Can You Pressure Wash a Roof? Short Answer: No

No. You can point a pressure washer at a roof and it will look better for a season, but asphalt shingles are not built to take it. The pressure strips off the mineral granules that protect the asphalt underneath, and it drives water up under the shingle courses where it has nowhere to go. Neither one shows up the day you finish, which is why the job keeps getting done.

What the pressure does to a shingle

An asphalt shingle is a mat saturated in asphalt with crushed mineral granules pressed into the top face. Those granules are the sunscreen. They take the UV so the asphalt underneath stays flexible instead of drying out and cracking. Any pressure high enough to move algae off the surface is also high enough to move granules, and after a roof gets washed that way you can usually find a pile of them at the bottom of the downspout. They do not grow back.

The second problem is direction. A roof sheds water one way, down the slope, and every course overlaps the one below it. Spraying up the slope pushes water backward under that overlap, past the point the shingle is designed to seal. On a Minnesota house that water goes into a cold cavity in October and is still there in March.

What the black streaks actually are

The dark streaks running down a shaded roof slope are not dirt and not mold. They are an algae that feeds on the limestone filler in the shingle. It spreads downhill with runoff, which is why the streaking runs in vertical bands and why it is always worst on the north slope and under overhanging trees. Same organism family, same damp-and-shaded conditions, as the green that grows on the north side of a house.

It is a cosmetic problem. Algae streaking does not make a roof leak and does not take years off it. If the streaks do not bother you and you are not about to list the house, leaving them alone is a legitimate answer.

What takes it off without wrecking the roof

Chemistry, not force. The method that does not damage shingles is a cleaning solution applied at roughly garden hose pressure, given time to work, then rinsed. The solution kills the algae and the roof fades back over the following weeks rather than coming clean in an afternoon. That is the trade-off, and it is the right one.

It also comes back. Nothing applied to a roof keeps algae off permanently in a climate that stays damp from October through April. The usual way to slow it down is a strip of zinc or copper run along the ridge, so every rain carries a small amount of metal down the slope. Houses that already have a galvanized ridge cap often have a clean band under it for exactly that reason.

This is not a job I take

Being straight about it: I clean windows, gutters and screens, and I pressure wash flat exterior surfaces. I do not wash roofs. If your roof is streaked badly enough that you want it dealt with, you want somebody who soft washes roofs specifically and is set up to work at that pitch. Hiring a pressure washing company to do it is how roofs get damaged, because the tool they bring is the wrong one for that surface.

One thing worth checking while you are looking up there: a roof growing algae is usually a roof over gutters holding grit and shingle granules. That part is ordinary maintenance, and it matters more than the streaking does. More on that in gutter cleaning.

What a pressure washer is right for

Hard, flat, non-layered surfaces. Concrete driveways, sidewalks, patios, steps, retaining walls, fences. Those take pressure well and there is no overlap for water to get pushed under. Siding is a middle case: it gets washed with solution at low pressure, not blasted, for the same reason a roof does. The rundown of what gets which treatment is on the pressure washing page, and the local version is pressure washing in St. Paul.

If you want concrete or siding looked at before the snow comes, you can get a quote or book a visit.