Every few years, not every year. Because professional roof cleaning kills the organism rather than rinsing its surface, a properly treated roof stays clean far longer than a washed wall — which changes the maintenance rhythm entirely.
Why roofs run on a longer cycle
A soft-washed roof is cleared of living colonies at the root. Recolonisation has to start over from airborne spores, establish in the granule texture, and grow to visibility — a multi-year process on most roofs.
Contrast that with siding, where annual washing is normal. The difference is the treatment chemistry and the fact that a roof, unlike a wall, is not being re-wetted by sprinklers and shade-trapped dew every morning — though shaded roofs narrow the gap.
What shortens the interval
Heavy tree cover is the main factor: shade slows drying, and overhanging branches drop debris that holds moisture and seeds growth. North-facing slopes recolonise first, and roofs near woods see more spore pressure than roofs in open subdivisions.
If your last cleaning was years ago and faint streaking is reappearing on the north slope, that is the early signal — treatment at that stage is quicker and cheaper than waiting for full-roof streaking.
The maintenance mindset
Trimming back overhanging branches is the single best thing you can do between cleanings — more sun, faster drying, less debris. Keeping gutters clear helps the roof edge dry properly too.
Then simply watch the north slope each spring. It is your early-warning system: whatever is coming for the roof shows up there first.
Not sure where your roof is in the cycle? A photo of the north slope tells us most of what we need. Read more about professional roof cleaning →