If your house is under or near trees you will probably need to clean the debris off your roof periodically. Leaves, branches and debris have a tendency of piling up in valley areas, and on roofs with a low slope. This debris, which is organic material, retains moisture and over time will break down. (Imagine having a compost pile on your roof.) This has a damaging effect on roofing materials, especially asphalt shingles. This situation can make a roof more prone to leaks and can shorten the life of your roof. The frequency with which you clean the debris off your roof depends on your house. Determining factors include the proximity of trees to the roof line and the type of trees they are. On average, houses that have trees within 20 feet should be cleaned a least once a year or when leaves start building up, and houses with trees growing over the roof may need to be cleaned twice a year at a minimum. Climbing ladders and walking on roofs can be dangerous, so take the normal precautions.