The house is 50 yrs. plus, 2 storey, the roof is under 10 years old. Two dormers at the front of the house, the only 2 bedrooms upstairs. The dormers are gable type having 3 visible fascias and do not have gutters or downspouts. Should the dormers have gutters and downspouts.
Runoff from the dormers will slowly erode granules of the main roof at the dripline, especially where the dormer eves are highest and runoff falls the farthest before hitting the main roof (where runoff will have the greatest “impact energy”).
Runoff from dormer downspouts will slowly erode granules of the main roof too and although runoff will have less impact energy (since the downspout curves and slows runoff where it empties onto the main roof), it will concentrate runoff volume from the dormer roof. If the dormer were near the front eve, downspouts could empty directly intothe main roof gutters, which would be the best. Otherwise, it’s a tradeoff.
I’ve not seen this really addressed anywhere.