Staining: The wash of water over copper gutter surfaces can have additional impact. Moisture in contact with copper gutter surfaces tends to pick up small quantities of copper salts. When this moisture contacts with absorbent material, such as marble or limestone, it will be absorbed. As the moisture evaporates, it leaves behind the copper salts as a stain on these materials. The green stain is particularly visible on light colored surfaces.
The condition does not occur with heavy rains or similar rapid run-off, since the dwell time of the moisture on the copper is short and little copper salt is picked up. Staining results from the slow bleeding action of copper laden moisture.
There are a number of ways to reduce staining or its visual impact. Two common methods are: collecting run-off in copper gutters and directing it away from the building via copper downspouts; and designing drip edges to a minimum of one inch, helping reduce the amount of copper laden moisture that comes into contact with material below. Coating the adjacent surface of the porous material with a clear silicone sealant can reduce staining by minimizing the amount of moisture absorbed into the surface. The use of lead-coated copper results in a black or gray stain, which may blend better with some building materials.