Why does my roof look streaky after rain?

Right after the shower, before the sun comes back, look at where the dark lines sit. Water does not wet a roof evenly for long. It gathers into runoff that follows the same paths every time: down the low lines of each tile course, off the lower edges of shingles, along valleys and toward the eave. Wherever that water runs, the surface stays darker than the roof around it, and a roof that looks streaky after rain is often showing you its own drainage map.

Growth can sit along those same paths, so wet streaks and growth streaks sometimes line up. The Asphalt Roofing Manufacturers Association (ARMA) writes that roof algae can form where “moisture tends to dwell on roof surfaces,” and a line that carries water after every shower is one place it can dwell.

Some growth also changes color in the rain. The U.S. Forest Service explains that a lichen turns on when wet, and that a lichen whose partner is a cyanobacterium takes on “a dark green, brown, or black color” once the algal cells show through. A gray fleck on a dry afternoon can read as a dark mark on a wet morning. The glossary entry for streaking covers the word as the glass trade uses it; on a roof, the marks have more than one source.

Lines that fade as the roof dries were mostly water, or water sitting on a thin film of dust, and a lichen that darkened in the rain pales again as it dries. Neither is something new that arrived with the shower.

A light pattern is a different signal. On concrete tile, Eagle Roofing Products' Florida owner information says: “Temporary light patterning may appear on darker tiles. This is caused by efflorescence and occurs naturally as seen on sidewalks and brick walls when alkaline deposits gravitate to a surface exposed to moisture.” Eagle adds that if it occurs, “it will disappear completely after additional exposure to rain.” Eagle's word is more rain, not an afternoon of drying. What that patterning is made of is explained under the white powder on roof tiles.

A pale cast that stays through rain after rain is not what Eagle describes. The Tile Roofing Industry Alliance writes that slurry-coated concrete tile can oxidize and turn chalky before gradually wearing down to the concrete base, a slow change in the tile's surface rather than anything rain puts on it.

Dark marks that stay after the roof is fully dry were there before the shower. ARMA's algae and moss bulletin says that by the time algae is visible it has likely been present for several months or longer, so a rainstorm did not put a black streak there in an afternoon.

Why the black streaks on a roof form in the first place has its own answer, and the roof cleaning page quotes what the roofing makers name as the organism. Short dark lines hanging below the edge of each tile course are a narrower pattern, answered under dark drip lines under roof tiles. What any particular streak is on a given roof is a question for a licensed roofing contractor.

One pattern shows most clearly in rain: a single heavy stripe on a lower roof, directly below where an upper roof's water comes down. ARMA addresses it in its algae and moss bulletin: “Do not allow gutters from an upper roof to drain directly on a lower roof; extend the downspout from the upper roof into the lower gutter.”

ARMA lists that instruction among its steps that discourage moss and algae growth, under keeping gutters clean to promote proper water drainage. A lower roof that takes an upper roof's runoff carries far more water along one strip than across the rest of its surface, and ARMA writes that algae can form where moisture tends to dwell. Watching the water during a shower is a simple way to see whether a house has that layout.

Where a downspout empties, and whether it should be extended, is a question for a roofing or gutter contractor. The layout is part of the house, and it tends to put the same stripe in the same place until the water goes somewhere else.