Why does my roof get dirty under the trees?

Picture a roof at dawn. On a night with dew, it settles across every plane, and GAF names dew as the primary moisture source for blue-green algae. As the sun climbs, the open parts of the roof warm and dry. The patch under a branch stays in shade, so its dew lingers, and on some mornings it is still damp when the rest of the roof is dry.

Then the branch adds a second thing. Leaves, twigs and whatever else the tree sheds land on that same shaded patch and sit there. A layer of debris holds water the way a wet towel does, so the surface beneath it spends even longer damp.

The Asphalt Roofing Manufacturers Association (ARMA) wrote both pieces into one sentence: “Overhanging tree branches provide additional shade and drop debris on the roof that further holds in moisture and acts as a food source for moss.” That sentence is about moss on asphalt roofing, and it names both halves of why a roof gets dirty under the trees.

Debris does not stay spread evenly across a roof. It slides and washes toward the low lines, lodging in a valley where two roof planes meet and banking up along the eave where the roof ends. Those are the same lines rainwater follows, so the debris that collects there tends to be wet more often than debris on the open slope.

That is why ARMA's own steps against moss and algae include clearing debris and keeping the drainage working. ARMA writes that debris “should be cleared regularly with a leaf blower or similar, non-abrasive method,” and, in a separate line, “Keep gutters clean to promote proper water drainage.” The word regularly is ARMA's, and ARMA attaches no interval to it.

The Tile Roofing Industry Alliance (TRI Alliance) explains what growth needs on tile, and it is not the tile. “Mildew, algae and moss do not grow directly on tile but on the accumulation of dust and other organic materials that settle on tile,” its maintenance guide says, and with enough cycles of heat, moisture and light, spores start to grow.

A tree overhead supplies two of those ingredients at once: organic material falling onto the tile, and shade that stretches out the moisture. TRI Alliance names trees directly in its list of what drives this growth: “location of the structure, the direction the roof is oriented to the sun and the quantity of trees, canals and lakes in the area.” TRI gives no ranking among those variables, and neither does this page.

ARMA separates the two in its algae bulletin: “The algae discolorations should not be confused with moss or tree droppings, which typically produce only localized discoloration.” Algae, in ARMA's description, can darken a roof uniformly or in streaks, and GAF writes that blue-green algae stains usually cover more area on the lower section of a roof and taper toward the ridge.

The two can share a roof. CertainTeed writes that “shaded areas serve as excellent breeding grounds” for algae and that “wind carries algae spores from roof to roof,” and the patch under a canopy is one of those shaded areas. What those black streaks on a Naples roof are, and which organism the roofing makers name, is explained on the roof cleaning page. Which slope of a roof tends to streak first is its own question, answered under why roof algae grows on the north side.

ARMA's algae and moss bulletin lists what makes a roof less inviting to growth, and the first item is about the tree itself: “Trimming tree branches back to allow more sunlight to reach the roof and minimize debris accumulation will provide an environment less inviting for such growth.”

Trimming branches back is work for an arborist or a tree service. Two more of ARMA's steps, clearing debris and keeping gutters clean for drainage, are quoted above with valleys and eaves.

Two limits come with that list. ARMA's bulletin is written for asphalt roofing, so its steps are ARMA's steps for a shingle roof, not a tile maker's instruction. And ARMA's claim is modest: less inviting, not immune. Trimming changes the conditions on the roof; it does not change what moss needs, and ARMA says moss must have a moist environment to survive.

A palm is a tree with its own shedding habits, and whether palm trees make a roof dirtier than other trees is answered on its own page.

Under any canopy the shade and the falling leaves are still there after a cleaning, and why growth returns to a cleaned roof is answered under why a roof is dirty again after a year.

A cleaning also does not judge the roof under the debris. A cracked tile beneath a fallen limb or a shingle edge that moss has lifted is a question for a licensed roofing contractor, and whether a branch should come down is a question for an arborist.