Black streaking, green film, and lichen patches are surface growth and respond to cleaning. Stains that follow a flashing line, sit in a defined patch below a penetration, appear as a darker area with visible granule loss, or show up on the ceiling inside are damage indicators. Cleaning a roof that is leaking does not fix the leak, and it delays the diagnosis.
Key Takeaways
- Growth spreads along water flow paths; damage stains concentrate at a fixed point
- Granule loss visible as a shinier or darker patch is wear, not dirt
- Any interior ceiling stain moves this from a cleaning question to a roofing one
- Stains around chimneys, skylights, and valleys are flashing suspects until proven otherwise
- A cleaning contractor should tell you when the answer is a roofer
Cleaning Problem or Repair Problem
Most roof discoloration in this region is biological and cosmetic. Black streaking is algae, green film and cushions are algae and moss, crusty gray-green or orange patches are lichen. All three sit on the roof rather than in it, and all three respond to a properly applied low-pressure wash.
A smaller set of stains means something else. They tend to be localized rather than spreading, tied to a specific feature rather than to the slope, and they often have a corresponding sign somewhere else, in the attic, on a ceiling, in the gutter.
The distinction matters for a practical reason beyond money. Washing a roof that has an active water problem removes the visual evidence that would have helped diagnose it, and it puts several hundred gallons of water onto a surface that is already failing to keep water out.
What Roof Stains Can Indicate
Spreading dark streaks down the slope. Algae. Cosmetic. Cleaning is the answer. See our guide on identifying roof algae, moss, lichen, and black streaks.
Green cushions at the laps and in valleys. Moss. Holds water against the shingle and lifts edges as it grows. Cleaning is the answer, and sooner rather than later.
A defined dark patch below a chimney, skylight, vent, or valley. Suspect flashing. Water is being concentrated and shed onto one area, or getting past a joint. This is a roofer's question first.
A patch that looks shinier, smoother, or a different shade than the shingle around it. Granule loss. The protective granule layer has worn away, exposing the asphalt mat. Cleaning will not restore it and aggressive cleaning accelerates it.
Rust-colored streaking below a fastener, vent, or metal component. Corrosion bleed. Cosmetic on the roof surface, but it tells you a metal component is failing.
A dark, wet-looking area that stays darker than the rest after several dry days. Retained moisture, which suggests the deck below is holding water.
Sagging, waviness, or a soft-looking plane. Structural or deck problem. Nothing to do with cleaning.
White or light powdery deposits on masonry chimneys. Efflorescence, indicating moisture moving through the masonry.
Visual Checks You Can Make From the Ground
All of this is assessable with binoculars and a phone camera on an overcast day. Do not climb.
- Does the stain spread or concentrate? Growth spreads and follows the flow path. Damage concentrates around a fixed feature.
- Is there a penetration directly above it? Chimney, skylight, vent stack, satellite mount, dormer. If yes, treat the flashing as the suspect.
- Does the affected area look textured or smooth? Growth adds texture. Granule loss removes it.
- Compare elevations. Growth concentrates on the shaded side. A stain on the sunny side is more likely to be something else.
- Check the gutters and the ground below. Granules accumulating in the gutter or at the downspout outlet mean the shingle surface is shedding, which is wear rather than dirt.
- Look at the eave line and fascia. Staining, peeling paint, or soft wood at the eave often means gutter overflow, which is a gutter problem showing on the roof.
- Go inside. Check the attic in daylight for light coming through, dark staining on the sheathing, or damp insulation. Check ceilings in the rooms below.
- Look at the ridge and hip lines. Missing or lifted cap shingles are a repair item, not a cleaning one.
When to Pause Cleaning and Call a Roofer
Stop and get a roofing assessment before any cleaning if you see:
- Any interior ceiling or wall stain, current or historic, below the affected roof area
- Daylight visible through the attic, or dark staining on the underside of the sheathing
- Missing, cracked, lifted, or curled shingles
- Exposed or lifted flashing at a chimney, skylight, valley, or wall junction
- Visible granule loss over an area rather than at isolated spots
- Sagging or waviness in the roof plane
- Soft or rotted fascia, soffit, or eave trim
- A roof at or past its expected service life, where cleaning is money spent on a surface being replaced soon
None of these rule out cleaning permanently. They change the order. Repair first, then clean, or in some cases replace and skip the cleaning entirely. A contractor who takes the cleaning job without mentioning any of this is not doing you a favor.
How a Cleaning Professional Assesses Access, Materials, and Runoff
When the answer is cleaning, the assessment should still cover three things beyond the stain itself.
Material. Asphalt architectural, three-tab, slate, cedar shake, metal, synthetic, or tile. Each takes different chemistry and different handling. Slate and cedar in particular are their own problems; see why slate roof cleaning needs a different approach.
Access. How the roof gets treated without anyone walking the field. Pressure washing a roof is never appropriate, and walking a wet, mossy, or steep roof is both unsafe and damaging. Ladders, staging, lift, or aerial assessment depending on the property.
Runoff. Everything applied to a roof ends up on the walls, windows, walkways, and plantings below it. That needs a plan: pre-wetting landscaping, protecting or rinsing the elevations beneath, and accounting for where the water finally goes. A roof clean that leaves streaked siding has traded one problem for another.
Questions to Ask Before Approving Work
- What is your read on this stain specifically, and what makes you confident it is growth rather than damage?
- Will anyone walk on the roof, and if so where and how?
- What pressure and what solution, and how does that suit this roof material?
- What happens to the siding, windows, and plantings below during the wash?
- What will the roof realistically look like afterward? Lichen in particular often does not fully clear
- If you find damage once you are up close, what happens?
- Does this roof have enough remaining service life to be worth cleaning?
That last question is worth asking plainly. A roof three years from replacement is usually not worth the spend, and a contractor willing to say so is one worth using on the roof you replace it with.
Request a Roof Assessment
WashLabs Exteriors assesses roof staining before proposing anything, uses low-pressure soft washing rather than pressure on any roof surface, and will tell you plainly when what you are looking at is a repair rather than a cleaning job.
Call (610) 665-3995 or request a free assessment. See our roof cleaning and gutter cleaning services.