Roof Reflectivity Part I – What You Need To Know

Here’s Everything You Need To Know About Roof Reflectivity

In the Winter months it’s easy to feel the effects of the bitter cold through the historic Washington DC rowhome building assemblies. These buildings were built before the invention of the majority of modern insulative materials. These buildings are also simply very drafty. They leak air, even and through under roof assemblies which are largely free of precipitation leaks. There are lapped elements in roofs which can allow air (and heavy wind driven rains and water in ice damming conditions) to pass relatively freely.

The picture below shows an example of three side-by-side garage rooftops. These garages happen to be behind a series of joined rowhomes, and the roof membranes at each of the rooftops happened to be similar to the respective main house roof membranes. In this case, all three rooftops are made from modified bitumen roof materials. These roofs are built contrary to proper installation and/or upkeep and maintenance principles for modified bitumen roof membrane systems. These rooftops were supposed to be coated with a moderate to high reflectivity roll-applied coating but were never properly coated. Coating a modified bitumen smooth roof system like this is required because the rooftop alone does not include a coating or covering later which will protect it from harmful ultraviolet rays. Ultraviolet rays eat through a rooftop material like this, over time, and can cause significantly accelerated deterioration.

roof reflectivity

For clarification and further understanding of modified bitchman roof systems, it’s good to also understand that there’s a second type of category of modify the bitumen roof systems called granulated roof membranes. These systems can also be single-ply membranes, but the roll sheeting has thousands of small ceramic granules covering the entire exposed surface. The ceramic granules, like many stone and other non-organic materials happen to do a decent job resisting the harmful effects of ultraviolet rays. This type of membrane requires no roll-applied residual coatings, but there are also shortcomings in this type of material, there are always tradeoffs. In the case of granulated modified bitumen systems, over many years, the ceramic granules can begin to loose their adhesion.

In the summertime, traditional rooftops, in direct exposure to sunlight in our geographic area of Washington, DC, could heat the attic so much that it can cook like a mild oven. Attics can reach temperatures over a 150゚fahrenheit in the Washington, DC area in the midsummer, especially at rooftops with black or dark colored roofs. Historic roofs were generally dark colored, most often black like most bitumen based materials and had extremely low reflectivity. Low roof reflectivity means that the solar energy from the sun will be absorbed by the rooftop and heat the roof surface up immensely. A temperature of 150゚F is literally hot enough to cook an egg. If the rooftop gets that hot then that means the attic can also get that hot.

In a future article we’ll talk more about the environmental impacts of rooftop reflectivity and alternative membrane types such as EPDM and TPO membranes.

Smart proactive replacement, construction, upkeep and maintenance of low slope roof systems requires an enthusiastic interest and understanding of waterproofing principles and building science. Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building.

We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company. You can learn a lot more on our blog. Feel free to check it out. If you have questions about the roof and related systems of your building in Washington DC, fill out the webform below and drop us a line. We will be in touch if we can help.

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