Last summer I installed a peel-and-stick underlayment topped with a white, mineral rolled roofing cap sheet to the new roof on the extension my son-in-law Rob and I added to our covered porch. When I asked my granddaughter Delaney (11 years old at the time) to help me apply a white elastomeric coating on the original tan roof, she replied: “Cool! I’d love to help!”
Cool, indeed. On a typical summer afternoon, a clean white roof that reflects 80 per cent of sunlight will stay about 31°C cooler than a grey roof that reflects only 20 per cent of sunlight. Putting a white coating on our roof probably gave it reflectance properties that would classify it as a cool roof.
Actually, the first roofing project I undertook was about 55 years ago, when my dad and I re-shingled the roof on our house with white three-tab asphalt shingles. I can personally testify that white materials have been a popular cool option for roofs for quite some time.
But a cool roof does not necessarily have to be white. Sunshine includes ultraviolet, visible and near-infrared light, and about half of the sun’s energy is invisible near-infrared light. There are new cool-coloured dark roofs that look like traditional dark roofs but better reflect near-infrared light.
So how does aggregate fit into the picture? The answer probably is obvious to many of you.
{{image2-a:r-w:200}}Coarse sand, either natural or manufactured sand from crushed stone, serves as granules for roofing material – including shingles, like on my childhood house, or rolled roofing like on our new porch roof. Some granules are coated, others are natural. But most start with coarse sand.
Slightly larger aggregate is used on flat, built-up roofs (BURs). Commonly referred to as “tar and gravel” roofs, BURs are composed of alternating layers of bitumen and reinforcing fabrics. They are topped with a layer of pea stone, either natural or crushed. And if the stone is white, it can be a cool roof.
Stone-ballasted roof systems, which began appearing in the 1970s, are another way to cover flat roofs. Although stone-ballasted roofs appear similar to BURs, there are major differences. Ballasted roofs are “loose laid”, which means the membrane, thermal barrier and rock are all laid down without fastening them to each other.
Obviously, the membrane seams are sealed and it is secured to the parapet and at roof penetrations, but it is not adhered to the roof deck or the layers beneath it.
The key to the entire system is the aggregate ballast that is placed on top of the membrane, weighing down all the components to hold them in place. The stones in a ballasted roof are at least 25mm in diameter and applied much more heavily than a BUR.
The weight can vary from about 50 kilograms per square meter (the minimum allowed by the code) to 120kpsm or more.
Unless the aggregate is white, ballasted roofs do not meet the traditional requirements of reflectivity and, therefore, have not been officially recognised as cool roofs. However, the mass of the ballast acts as a heat sink and simulates the performance of a traditional cool-roof surface with high solar reflectance and high thermal emittance.
So, next time you are looking for an environmentally friendly application of aggregate, the answer might be lying right over your head. How cool is that?