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It seems roofing evolution is measured in colors from black pitch to silver, then green—and now it's time for blue. Blue roofs hold water without plants but they’re still considered green. What’s in their favor is that they’re cheaper, lighter, hold more water per square foot, and can be installed almost anywhere. They may require reinforcing the roof rafters, but not always. They’re not the lush landscapes we expect from the variety of plants and hardscapes that make a classical green roof but they do a great job of keeping water off the ground.
Blue roofs are designed to retain or detain the rain. Most city storm water falls on paved or non-porous areas. To get rid of it, it has to drain into underground culverts and pipes. In most of the city, this is where it gets mixed with sewage and human waste. All of this has to be processed and cleaned up. This is very pricy for a crowded town with many buildings and nice concrete (mostly) sidewalks and streets but no place for outside for stormwater to go. That’s an awful lot of dirty water to process and cleaning it up isn’t cheap. Blue roofs take some of that storm water as soon as it leaves the sky, while it’s still clean and holds it up on top of the buildings that are catching it. Sounds pretty simple. It should because it is, and it doesn't adversely affect your roof while it’s up there.
There are several varieties of blue roofs, all of which will hold water—some for much longer than others. The easiest and most universally understood blue roof occurs when the drain gets partially blocked with leaves or other unintended debris. The result is a pond of water at the lowest point of the roof that slowly trickles away. It’s the simplest and most common of blue roofs. If slowing the drainage is deliberate by surrounding the drain opening with a restrictive device to control the flow of the water, it is called a weir dam. Before they made it to the rooftops, weirs were most often used as a means to catch eels as they migrate up the shallows of rivers.
The rooftop weir method has the advantages of being cheap, quickly installed and easily removed if necessary, which will be often because not just water flows into the drains. Wind driven rain has plenty of dust and sandy particles blowing around mixed with pollen, leaves, feathers, food wrappers, rooftop utility parts- you get the picture. Also, weirs are placed at the lowest points of the roof, where all the water is and not coincidentally, where most roof leaks occur. On most buildings it might be better to leave weirs to those hearty folks who catch eels and put something else on roofs.
Another flavor of blue roof is the container or tray. For now, it’s a big, plastic pizza box-like container with a layer of stones in it that holds water until it evaporates or slowly drips out. They are easy to install—just set and forget. Tray systems can be anything that holds water and doesn’t blow off the roof when the wind is screaming—which it often is. Trays are easy to move and should be installed on a protective sheet of something pretty durable and if properly placed, won’t hurt the roof. Their presence actually protects the roof surface as it shields it from the sun so the roofs under them leak less often. They tend to be expensive, they are heavy to start with and will get a lot heavier when they fill with water when the weep holes clog up. In some conditions they can create breeding habitat for mosquitos, but that is something that can be addressed. While all trays are containers, not all containers are trays. New forms of rooftop water storage are being developed all the time. We've designed and installed the bladder bags show here to allow multiple uses of retained rain.
Still another blue is the check dam. A neat simple means using low horizontal barriers that keep the water on the roof surface in parallel lines following the slope of the roof. Think of check dams as long, tiny barriers a few inches high every few feet up the roof. The water would pond in long thin triangular shapes with the deepest side against the dam. The sun evaporates the water pretty quickly because it’s not very deep, it cools the house and the layer of water helps protect the roof membrane from some U/V damage—the silent killer of roofs. The advantage for this system is that it is usually the least expensive, cools the house and protects the roof. It also will probably leak if not properly installed. A check dam system can easily be tweaked to hold more or less water in specific conditions. and if done correctly, can direct some of the water away from places it shouldn’t be.
The last of our blue roof types is the off-roof container. Here a bit of imagination becomes useful. Containers for water can be plastic, metal, little or big, underground cisterns, tanks and barrels. All that is really required is something to store the water once it is captured. When storing storm water on the ground gravity and weight load isn’t so important as it is when all this water is parked over your family’s heads as you sleep through a killer thunderstorm. The ground based water containers can be bigger and heavier, but they are not set and forget. Each system requires vigilance, common sense and maintenance that includes a regularly scheduled inspection. Ground containers, rain barrels, cisterns, oversized downspouts etc.—all that stuff needs attention as well. They must be kept clean and secure for the water to flow happily. You don’t want anything (or anyone) to drown in you cistern or rain barrel or have a badly flooded basement you could have avoided because you haven’t been neglecting maintenance. You might also want a pump to make best use of your stored water.
With the exception of ground storage, all of the above rooftop methods require a few things to start with:
First is a roof strong enough to support the weight. Water is heavy and a lot of it can show up in a very short time.
Second is a roof that is in very good condition. Your roof has to stay leak free for a long time or a lot of stuff has to get moved before you can fix it.
Blue roofs mitigate stormwater runoff.
Blue roofs can cool your home by evaporation.
Blue roofs can provide a source of pressurized, free water for garden irrigation, flushing toilets, washing cars and many other uses.
Rooftop water is not poisonous but it’s not treated either. It probably gets more than its share of dead bugs, bird poop and windblown trash, so don’t consider it drinkable. Remember: DON'T DRINK IT!
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