I built our family home in 1995. There are eight planes that make up the roof. About 60 per cent of the roof is a slope of 6⁄12 and the remainder is 12⁄12. On two of the 12⁄12 sections there is dark streaking forming. There are a number of trees in our back yard. The roof is dual grey 25-year, three-tab shingles. I have noticed that dark streaks seem to be common on grey roofs. Will zinc strips under the ridge capping help? Any other ideas?

The discoloration you see is typically caused by various fungus or algae growths on the shingles. It may not matter what type of roof pitch you have. It's more to do with perfect climate conditions for these types of organisms to live and grow (shady, damp and cool).

There are many products that are available to you. They all have varying degrees of effectiveness and longevity. Some require regular maintenance every year; some are toxic, and some are more environmentally friendly. Products that are sold to help control fungus and algae can be in powder, liquid or sheet metal forms and can have side effects on other building materials or vegetation and animal life if they run off the roof and onto the ground. Carefully check to see if any of the negatives aspects will affect how you feel about usage.

Some roofing shingle manufacturers incorporate copper into some of their products. These shingles are installed on ridges and other strategic locations. Rainwater washes over the shingles and copper is
released over time to control growths. Copper is said to be more effective than zinc but both may be more harmful because we aren’t able to control runoff and how it impacts on the environment. Look into
metal roofing as an option next time you re-roof. It may be easier to keep clean.