al–Zn alloy-coated steel roofing systems, the prediction of service life is complicated by the fact that the product has been commercially available for under 40 years, and end of life has rarely been observed. In this paper, we combine field inspection procedures with a novel technique for securing roof samples for subsequent laboratory analysis in order to enable the prediction of roof service life for a 55 % Al–Zn alloy-coated steel roof in Denver, CO. The performance of other roof components is also quantified. A methodology is then presented that utilizes the current best practices, together with the results of this roof analysis, to predict the total roof service life of similar constructions when built using today’s practices. This work confirms previously published work that used experimental corrosion specimens to establish the long-term durability of 55 % Al–Zn alloy-coated steel. It is suggested that this work can be replicated in order to develop meaningful corrosion and material performance data for any climate in which this type of roof is constructed.