The PVDF coatings used on quality metal roofs consist of a very tight molecular bond that encourages dirt to wash off of the roof. In most environmental conditions, these roofs stay very clean over the long term. However, while these coatings will not support the biological growth that leads to dark streaks and stains on granular surface and shingle roofs, there are certain environmental conditions which can cause dirt to collect on the roof. The worst situations are in areas with surrounding trees, and this is made worse by trees that lose sap which can stick to the roof and then hold dirt or even support biological growth over time. Additionally, heavy pollen or airborne fungus can land on a roof and stick there. Areas prone to heavy and frequent dew (such as waterfront or near-waterfront properties) can also exacerbate this issue by creating a sticky or gummy coating on the roof. We always tell folks to imagine letting your car sit out in the area of your roof and not washing it. The results on it, over time, will be similar to what will happen on your roof. In some cases, folks like this because it helps the roof to take on a more “woodsy” or “natural” appearance. The resulting grime on the roof, while it can be cleaned by scrubbing or light power washing, will not normally be something that damages the integrity of the roof system or its finish.