Understanding Exterior Mold and Growth on Siding, Roofing, Concrete, and Wood Surfaces in the Pacific Northwest
- PacWest Pressure Washing
- May 10
- 5 min read
Your guide to identifying and tackling the most common organic growths that threaten buildings and surfaces in the Pacific Northwest.
The Pacific Northwest (PNW) is known for its lush landscapes, consistent rainfall, and mild temperatures. But these beautiful conditions also create an ideal environment for various types of mold, mildew, algae, lichens, and moss to grow on a building’s exterior. Left unchecked, these growths can cause aesthetic damage, degrade building materials, and contribute to health problems. This guide will help you identify different types of exterior growths commonly found in the PNW, explain the species involved, and provide tips for prevention and treatment.
Types of Exterior Mold and Growth
1. Algae
Algae are simple, photosynthetic organisms that often form slimy, green, or black streaks on roofing, siding, and other exterior surfaces. Algae thrive in shaded, moisture-rich environments, making the PNW climate especially favorable for their growth.
Examples of Algae Species:
Gloeocapsa Magma
This black algae species is commonly found on roofing, appearing as dark streaks that accumulate over time. It thrives in areas with high moisture and limited sunlight.
Chlorella Vulgaris
This green algae species forms thick mats on shaded surfaces such as siding, concrete, and roofing. Left untreated, it can discolor and damage surfaces.
Trentepohlia
This red or orange algae species is typically found on damp, shaded exteriors. It can appear on roofing, siding, and other surfaces, giving them a stained or discolored look.
2. Mold
Mold is a type of fungus that grows in damp conditions and can appear on a variety of surfaces, including wood, siding, roofing, and concrete. Mold can cause significant damage if left untreated and may pose health risks to sensitive individuals, such as those with respiratory issues.
Examples of Mold Species:
Stachybotrys Chartarum (Black Mold)
Often referred to as “toxic mold,” Stachybotrys is commonly found on water-damaged materials, such as wood and drywall. It thrives in areas with high humidity and poor ventilation.
Cladosporium Cladosporioides
This dark green or black mold species is common on exterior surfaces like wood, siding, and roofing. It can spread quickly under the right conditions and lead to structural damage if left untreated.
Aspergillus Fumigatus
Typically found in environments with chronic moisture problems, Aspergillus can affect exterior surfaces such as roofing and siding. Though not as commonly found on the exterior, it can still be dangerous if it spreads inside the home.
3. Mildew
Mildew is a type of fungus that typically grows in the form of a white, grayish, or powdery coating on moist, flat surfaces. Like mold, mildew thrives in humid, poorly ventilated areas. Mildew is generally easier to remove than mold but can still cause long-term damage if left untreated.
Examples of Mildew Species:
Bipolaris Spicifera
This mildew species appears as grayish spots and commonly affects wet surfaces, such as wood, roofing, and siding.
Erysiphe Cichoracearum
Known as powdery mildew, this species often appears as a white, powdery coating on roofing, concrete, and wood surfaces.
Peronospora Belbahrii
A downy mildew species found in moist conditions, it can spread on surfaces like siding, roofing, and concrete when the area is shaded and damp.
4. Lichens
Lichens are composite organisms made up of a symbiotic relationship between fungi and algae or cyanobacteria. These growths appear as crusty, leafy, or gelatinous patches on surfaces like roofing, wood, and concrete. Lichens are not harmful to humans but can contribute to surface degradation over time.
Examples of Lichen Species:
Xanthoria Parietina
Commonly known as the "yellow scale," this lichen appears as yellow or orange patches, typically on shaded, moisture-rich roofing and siding.
Cladonia Cristatella
Also known as “reindeer lichen,” this species grows on roofing and concrete surfaces in the PNW. It often appears as pale gray or white crusty growths.
Usnea Longissima
This beard lichen grows in the damp, shaded environments common in the PNW. It typically forms long, hair-like strands on wood, siding, and tree surfaces.
5. Moss
Moss is a small, non-vascular plant that grows in dense, mat-like clumps on moist surfaces. Moss thrives in the cool, wet conditions of the PNW and can be found on roofing, concrete, wood, and even tree trunks. While moss itself isn’t typically harmful to human health, it can damage roofing and other exterior surfaces by retaining moisture and promoting further growth of algae or mold.
Examples of Moss Species:
Sphagnum Moss (Peat Moss)
Sphagnum moss is commonly found on roofs, concrete, and wood in the Pacific Northwest. Its ability to hold moisture can cause shingles or tiles to lift and crack over time.
Bryum Argentum (Silver Bryum)
This species is prevalent on roofs, concrete, and sidewalks in areas with persistent moisture. Its growth can contribute to surface damage by trapping moisture.
Plagiomnium Cuspidatum (Cushion Moss)
Known for forming dense, cushion-like clumps, this moss species is commonly found on shaded wood, concrete, and roof surfaces, where it holds moisture and can encourage further growth of algae and mildew.
Why Does Exterior Growth Thrive in the Pacific Northwest?
The Pacific Northwest’s climate provides an ideal environment for the growth of algae, mold, mildew, lichens, and moss. Several factors contribute to the prevalence of these growths:
High Humidity: Consistently high humidity levels create an ideal environment for fungal and algae growth.
Frequent Rainfall: Continuous moisture from rainfall allows growths to thrive, particularly on roofs, siding, and concrete surfaces.
Shade and Lack of Sunlight: Many homes in the PNW are surrounded by trees or are positioned in areas with limited sunlight. This lack of sunlight prevents surfaces from drying properly, allowing growths to persist.
Mild Temperatures: The moderate climate, neither too hot nor too cold, allows these organisms to grow year-round.
Prevention and Treatment
1. Regular Cleaning and Maintenance
Regular cleaning of your roof, siding, concrete, and wood surfaces can prevent the build-up of mold, algae, and other growths. Using non-toxic cleaners or professional pressure washing services can help keep surfaces free of contaminants.
2. Improve Drainage
Ensure your gutters and downspouts are clear to avoid water accumulation on your roof, siding, or concrete surfaces. Proper drainage around your home’s foundation is essential for preventing moisture-related growth.
3. Increase Ventilation
Areas with poor airflow are more likely to foster mold and mildew growth. Proper ventilation in attics, crawl spaces, and basements will help reduce moisture and prevent growth.
4. Repair Leaks Leaking roofs or pipes provide an ideal environment for mold, mildew, and algae. Ensure that leaks are repaired promptly to prevent moisture from accumulating on exterior surfaces.
5. Anti-Growth Treatments
There are commercial treatments available to prevent the growth of moss, mold, and algae. These treatments can be applied to roofing, siding, and concrete surfaces to help minimize future growth.
When to Call a Professional
While many exterior growth issues can be managed through regular maintenance, severe or persistent growth may require professional cleaning services. PacWest Pressure Washing specializes in removing mold, mildew, algae, and moss from all exterior surfaces using eco-friendly methods. If you're dealing with extensive growth or difficult-to-remove stains, a professional service may be necessary to restore the appearance and integrity of your business.
By understanding the different species of exterior growths, business owners in the Pacific Northwest can better identify and address the unique challenges posed by the region's climate. Regular maintenance, moisture control, and professional services can help keep your property in top shape and free from harmful exterior growths.
Call us today. 866-848-9274
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References:
University of Washington: "Pacific Northwest Climate and Mold Growth" – https://www.washington.edu
EPA Mold Resources – https://www.epa.gov/mold
National Pesticide Information Center: "Algae Control and Prevention" – https://npic.orst.edu
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