Yes. Surface mildew can reduce the solar reflectivity of a commercial roof and, over time, diminish some of the energy-saving benefits of a reflective or “cool” roofing system. The effect is particularly relevant for white roof membranes and reflective roof coatings, where maintaining a clean, highly reflective surface is an important part of long-term performance.
How Mildew Affects Roof Reflectivity
A reflective roof works by sending a significant portion of incoming sunlight back into the atmosphere instead of absorbing it as heat. The higher the roof’s solar reflectance, the less solar energy the roof absorbs. This can reduce roof temperatures and lower the cooling load on the building. The U.S. Department of Energy identifies high solar reflectance and the ability to maintain that reflectance over time as important characteristics of a cool roof.
Mildew, algae, dirt, biological growth, and other surface contamination can darken or cover portions of the roof. A darker, contaminated surface generally reflects less sunlight than a clean white or light-colored surface. DOE guidance specifically notes that mold or algae growth can decrease the reflectance of cool roofs and increase roof temperatures.
This does not mean that every small amount of mildew will cause a noticeable increase in a building’s energy bills. The impact depends on how extensive the growth is, how much of the roof is affected, the roof’s original reflectance, the building’s insulation, climate, HVAC efficiency, and other factors.
Can Mildew Reduce Energy Savings?
Potentially, yes. When surface contamination lowers solar reflectance, the roof can absorb more solar heat. That can increase heat transfer through the roof assembly and potentially increase the workload on the building’s air-conditioning system.
The effect is most important on buildings that depend heavily on cooling and use highly reflective roofing systems. DOE research has found that roofing products that soil rapidly can lose a significant portion of their potential energy savings, demonstrating why maintaining aged reflectance is important.
However, building owners should not assume that cleaning a roof will automatically produce enough energy savings to justify cleaning solely for energy purposes. DOE guidance recommends incorporating roof cleaning into an overall maintenance program rather than treating energy savings as the only reason for cleaning.
Why This Matters in Florida
Florida’s hot, humid climate can create conditions favorable to biological growth on some roof surfaces. Moisture, shade, accumulated debris, and poor drainage can contribute to mildew and algae development.
For Florida commercial properties with white or reflective roof coatings, maintaining surface cleanliness can therefore be part of protecting the roof’s intended reflective performance. A roof inspection should determine whether discoloration is simply superficial or indicates a broader moisture, drainage, coating, or membrane issue.
Should a Commercial Roof Be Cleaned?
Cleaning can restore some lost reflectivity, but the method matters. Aggressive pressure washing, abrasive techniques, or incompatible chemicals can damage certain roofing membranes and coatings. DOE recommends following the roofing manufacturer’s cleaning requirements.
Professional commercial roof maintenance can evaluate the roof condition, identify mildew or algae growth, check drainage and ponding areas, and determine whether cleaning, treatment, coating maintenance, or a larger restoration project is appropriate.
For commercial building owners, the key takeaway is simple: surface mildew can reduce roof reflectivity and potentially reduce cooling-energy savings, but the actual impact depends on the extent of contamination and the overall roofing system. Keeping a reflective roof properly maintained helps preserve the performance for which the roof was originally designed.
If a Florida commercial roof has visible mildew, algae, dark staining, or declining coating performance, a professional roof inspection can help determine whether cleaning or commercial roof restoration is the more appropriate solution.
Related Questions
- How do you safely clean a reflective roof without losing reflectivity?
- In Florida, does insulation upgrade or reflectivity deliver greater savings?
- How do you calculate energy savings from a reflective roof in Florida?
- How is “cool roof” performance measured and verified?
- Do cool roofs require special maintenance compared to traditional roofs?
- What is the Solar Reflectance Index (SRI) and why is it important for cool roofs?
Commercial Roof Replacement / Re-Roofing
