What causes the dark drip lines under roof tiles?

A raindrop landing on a tile roof does not stop where it lands. It runs down the curve of the tile, along the overlap with the course below, and hangs for a moment at the low edge before it drips or moves on.

Dew adds to it. GAF's bulletin on shingled roofs says blue-green algae requires moisture and proper temperatures to grow and that dew is the primary moisture source, so the water that marks a tile course is not only rain. The Tile Roofing Industry Alliance explains what happens on tile: mildew, algae and moss do not grow directly on the tile but on the accumulation of dust and other organic material that settles on it, and with enough cycles of heat, moisture and light, spores start to grow.

ARMA's algae and moss bulletin, written for asphalt roofing, says algae can form where moisture tends to dwell on roof surfaces. A tile course that sheds water to the same low edge every time is a place where moisture dwells, and dust that collects there is caught again by the next runoff, course after course.

A pattern that shows up in the same place on every course is not random, and the mechanism behind attachment explains why.

An article on biofilms in the CDC's journal Emerging Infectious Diseases says microorganisms attach to surfaces and develop biofilms, and that attachment occurs most readily on surfaces that are rougher, more hydrophobic and coated by conditioning films. The article is about biofilms in general, and settled dust is not the conditioning film it describes, so reading a dusty tile edge that way is a bridge, not a sentence any roofing document wrote. What the bridge suggests is that each new wetting lands on what the last one left rather than on a clean slate.

Where water is concentrated matters on a roof. ARMA's algae and moss bulletin says gutters from an upper roof should not drain directly on a lower roof and that the downspout should be extended into the lower gutter. A tile course does a small version of that concentrating by its shape, sending water to the same edge on every rain, which is why a line under it tends to stay in one place. That edge of a tile course is not the same thing as a drip edge, the roof part with its own glossary entry.

The mechanism above explains why a line forms where it does. It does not identify what a specific line is made of.

Eagle Roofing Products' Florida owner information describes a different pattern: temporary light patterning that may appear on darker tiles, caused by efflorescence when alkaline deposits gravitate to a surface exposed to moisture, which Eagle says will disappear completely after additional exposure to rain. That is a pale mark on dark tile, not a dark line, and the two are not the same condition. What the white powder is on its own is set out on the white powder on roof tiles.

Rust is another possibility at a metal fastener or flashing. Cambridge Dictionary says a substance oxidizes when it combines with oxygen and loses hydrogen to form another substance, with iron oxidizing to form rust as its example, a chemical change rather than a growth. Which of these a given line is, and any flashing or fastener question, belongs with a licensed roofing contractor.

The broader black streaks across a roof are explained on the black streaks on a Naples roof, why a roof looks streakier while wet is answered on why a roof looks streaky after rain, and how a white tile goes dark is answered on why a white tile roof turns black. The S-tile profile has its own glossary entry.