Why is one side of my roof darker?

A darker slope can show three different looks from the street: dark streaks or patches across one plane from the eave up to the ridge, a dark patch that stops where a tree's canopy stops, or, on painted metal, one slope gone lighter and duller so the other reads darker by comparison.

Each look has its own documents. The streaked plane is the exposure question GAF and CertainTeed answer differently for algae, and ARMA answers for moss. The canopy patch is the tree mechanism ARMA and the Tile Roofing Industry Alliance describe. The lighter, duller metal slope is chalk, which U.S. Steel describes for painted metal.

GAF's current bulletin on shingled roofs says blue-green algae generally grows on the northern exposures of the roof, since that exposure generally receives less sunlight. CertainTeed's page on algae-resistant shingles says algae discoloration most often occurs on east-facing surfaces but can show up anywhere on the roof.

Two makers, two exposures, and neither cites a measurement. ARMA's north-facing line is about moss, not algae, and why the makers disagree is set out on why roof algae grows on the north side.

On painted metal, the sunnier slope can go lighter rather than the shaded one going darker, and the mechanism is not biological.

U.S. Steel's guide to cleaning painted metal buildings describes chalk as a natural white by-product of ultraviolet degradation of the paint system, a light film that builds over several years, makes colors appear lighter and reduces gloss. Chalking increases with greater exposure to the sun, the guide says, so the roof, the south side of a building and buildings in southern climates may chalk more, and chalking is more noticeable on darker colors than on lighter ones. The same guide puts mildew on the other side of the pattern, on surfaces that remain moist for extended periods such as north walls, under eaves or sheltered areas. What UV degradation means is defined on its own page, and chalk across roof materials is answered on why a roof looks chalky.

A roof does not need a compass to explain a darker slope when a canopy sits over just one part of it.

ARMA writes that overhanging tree branches provide additional shade and drop debris that holds in moisture and acts as a food source for moss. TRI Alliance lists the quantity of trees in the area among the variables that affect growth on tile, alongside the direction the roof faces. A slope under a canopy the other slope does not have can darken whichever way the house points. That mechanism in more detail is on why a roof gets dirty under the trees.

One more document bears on color, and it is about individual tiles, not whole slopes.

Eagle Roofing Products' Florida owner information, as archived, says that on single-color roofs there will be a variation in hue from one tile to another, because concrete tiles are made from natural raw materials that vary in color, and that the variation tends to be less noticeable as the roof ages. Eagle also says it does not warrant color due to the tile's exposure to the elements. That describes a tile-to-tile patchwork on Eagle's concrete tile, not one slope darker than another, and no source read for this page says it explains a whole slope. What Eagle's text means for a given roof is a question for that text and for a roofing contractor.

The three looks can overlap on one roof, and telling them apart from the ground or from a photo is not something a cleaning settles.

A cleaner is not an inspector, and what an exterior cleaner is not sets out that boundary. Whether a darker slope also runs warmer is a separate question with its own limits, answered on whether a dirty roof makes a house hotter. Which pattern explains one side of a given roof is a licensed roofing contractor's call.