I live in a very moderate southern California climate with an average rainfall of only 15 inches (38cm) / year. My roof is asphalt shingle and the pitch is 1/3. In two places I have vines growing up walls and then spreading out on the roof. The vines lie directly on the shingles and cover sizable areas. The vines do not adhere to the roof - as ivy would - but are held in place by gravity and friction. My questions are: 1. Do the vines impede water shedding and so promote roof failure? I haven't had any leaks and the vines have been there for years. 2. Are the vines shortening the life of the roof somehow or actually prolonging it? 3. Do the vines have any significant insulation value? 4. What other concerns might I have? Thanks.