Question
Hi John, my problem is my neighbors deodara cedar has been dropping its needles on my roof for years and for years I have been removing these needles. The needles collect in the roof valleys. After cleaning these needles from my roof I've noticed that the asphalt roof shingle are breaking down. Will this also cause the sub roofing and rafters to decay like dry rot or some other fungus.

Answer
Hi Richard,

Having to deal with the plant residue of neighbors plants is a consequence of living in the city, I suppose.  


Many gymnosperm species produce a resinous chemical which very effectively degrades asphalt.  By and large, there is not an increased risk because of the needles themselves producing a higher risk of rot diseases or fungal infestations.  To the contrary, there are many species of plants which produce allelochemicals which inhibit the establishment of bacteria, fungi, and plants.  

There is however, the associated risk of fungal infestations in the roof timbers when there is no covering such as the subroof or asphalt shingles.

The best remedy is to remove the needles from the roof regularly.  These chemicals result from the decomposition of the needles.  (The needles make great mulch for flower beds, especially around azaleas and acid loving plants.)