On an asphalt shingle roof, the Lehigh Valley’s most common roofing material, humidity does its harm from two sides. Outside, damp that lingers on a shady slope lets algae and moss grow. Inside, moisture from the house works its way up into the attic, and when it meets a cold surface it turns back into water under the roof. That inside moisture is often a cause of roofing problems, and you can’t see it from the yard.
Streaks and moss on the shady side
Algae can form where moisture tends to stay on a roof, and coastal and humid climates suit it best. It usually starts as small spots and soon spreads into streaks, so by the time you notice it, it has likely been there for several months or more. The spores travel on the wind and on animals. That’s why the same dark streaks often turn up on roof after roof in a neighborhood, row houses included. They show most on light-colored shingles.
If you chose reflective “cool” shingles to keep the attic cooler, algae is more than an eyesore. On a reflective roof it counts against how well the roof works over the long run.
Unlike algae, moss can hurt how the shingles themselves perform. Both need damp, so trim back the branches over the roof. That lets the shingles dry, and a drier roof helps keep algae, moss and lichen down. Our shingle roof lifespan guide covers what each growth does to the shingles, and why a pressure washer should never be used to clean them off.
Damp coming up from the house
Cooking, baths and laundry all put water vapor into the air indoors, and some of it migrates up into the attic. In winter, warm and moist air tends to rise through the ceiling, especially through the gaps where wiring and pipes pass through it. Then, in a cold attic, it condenses on the rafters, the nails and the underside of the deck. That water can corrode the nails and rot the wood, and in some cases mold may begin to grow.
The shingles can show it too. As the deck takes on moisture and dries out again, the wood can swell and shrink, and the shingles on top can buckle. Buckling from a damp deck tends to run in lines that follow the joints in the deck. On a deck made of boards rather than sheets, each board can move on its own as its moisture changes. A bump here and there in no pattern usually points to something else, such as wrinkled underlayment or a nail that wasn’t driven flush.
Summer adds heat to the damp. An attic that holds its heat may age the shingles above it faster. The lifespan guide also goes through what else shortens a roof’s life.
Keeping the attic dry
Start where the moisture starts. Vent the clothes dryer and the stove to the outside where you can, and run the bathroom fan when you shower. Then check where each fan’s duct ends. One that stops in the attic just moves the bathroom’s steam up under the roof.
Sealing the gaps around wiring and pipes, and around the attic hatch, can cut down how much damp air gets up there.
The rest comes down to ventilation. A well-vented attic is drier in winter and cooler in summer. The vents themselves, and what keeps them from working, are covered in types of roof ventilation systems for homes.
Frost on the nails and other signs of damp
Look from the yard, since nobody but a roofer should be up on the roof. Binoculars can pick out streaks, moss on the shady slopes and any wavy line running across the shingles.
If it’s safe to get into the attic, go up on a cold winter morning, when the damp shows the most. Watch for:
- Frost on the tips of the roofing nails that poke through the deck
- Rust on those same nails
- Insulation that feels damp or has packed down
- Dark stains or mold on the underside of the deck
- A musty smell
Any of them can mean moisture is getting into the attic faster than the air carries it out.
If you can’t tell whether it’s condensation or a leak, start with a roof inspection, because it takes in the attic and not just the shingles. Call (484) 553-0213 to book it, or send the details through our contact page.