Zinc and Copper Strips

Zinc and copper strips are narrow bands of sheet metal installed along or just below a roof's ridge to slow the growth of moss, algae and lichen. When rain runs over the metal, small amounts of zinc or copper dissolve and are carried down the roof, where the metal ions inhibit the organisms. The same effect explains why areas directly below galvanised flashings, copper vents or chimney flashings often stay clean while the rest of the roof turns green or black.

Installation

Strips are usually fixed along both sides of the ridge, with the lower edge exposed to rain. Guidance from horticultural libraries describes nailing them roughly every foot (about 300 mm), and fasteners should be placed where they are covered or sealed to avoid leaks. On long slopes, some installers add a second row lower down, because the effect weakens with distance from the strip. Copper and zinc should not be placed where their runoff will reach incompatible metals, such as aluminium or galvanised gutters in the case of copper.

Effectiveness

• Strips work best as prevention on a clean roof or after existing moss has been removed; they are slow to clear established growth.
• Effectiveness varies with the amount of metal exposed, rainfall and roof length. University of Washington library material notes that many commercial strips work for around a year, while larger galvanised flashings can continue for decades.
• Copper is generally considered more toxic to moss and algae than zinc, but is more expensive.

Environmental notes

Zinc and copper in roof runoff are pollutants, and both are toxic to aquatic life at sufficient concentrations. Extension and environmental sources advise directing runoff away from ponds, streams and sensitive plants, and avoiding use with strong acids or bases. Some local authorities regulate metal-bearing runoff or discourage zinc products near waterways. The US Environmental Protection Agency lists zinc metal strips among minimum-risk pesticides.

Related products

Algae-resistant asphalt shingles use copper-treated granules for a similar effect spread across the whole roof.