The north side of most roofs receives less sunlight and is good breeding ground for mold, mildew, fungus and algae. Shingles can be cleaned easily and effectively by using Acry-Tech Roof & Deck Cleaner. Dilute the product, spray on with a pump up type garden sprayer, and rinse off with a garden hose nozzle set to a sharp stream, or use a pressure washer. Be careful to back off from the surface if using a pressure washer, because you don't want to damage the shingles with high pressure.
After the roof is cleaned, you can apply a clear sealer containing biocides to keep the mildew at bay for many years.
visit www.acrytech.com for the products mentioned