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Roofing Tips, Tricks and Ideas

Mold, Mildew and Algae Resistant Roofing

Published by Pickard Roofing at

In warm and humid climates, such as North Carolina’s, roof algae is a prevalent and sometimes persistent problem. If you notice stains on your building’s roof or other signs of algae, it’s important to know what issues the algae growth does and doesn’t present.


What is Roof Algae?

Roof algae is algae growth that forms on the building or a roof. The growth is an algae organism, similar to what you’d find in a pond or on an uncleaned fish tank, but it develops on the roof of a building instead. Like pond and fish tank algae, roof algae requires a lot of warmth and water to form. Thus, it’s most common in warm and humid locations like Raleigh, NC. Roof algae may or may not impact your roof. In addition, the solution may be different for sloped and low-sloped or flat roofing materials.  


Effects of Roof Algae and Mildew on Your Sloped Shingle Roof

For a sloped shingle roof, it’s more common to see algae on the east side of a building, because the sun deters algae growth and shines more on south and westward facing roofs. Shaded areas where large tree overhangs the roof can also be a common area.  If drainage of the sloped roof is adequate and there is not any ponding of water on the roof, algae may not be impacting the current performance of the roof. It may come down to aesthetic appearance only. An experienced roofing contractor will be able to provide guidance to any potential issues. 

Algae Should Never Be Pressure-Washed Off of a Shingle Roof

As for solutions, many building owners first turn to pressure washing. This shouldn’t be done under any circumstances with shingle applications, as it causes a few issues.

First, pressure washing doesn’t kill algae but merely displaces it. More specifically, pressure washing spreads algae all over a roof and the organism can begin to grow in other places.

Second, pressure washing can remove surface granules on a roof -- particularly on a shingle roof. This can weaken the roof and ultimately, shorten the roof’s life span.


Effects of Roof Algae and Mildew on Your Low-sloped or Flat Roof

TPO roofs  are made with thermoplastic polyolefin membrane that covers the surface of the roof. These roofs are commonly used both in commercial and residential settings where a building’s roof is low sloped or flat. When algae forms on a TPO roof, there may be areas where water is pooling due to structural design and/or an actual roof drainage problem. Your professional roofing contractor will be able to assess the root cause of the water ponding and recommend a solution. The roofing contractor will also be able to recommend the best way to clean the roof of the algae and mildew. 


Other Effects of Roof Algae and Mildew on Your Roof

Most importantly, any visible algae or mildew growth on a roof is likely extremely slippery. Algae growth is generally slippery to walk on, but it’s especially important to avoid if you’re walking on a surface that’s high above the ground -- and especially if that surface is sloped.

Additionally, when the algae dark spots are highly visible, it can lower a building’s property value. Whether and how significantly algae affects a building’s value depends on the overall condition of the roof, the general aesthetic of the building, and the location of the roof algae and mildew.


Structural Effects of Roof Algae and Mildew

Roof algae and mildew don’t necessarily pose the same toxicity dangers as some molds do, but they still can be a sign of a bigger problem.

To accurately assess algae’s impact on a roof’s structure and performance, building owners should contact a roofing professional. Only a professional has the expertise to determine whether algae is only aesthetically displeasing or is actually pointing to a structural issue in a roof.

Professional Roofing Solutions

A roofing professional can recommend a solution that will work for a specific roof algae problem. Depending on the specifics of an issue, a professional might recommend addressing drainage or pooling issues, cleaning a roof without pressure washing, clearing trees, and installing algae-resistant roof shingles (for shingle roofs).

Algae-resistant shingles are non-toxic shingles that use copper granules to reduce algae, mildew and mold growth. They’re an affordable and low-maintenance solution in many cases.

To speak with an expert roofer, contact us. We’ve helped many residential and commercial building owners in the Raleigh region, and we’re familiar with the best roofing materials available. If your roof has algae growth, our professionals will know how to address the problem and prevent it from coming back.


At Pickard Roofing, we think Design. Craft. Renew. We are experienced in all types of roofing systems- and provide the right roofing design and selective craftsmanship for renewing our customer’s assets. We invite sustainable building customers to give us a call for your free evaluation. For over 90 years Pickard Roofing Company has installed countless metal, tile, slate, TPO, EPDM, and all types of shingle roofing in the Triangle. Our services include residential, commercial, custom architectural metal fabrication, and historical renovation.