What does the white staining below the vent of this two-story house imply?
a. Electrolysis de-zincification of the low-quality galvanized vent is occurring (notice the difference in color compared to other vents).
b. The asphalt shingles have a fairly uniform coating of algae.
c. A white sealant applied to help prevent roof leakage is being dissolved by roof drainage.
d. There is a moisture problem caused by combustion appliance exhaust venting.