Why does my roof look worse after a wet winter?

Start with what the calendar normally does. Naples Municipal Airport's 1991 to 2020 normals mark out four dry months.

November averages 1.39 inches of normal rain, December 1.32, January 1.68 and February 1.40, every one of them under 1.7 inches, against 7.41 to 8.89 inches a month in the rainy season from June through September. The same normals put November's average temperature at 72.4 F, December's at 68.2, January's at 65.3 and February's at 67.5. GAF says blue-green algae needs “proper temperatures” and does not put a number on them; what the normals show is four months under 1.7 inches of rain, none averaging below 65.3 F.

A wet winter does not change the temperature normals; it puts more water on a roof in months that normally see under 1.7 inches of rain.

GAF's bulletin, written for shingled roofs, names “moisture and proper temperatures” as what blue-green algae requires and dew as “the primary moisture source.” ARMA's algae and moss page says algae can form where “moisture tends to dwell on roof surfaces,” and that “coastal or humid climates create the most conducive conditions for algae growth.” Extra rain in those months adds to the moisture side of GAF's pair. NOAA's normals record rain, not dew or the dew point, and they say nothing about how warm a particular wet winter runs.

Algae discoloration does not show the day conditions turn favorable, which is part of why a wet winter's effect can read as a spring surprise.

ARMA's algae and moss page describes the sequence: the organism “protects itself from damaging ultraviolet rays by producing a dark pigmented sheath,” discoloration “typically begins as small spots then quickly transforms into streaks on the roof plane,” and “by the time the algae is visible, it likely has been present for several months or longer.” A roof that looks worse once the dry season ends can be showing growth that began, out of sight, while the extra moisture was there. ARMA's page is written for asphalt roofing, and no source read times this on a Naples roof.

Concrete and clay tile bring in two more documents, one about growth and one about a change that is not growth at all.

TRI Alliance says growth on tile starts “with enough cycles of heat, moisture and light,” and a wet winter adds to the moisture in that list. Separately, Eagle Roofing Products' Florida owner information for its concrete tile, as archived, describes “temporary light patterning” on darker tiles caused by efflorescence on concrete roof tile and says that “if it should occur on your roof, it will disappear completely after additional exposure to rain.” Which change a given roof is showing is not something these documents settle; a roofing contractor can look at the tile.

A wet dry season is one departure from a normal that runs warm every month.

The twelve months of Naples normals show the average each month keeps to, which is the baseline a wet winter departs from. How Naples compares with algae up north is a separate question, as is why Naples roofs stain so quickly in an ordinary year, and why a roof ends up turning green rather than dark. The normals behind the dry season are thirty-year averages, and any single year can run wetter or drier.