On homes shaded by pines and hardwoods, or on roof slopes that face north and receive little direct sunlight, we often see algae and moss begin to develop on the shingles. These areas stay damp longer after rain and morning dew, which creates the conditions both types of growth need.
Algae usually appears as dark or black streaks running down the roof, while moss forms thicker green patches that can build up around the edges of shingles. Although they are often discussed together, they affect a roof in different ways.
Algae is generally more of an appearance issue on asphalt shingles, while established moss can grow around and beneath shingle edges and interfere with how water drains from the roof.
In this guide, we’ll explain how to tell moss and algae apart, what causes them to grow, whether they can damage your roof, and how to remove them safely.
Is It Moss, Algae, or Lichen on Your Roof?
Algae is usually what causes the dark or black streaks you see running down asphalt shingles.
It develops on parts of the roof that stay damp longer, including shaded slopes and areas beneath or near tree cover. On some homes, one side of the roof can have noticeable staining while the sunnier side remains relatively clean.
Moss is easier to recognize because it forms raised green growth rather than flat staining. It often starts in shaded areas, along shingle edges, or in places where leaves and other debris collect and hold moisture against the roof.
You may also see lichen, which typically appears as flat or crusty patches in shades of green, gray, white, or yellow. Lichen attaches more firmly to the shingle surface and can be harder to remove without disturbing the roofing material.
All three are more likely to develop where the roof receives limited sunlight or takes longer to dry after rain and morning dew. If you have overhanging branches or accumulated debris, these areas are more favorable for their growth.
Can Moss and Algae Damage Your Roof?
Algae on asphalt shingles is usually more of an appearance issue than a sign of roof damage.
The Asphalt Roofing Manufacturers Association states that there is no scientific evidence that algae damages asphalt shingles, although the dark streaks can become widespread and make the roof look older or dirtier than it is.
Moss can become more concerning as it gets thicker and spreads across the roof. If it grows around and beneath the edges of shingles, it can cause them to lift and create places where water can move underneath the roofing material.
Thick moss, leaves, and other debris can also keep parts of the roof damp for longer periods. If moss is growing on shingles that are already curled, cracked, loose, or deteriorated, the roof may need more than cleaning alone.
A small amount of moss or algae does not necessarily mean the roof needs to be replaced. The condition of the shingles underneath the growth is usually more important when determining whether the roof only needs cleaning or whether repairs may also be necessary.
How to Remove Moss and Algae From a Roof
We use different methods to remove moss and algae without damaging the shingles.
For algae, the Asphalt Roofing Manufacturers Association recommends applying a bleach-and-water solution gently to the affected shingles, allowing it to remain on the surface for about 15 to 20 minutes, and then rinsing with low-pressure water.
Do not scrub or pressure-wash the roof, since either method can loosen the protective granules from asphalt shingles.
Moss may take longer to come away from the roof. Once treated, the dead growth will gradually loosen and can wash away with rain or be removed using a non-abrasive method.
Pulling or scraping established moss from the shingles can damage the surface or disturb shingles that have already started to lift.
Cleaning also does not necessarily keep the growth from returning. If tree branches heavily shade the roof, trimming them back where practical can allow more sunlight and airflow to reach the shingles.
Keep leaves, pine needles, and other debris from accumulating on the roof, and keep gutters clear so water can drain properly.
If algae has been a recurring problem and the roof is approaching replacement, algae-resistant shingles are another option.
These shingles are manufactured with technology designed to inhibit algae growth rather than relying on treatments applied to an existing roof.
If moss has spread across large areas of the roof, look for shingles that are lifting, curling, cracked, loose, or missing. Established moss can lift shingle edges, and severe buildup can allow water to move laterally beneath the roofing rather than continuing down the roof as intended.
If you see signs of a leak, damaged flashing, deteriorated shingles, or extensive moss on an older roof, these are also reasons to have the roof inspected rather than treating it only as a cleaning problem.
In those cases, some areas may need repair even after the moss or algae is removed.
Have Moss or Algae on Your Roof?
Moss and algae are common on roofs that stay shaded or damp, but the type of growth and the condition of the shingles underneath determine how much attention the roof needs.
If you see heavy moss growth, lifted shingles, or staining and aren’t sure whether the roof simply needs cleaning or if there may be damage underneath, SureFlow Roofing can inspect the roof and help you determine the best next step.
Contact us or call (864) 903-7873 to schedule a roof inspection or request a quote.