Black streaks running down an asphalt roof can look like water damage, burned shingles, or a sign that the roof is failing. On many Creve Coeur houses, the streaks are algae growing across the surface of the shingles. That distinction matters because algae is mainly an appearance and maintenance issue. A leak, missing shingle, or worn-out roof calls for a different response.
The growth is commonly called blue-green algae, although it usually looks black once it spreads across a roof. It tends to be easier to see on light gray, tan, and weathered shingles. The streak pattern develops because rain carries the growth and its residue down the slope.
You do not have to decide what it is from the driveway. A close inspection can separate surface staining from damage to the roofing assembly. The useful question is not simply whether the roof looks dirty. It is whether the shingles beneath the staining are still lying flat, holding their granules, and keeping water out.
Town roofing
Creve Coeur gives roof algae several places to settle. Mature trees shade sections of many houses, and neighboring roof planes can block direct sunlight for part of the day. Shaded shingles stay damp longer after rain, dew, and melting frost. Organic debris in valleys or behind chimneys can hold additional moisture against the roof surface.
The pattern often appears more heavily on a roof plane that receives less sun, but orientation alone does not settle the diagnosis. A shaded section beneath an oak may stain while a nearby slope remains clear. Metal runoff can also interrupt the growth. Clean-looking strips below copper or zinc flashing do not mean those shingles are newer. Small amounts of metal washing down the roof can discourage algae in that path.
Algae staining is different from moss. Algae generally creates flat discoloration. Moss forms a thicker growth that can lift shingle edges and trap water. Lichen often appears as crusty, pale, or green patches attached to the granule surface. Pulling moss or lichen away by hand can remove granules with it, so identification should come before cleaning.
St. Louis weather adds another concern. Water held around raised growth can freeze, thaw, and freeze again. That does not mean one patch of moss has ruined the roof, but repeated moisture and movement are reasons not to let established moss sit indefinitely. Ice damming at an eave is a separate condition caused by heat loss, snow melt, and refreezing. Cleaning a stained roof will not correct an attic or insulation problem that produces ice at the edge.
Black staining also should not hide other defects. Look separately for shingles that are cracked, curled, missing, or sliding out of position. Check pipe boots, wall flashing, valleys, chimney flashing, and the transition between a main roof and an addition. Hail damage is not diagnosed by the presence of algae. It requires inspection of the shingle surface and other exterior components for a consistent damage pattern.
Cleaning without shortening the roof's life
Pressure washing is the wrong starting point for an asphalt shingle roof. A concentrated stream can strip protective granules and force water beneath shingle edges. Aggressive brushing can produce similar damage. A person walking across a warm, brittle, or steep roof can also break tabs while trying to improve its appearance.
A cleaning proposal should identify the solution, how it will be applied, and how plants, painted surfaces, gutters, and runoff areas will be protected. It should also state whether the roof will be walked and how workers will reach steep or high sections. Shingle manufacturer instructions matter here. A cleaner that ignores those instructions can create a larger problem than the staining.
Do not expect every stain to disappear the moment a treatment is applied. Depending on the approved process, dead growth and residue may release gradually through later rain. Heavy moss may require staged treatment rather than scraping. Ask the cleaner to explain the expected result before the work begins.
What it costs
The first cost decision is whether cleaning is worth doing on the roof you already have. Cleaning a sound roof can improve its appearance and remove growth that holds moisture. Cleaning a roof near the end of its useful life may spend money on shingles that will soon need replacement for unrelated reasons.
Age is only one part of that decision. A three-tab roof that has become brittle, has widespread granule loss, and no longer tolerates lifting for a repair may not be a sensible cleaning candidate. Once an older three-tab shingle breaks when a roofer gently lifts the surrounding tabs, even a small repair can spread into a larger disturbed area. A younger architectural shingle roof with intact edges and uniform granule coverage presents a different calculation.
The cleaning price changes with roof size, pitch, height, access, number of stained slopes, and the amount of moss or lichen present. Landscaping protection and controlled runoff can add labor. A house with one accessible stained plane is a different job from a two-story house with dormers, valleys, and steep rear slopes.
Ask for cleaning and roofing work to be priced separately. A vague package can make it difficult to tell whether you are paying for treatment, repairs, or both. If repairs are recommended, the written scope should name the location and defect. Examples include a split pipe boot, loose flashing, missing tab, or exposed fastener. The words roof treatment do not describe a repair.
Replacement should not be proposed solely because dark streaks are visible. It becomes a reasonable discussion when inspection finds broader conditions such as extensive cracking, repeated failed repairs, widespread loss of granules, brittle shingles, deteriorated flashing, or decking problems. If replacement is appropriate, Keys installs Atlas and Malarkey shingles. The proposal should identify the selected product and the complete roof assembly rather than listing only a color.
What to ask
Start by asking the inspector to show you close photographs. One wide photograph establishes the roof plane. Detail photographs should show the surface of the stained shingles, their lower edges, and any moss or lichen attached to them. Photos of flashing and penetrations help separate the staining question from possible leak paths.
Then ask direct questions:
- Is this flat algae staining, raised moss, lichen, or actual shingle deterioration?
- Are the shingles still flexible enough to walk and work around without breaking?
- Is granule loss limited to normal aging, or are there exposed asphalt areas?
- Does the cleaning method follow the written instructions for this shingle?
- How will landscaping, siding, metal, and roof runoff be protected?
- Are any repairs being recommended independently of the staining?
- Will I receive photographs and a written scope before deciding?
Also ask what will prevent the same conditions from returning quickly. Trimming branches may increase sunlight and reduce falling debris, but tree work should be based on clearance and tree health, not a promise that algae can never return. Keeping valleys clear and making sure gutters drain freely can shorten the time water remains around the roof edge. Zinc or copper accessories may be considered in some installations, but their placement, compatibility, and runoff staining should be discussed before anything is fastened through the roof.
If the staining appeared after a storm, do not assume the storm created it. Algae usually develops over time, although wet weather can make it more visible. A storm inspection should document storm-related conditions separately from age, maintenance, and biological growth. That keeps one dark roof plane from turning into an unsupported conclusion about the whole roof.
For a Creve Coeur roof inspection, call Keys Roofing at (314) 220-2333. Ask for photographs of the stained surface, the surrounding shingles, and any condition that is being recommended for repair. The next step should follow what those photographs show: leave sound shingles alone, clean suitable shingles correctly, repair a defined defect, or price replacement when the roof itself is no longer a practical repair candidate.