You walk past the side of the building after a wet weekend and notice it for the first time. A pale, almost dusty film smeared across the brick about three feet up from the ground. It wasn't there last month. You run your finger across it and it leaves a chalky white streak on your fingertip — like sidewalk chalk, but finer. Brush at it with your hand, and most of it falls off. A week later, it's back, in roughly the same place, after the next rain dries off.

You drive past the office building every morning. Black streaks run halfway down the north wall, dark stains spread under every HVAC penetration, and a green tint creeps up from the foundation. Then you turn the corner into your subdivision. The houses look fine. Same neighborhood, same humidity, same weather. The office building always looks worse.

You paid for a pressure wash. The crew showed up, blasted the roof, the siding, the patio — the whole property looked like a different home by the time they pulled out of the driveway. Six weeks later, the green tint is back on the north side of the house. Eight weeks in, the patio is starting to feel slick again. By the third month, the roof streaks are visible again from the driveway.

It's the first warm Saturday in April. You are standing in the driveway looking up at the roof, and they are back — those long dark streaks running straight down the shingles. They were not this bad last year. You can tell by how far they reach now, past the chimney, almost to the gutter line. The neighbor's roof has them, too. So does the one across the street.