If you have moved to the Gulf Coast from anywhere further north, the speed at which an exterior goes dirty here is genuinely surprising. A driveway that looked fine in spring is green by mid-summer. A north-facing wall grows a dark film. The roof develops black streaks that were not there last year. None of this means the house is neglected — it means the climate is doing exactly what this climate does.
Three things drive it. Heat and humidity give algae and mold close to ideal growing conditions for most of the year. Regular heavy rain keeps surfaces damp long enough for that growth to take hold rather than drying out. And near the coast, salt air leaves a fine film on everything, which holds moisture and gives spores something to sit in.
The black streaks running down a roof are a specific organism — a blue-green algae that feeds on the limestone filler used in asphalt shingles. It spreads by air, which is why it often appears on several houses in a street at once, and it always shows worst on the shaded, slower-drying slopes. It is not a stain sitting on top of the shingle; it is alive and rooted in it, and that is why simply rinsing a roof does very little.
The same logic applies to the green on a fence or the dark film on siding. What you can see is the visible part of something growing into the surface. Blasting the top layer off makes it look clean for a few weeks, then it returns faster than before because the roots were never touched. A cleaning solution that kills the growth is what buys you a year or more instead of a month.
Practically, that means most Gulf Coast properties want their exteriors looked at roughly annually, and shaded north-facing surfaces more often than sunny ones. It is far easier to keep on top of than to recover — heavy organic growth left for years starts holding moisture against the surface it is on, and at that point you are dealing with the material underneath as well.
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