This is mold growing on the roof sheathing visible in the attic. There are several reasons why mold can grow in an attic. In general, mold only grows when there is a water problem. Often times in an attic mold grows because of high humidity levels. When the humidity level gets to the 65% or higher level, mold will grow if present. Poor ventilation is often the cause of hot attics and high humidity. Also having bathroom exhaust fans that vent to the attic space instead of the exterior of the home often contributes to excessive humidity levels.
