Exposed asphalt shingles undergo chemical and physical changes as they weather. Here we focus on the resulting changes in solar reflectance. Most roofing granules employing inorganic metal oxide pigments are very stable. Initial reflectance changes are therefore due to changes in the asphalt itself, and the loss of processing oils coating the granules. Ultraviolet-induced photo-oxidation of these oils and exposed asphalt produces dark hydrophilic substances that are removed by rain, or in dry climates, transported by dew. After six months, changes in solar reflectance are small and (in California) mainly due an annual cycle of accumulation of atmospheric dust and its removal by rain. In hot humid climates cyanobacteria grow rapidly on granule surfaces, creating dark stains that reduce reflectance by as much as 0.06 at 3 years. We show that in these types of climates (exemplified by Houston) biocide additives such as Cu2O can be employed to maintain solar reflectance. When cyanobacteria are absent, solar reflectance changes over the first three years are on the order of 0.02 or less, and may be either positive or negative. (C) 2012 Elsevier B.V. All rights reserved.