What can cause asphalt shingles to prematurely lose aggregate?
I have a house built new in NJ in 1988. Roof is Tamko Heritage multilayer asphalt shingles. I believe that the warranty is 50 yrs. Shingles are over a plywood deck over an attic. Vents in each gable end, plus 2, 10-12" square roof vents. There also is an attic fan set to turn on when the attic temperature exceeds 85 degrees.
My problem is that the shingles along the ridge and some of the shingles near the ridge are losing their aggregate. Some near the east end of the ridge are just about completely stripped of aggregate. Initially I thought that it was staining (like mildew) because the underlying color is black. But then I noticed that the downspouts were depositing large amounts of shingle aggregate at their bases after a heavy rain. I then inspected using binoculars and it's clear that the black color is due to missing aggregate.
Otherwise the roof looks normal. Shingles are flat, not curled. All of the tabs are intact. From what I can see of the plywood from inside my attic, it looks flat, normally colored, and securely attached to the roof rafters. At present there are no leaks.
New Jersey is not an extreme weather area. No high-wind hurricanes (we get the rain at the tail end). Rare tornadoes (none that affected my house). Moderate temperatures compared to the South. The roof does not get walked on. In particular, not the ridge. So the roof has experienced a relatively mild environment.
The only thing I can think of is that we sometimes see birds sitting on our roof ridge. So their deposits could be having an effect. But that happens to our neighbor's houses and their roofs are OK. And I can't believe that shingles would not be designed to tolerate occasional bird droppings.
What do you think could be causing this? What is the best way to respond to the situation? I'm worried both about leaks and that the roof is an eyesore.
Since the roof should still be well under warranty, I don't want to just call a contractor and have him replace the roof at my expense. I have sent an email to Tamko informing them of the issue and asking for their recommendation on what I should do. But if past experience with other companies is any indication, I do not expect a quick response to a potential warranty claim. I do not know who installed the roof, as the house was built by a general contractor who handled all of the subcontracting.