Why does my neighbor's roof stay clean?

Some differences between two roofs are visible from across the fence. What either owner has had done to a roof is not.

• Roof color, light or dark.
• Roof material, tile, shingle or metal, each with its own maker's instructions by roof type.
• How many trees stand over each roof, and how close.
• Which way each roof plane faces.

The Tile Roofing Industry Alliance names overlapping variables for growth on tile: the location of the structure, the direction the roof is oriented to the sun, and the quantity of trees, canals and lakes in the area. Curb appeal is what shows from the street, and no document read for this page says a homeowner can tell from there whether a neighbor's roof has ever been washed.

ARMA is direct about this one: almost all types of roofing systems are susceptible to algae discoloration.

It is, ARMA continues, most readily visible on light-colored roofs and not so visible on darker shades. That is a statement about what the eye sees, not about which roof hosts more growth. ARMA names Gloeocapsa magma as probably the most prevalent of several algae species behind the discoloration, and nothing in its visibility sentence limits the organism to light roofs. A dark roof that looks clean may be carrying growth that simply does not show. Whether a lighter roof needs cleaning on a different schedule is answered on whether white roofs need cleaning more often.

If the neighbor's roof is newer, the shingle itself may be part of the difference.

ARMA says asphalt shingles that incorporate algae-resistant copper granules may be employed when it is time to replace a roof. CertainTeed's version uses copper-containing granules because copper has natural anti-algae properties, and GAF describes microsites that release algae-fighting copper over time. All three describe a shingle chosen at replacement, not something added to an existing roof. Whether that choice is worth considering on a Florida re-roof is answered on algae-resistant shingles in Florida, and which shingle goes on at replacement is a licensed roofing contractor's conversation.

TRI Alliance closes the door on one explanation before it opens: while research continues, there is currently no known manufacturing procedure or job-site coating with a long-term preventive effect on this growth.

So a neighbor's clean roof is not explained by a coating nobody publishes. ARMA writes that algae spores are carried by the wind or by animals and can spread from one rooftop to another, which is why growth is common across an affected neighborhood, and CertainTeed also says wind carries algae spores from roof to roof. How that spread works is set out on whether roof algae spreads from house to house. Exposure plays a part, though the makers name different sides: GAF says blue-green algae generally grows on the northern exposures, and CertainTeed says algae discoloration most often occurs on east-facing surfaces but can show up anywhere, a disagreement why roof algae grows on the north side sets out.

What is left, once color, shingle choice, exposure and trees are accounted for, is the variable nobody can see from the street: whether and when the neighbor's roof was last cleaned. A clean roof next door also says nothing about how warm either house runs, a question with its own limits on whether a dirty roof makes a house hotter, or about how either roof looks in a listing photo, answered on why a clean roof photographs differently.