Chemical and microbial effects of atmospheric particles on the performance of steep-slope roofing materials

The reflectivity of a roof is a critical component in design of strategy to reduce overall building energy usage. Airborne particulate matter that settles on a roof can either reflect or absorb incoming solar radiation. The light scattering and absorption processes occur within a few microns of the surface that affects the solar reflectance of the roof. The long-term loss of roof reflectivity appears driven by the ability of the atmospheric particulate matter to cling onto the roof and resist being washed off by wind and or rain. Contaminants collected from samples of roof products exposed at seven California sites for about one and a half years were analysed for major and trace elements and carbons to assist characterization of the chemical profile of the atmospheric particles that soil each roof sample. The chemical composition of the accumulated particles was very similar across the state of California; there was no clear distinction from one region to another. Elemental carbon did not contribute significantly to the loss of solar reflectance as initially expected. Dust particles and organic carbon compensated for the loss of solar reflectance due to elemental carbon possibly because some crystalline forms of these elements were light reflecting and contributed to the solar reflectance. Differences in microbial communities and biomass were seen between the various materials. Abundance of microbial biomass on roof tiles appears to be related to the composition/surface structure of the tile. Cyanobacteria or fungi represent the dominant player.