You'd want to 'finish' this job by reducing or eliminating the conditions under which the 'problem' thrives.
If you have trees close to the house, you need to get the sun in/on the roof. If you alter the conditions under which the problem thrives, you may eliminate it! Spring is coming. If you avoid this part of the job, you'll be back cleaning the roof. To me, personally, it's the kind of job I'd be asking myself, "Should I really be cleaning a roof?!" It's along the lines of vacuming your driveway. Yea, it's dirty but should-I do-I really want to be doing this?!