Those black streaks are algae, not age

What is actually growing up there

The dark streaks that run down a roof in vertical bands are a blue green algae. It feeds on the limestone filler used in asphalt shingles, it spreads by spores carried on the wind, and it holds moisture against the roof surface once it takes hold. That last part is the reason it is worth dealing with rather than ignoring.

Two things about it surprise people. First, it is not a sign the roof is old or failing. A roof five years into its life can streak badly if it faces north and sits under trees. Second, it spreads from house to house down a street, which is why one block will have it and the next will not.

Why pressure is never used on a roof

An asphalt shingle is a mat coated in asphalt with a layer of mineral granules pressed into the surface. Those granules are the roof. They shield the asphalt from ultraviolet light, and once enough of them are gone the asphalt underneath starts to harden and crack.

A pressure washer strips granules. Not gradually, immediately, and you can watch them wash into the gutter while it happens. That is why roofs are soft washed instead: a cleaning solution applied at low pressure kills the growth, and then weather takes the residue away over the following weeks. The roof keeps getting cleaner after Chris has left.

What is included

• The full roof plane, not only the elevation visible from the street
• Valleys and the areas under overhanging trees, where growth is always heaviest
• Gutters and downspouts rinsed so the residue is carried off rather than left sitting
• Landscaping wet down and rinsed before and after, same as a house wash
• A walk of the property afterwards so nothing is left where it should not be

Doing it alongside the house wash

Most people book the roof and the house together, and there is a practical reason beyond convenience. Roof runoff carries residue down onto the siding, so washing the walls first and the roof second means doing the walls twice. Chris works top down for that reason. If you are considering both, say so when you call and it gets sequenced properly.