You walk outside one day and notice dark streaks, green patches, or something that looks like moss growing across your shingles. Your first thought might be: Is this actually hurting my roof, or does it just look bad?

The answer depends on what’s growing up there.

Algae is often more of an appearance issue, while moss can become a bigger concern because of the way it grows on and between asphalt shingles. Either way, you shouldn’t automatically assume your roof needs to be replaced—and you definitely shouldn’t grab a pressure washer and try to clean it yourself.

At Barringer Brothers Roofing, we’ve served homeowners throughout the Metro East since 2004 and have earned 200+ five-star reviews. Here’s what homeowners should know about moss, algae, dark streaks, and other growth that can show up on an asphalt roof.

Is It Moss or Algae?

Although homeowners often use the terms interchangeably, moss and algae aren’t the same thing.

Algae commonly appears as dark or black streaks running down sections of a roof. It tends to be more noticeable on lighter-colored shingles and may make an otherwise healthy roof look older or dirtier than it really is.

Moss is thicker and more three-dimensional. It can appear as green clumps or patches growing across the shingles.

That difference matters because moss can physically interact with the shingles as it grows, while algae is often primarily a cosmetic concern.

Why Are There Black Streaks on My Roof?

Those black streaks aren’t necessarily dirt.

A common cause of dark streaking on asphalt shingles is algae growth. It tends to thrive in areas where moisture sticks around longer, which is why you might notice considerably more staining on one section of your roof than another.

A roof can have noticeable algae staining without having a leak or needing immediate replacement.

That said, if you’re unsure whether you’re looking at algae, aging shingles, granule loss, or another roofing issue, it’s worth having the roof checked before making assumptions.

Why Does Moss Grow on Some Roofs?

Moss likes moisture and shade.

Homes surrounded by mature trees can be particularly susceptible because portions of the roof may receive less direct sunlight throughout the day. Leaves and other organic debris can also collect on the roof and hold moisture.

You may notice moss developing more heavily on one side of your home simply because that section stays damp longer.

This doesn’t necessarily mean anything was wrong with the original roof installation. The environment surrounding your home can affect how your roof looks and ages.

Is Moss Actually Bad for Asphalt Shingles?

Moss is something homeowners should pay attention to.

As moss becomes established, it can grow around and underneath the edges of shingles. It also retains moisture, meaning affected areas of the roof may stay damp longer.

Over time, heavy moss growth can interfere with the way shingles are designed to shed water and may contribute to deterioration.

A tiny patch of moss doesn’t automatically mean you need a new roof, but significant or widespread growth shouldn’t simply be ignored.

Does Algae Damage a Roof?

Algae is generally less concerning than moss and is often primarily an appearance issue.

Those dark streaks can make a relatively good roof look much older than it actually is, which understandably worries homeowners.

That’s one reason it’s important not to judge the condition of an asphalt roof based only on its color from the driveway. Staining and actual shingle deterioration aren’t necessarily the same thing.

If the shingles themselves are still in good condition, visible algae doesn’t automatically mean the roofing system is failing.

Should I Pressure Wash My Roof?

No.

It might seem like the fastest way to make stained shingles look new again, but asphalt roofing isn’t a driveway or concrete patio.

Aggressive pressure washing can damage shingles and remove protective granules from their surfaces. Walking around on the roof with cleaning equipment also creates an unnecessary safety risk.

If you’re concerned about moss or algae, talk to a professional about the condition of the roof and appropriate cleaning options rather than blasting the shingles with a pressure washer.

Can I Scrape Moss Off My Shingles?

This is another DIY fix that can cause more harm than homeowners realize.

Scraping, brushing, or aggressively pulling moss from shingles can damage their surfaces. If moss has worked underneath shingle edges, forcing it off can potentially disturb the shingles themselves.

The goal shouldn’t simply be to make the green stuff disappear as quickly as possible. You want to address it without damaging the roofing material underneath.

Why Is Moss Only Growing on One Side of My Roof?

This is actually pretty common.

Different sections of the same roof experience different conditions throughout the day.

One side may receive hours of direct sunlight while another remains shaded by the house, nearby trees, or surrounding structures. The shaded side may stay damp considerably longer after rain or morning dew.

That’s why one roof slope can develop moss or algae while another looks completely clean.

It can also explain why certain sections of a roof appear to age differently even though all of the shingles were installed at the same time.

Do Trees Make Moss and Algae Worse?

They can contribute to the conditions that allow growth to thrive.

Large trees can shade the roof, reduce the amount of sunlight reaching the shingles, and drop leaves, twigs, and other debris onto the roof.

That doesn’t mean you need to remove every tree around your house. It simply means heavily shaded homes may require a little more attention.

Keeping branches from directly contacting the roof and preventing excessive debris from accumulating can help reduce some of those conditions.

Does Moss Mean I Need a New Roof?

Not necessarily.

The presence of moss alone doesn’t tell you whether a roof needs replacement.

A roofing contractor should consider the actual condition of the shingles and roofing system, including the age of the roof, amount of growth, condition of the shingles, previous damage, and any other signs of deterioration.

If the roof is otherwise in good condition, replacement may not be necessary.

If heavy moss is growing across an older roof that’s already showing significant wear, however, the conversation may be different.

What If My Roof Looks Dirty but Isn’t Leaking?

Don’t assume a dirty-looking roof is a failing roof.

Algae staining can dramatically change the appearance of shingles without necessarily causing an active roofing problem.

On the other hand, the absence of a leak doesn’t guarantee that every part of the roof is in perfect condition.

If you’re concerned about something you’re seeing, an inspection can help separate an appearance issue from an actual roofing issue.

When Should I Have It Checked?

Consider having your roof looked at if:

• Moss growth is becoming widespread
• Shingles appear lifted around the moss
• You’re noticing significant shingle deterioration
• The roof is older and you’re unsure of its condition
• You see missing or damaged shingles in addition to the growth
• You’re unsure whether the discoloration is algae or something else
• You’ve noticed signs of water intrusion inside the home

You don’t have to diagnose the roof yourself. If something has changed enough that you’re noticing it from the yard, it’s reasonable to have a professional take a closer look.

Frequently Asked Questions

Not Sure What’s Growing on Your Roof?

You shouldn’t have to climb onto your roof to figure it out.

If you’ve noticed moss, dark streaks, unusual discoloration, or another change in the appearance of your shingles, Barringer Brothers Roofing can take a closer look and help you understand the condition of your roof.

We’ve been serving homeowners throughout the Metro East since 2004 with roof inspections, repairs, and asphalt roof replacements.

Call (618) 703-3036 or fill out our online contact form.