Your roof is most likely to develop the appropriate environment for moss growth in the winter. Moss will grow on your roof shingles in shaded and damp areas. Mosses reproduce via airborne spores. If a roof is covered with moist organic matter or soil, and if minerals and fixed nitrogen are present in the dirt on the roof, moss can grow from the spores carried there. If the roof shingles are clean and free from dirt and debris, moss cannot take root.

For existing moss, cleaning with special formulations can help however, prevention is the best method. Keep your roof clear of debris, such as branches, leaves and pine needles and keep tree limbs cut back from over the roof to minimize shade. Trees produce sap and will naturally drop some of the sap onto a roof when in very close proximity to a home. Tree sap contains nutrients and sugars that are considered fine dining for moss. Sap from trees on a shaded roof area can accelerate micro-organism growth verses a roof not influenced by trees. Trees will also shade a roof from the sun, allowing moisture to be retained on a roof surface and, depending on the size and type of trees, may inhibit air flow across the roof. All of this combined with dropped tree leaves, branches and pine needles make a perfect environment for moss growth.

Also, use shingles with small amounts of copper or zinc in them.

Nice Idea....

Yes, I was asking some Dallas roofing contractors about this, and if you currently have moss on your roof, you can make a solution of the ratio 1 part zinc chloride and 9 parts water. You just apply the solution to the moss and wait 45 minutes in order to kill the moss, and then you can pressure wash the dead moss off the roof. Also, if your current shingles don't have copper or zinc in them and you don't want to replace them quite yet; you can put down zinc strips and copper wire along the length of the roof. This should help prevent future fungal growth.