Thanks for all your help and tips.
The roof looks great now. It was not washed in 10 years - so if the
grime takes that long to accumulate again - I will be happy. I plan to
replace the shingle in 5 years. I used a very soft bristle brush - like
a car brush. Just enough to agitate the sodium hypochlorite solution. I
also used a garden hose to rinse - not my pressure washer. I have lots
of big live oak trees and long needle pines shadowing my home - their
drippings seems to cause the problems. I am sure they will come back.
Harry