Ask Todd Miller *Metal Roofing Expert

What Is That Stuff On My Roof?

Written by Todd Miller on July 8, 2009 | Filed under: Articles

As asphalt and fiberglass shingles age, they can sometimes develop streaks of discoloration. This is especially prevalent in damp climates or shaded areas.

While these streaks are often seen as a sign of aging of the roof, they are actually biological growth. As dirt accumulates on standard shingles over time, it gets stuck in the nooks and crannies of the aggregate surface. That can then become a host for biological growth known loosely as “roof algae”. In areas where there is not a lot of moisture or where the roof gets frequent and regular sunshine, the algae does not thrive. But in shaded or damp areas, it can quickly overtake the surface of the roof, spreading vertically down the roof from the point where it first takes hold.

In worst cases scenarios, over time, the roof algae will even take on a green mossy appearance.

One benefit to many of today’s residential roofs is that they can be installed over old shingles. This is a “green” benefit as it avoids tearing off old shingles and disposing of them in landfills. However, when this is done, many folks wonder whether it is okay to install metal roofing over the biological growth on their old shingles.

It has been my observation over the years that this is not an issue. I always suggest a layer of underlayment over the old shingles. One purpose this serves is that it isolates the old shingles and any biological growth on them from moisture. This will cause the biological growth to harmlessly die.

Over my nearly 30 years in the industry I have been involved at least peripherally with tens of thousands of installations of metal roofing over old shingles, including those with algae, moss and fungus growth. Never have I later regretted that the old shingles were not removed.

Tags: , ,

Leave a Reply

Your email address will not be published. Required fields are marked *

About Todd

Todd's expertise comes from his years of experience in metal roofing manufacturing. As president of Isaiah Industries, based in Piqua Ohio, Todd runs a several of the world’s leading manufacturers of residential metal roofing; Classic Metal Roofing Systems, Kassel & Irons, and Green American Home. His personal goal is to educate homeowners of the long-life benefits of metal roofing.

Sometimes finding the right roofing product is the easy part.

Need help finding a local, quality metal roofing contractor or supplier? I can help.

Find a Contractor or Supplier

Before You Call a Roofing Contractor

dial cellphone

Unfortunately, most roofing contractors come in, measure, and simply give you a price. Learn more about what you want and what you need in new roof, before you invite the contractor into your home. My Roofing Profile Selector is easy to use, and it will help you find the very best roofing solution for you and your home.

Get started

Thinking about a new roof?

Educate yourself for a better experience when buying a new roof

This comprehensive eBook includes everything you'll need from "choosing a manufacturer" to a "metal roofing checklist," and is sure to be a great tool for helping you make the best investment in you home’s protection.

Get your copy!
classic metal roofing systems logo kassel and irons logo kassel and irons logo