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Purple-Roof is a Stormwater Tool that Doubles as a Green Roof
The Purple-Roof concept is far from a simple green roof upgrade. It might be clearer to describe the Purple-Roof concept as a stormwater tool that doubles as a green roof. The concept provides both stormwater retention and detention and thus fulfills the civil engineer’s requirements for stormwater management.
Generally, the main reason for installing a Purple-Roof compliant system is that you can reduce or even remove at-grade stormwater solutions and free up floor space for income-generating development. You are managing your stormwater on an otherwise unused and neglected area: the roof. Hence, there is a strong business case in densely populated urban areas where land is expensive.
Then add all the benefits you get from a green roof, such as cooling, reduced energy needs, pollution capture, increased biodiversity, and LEED points, and you also get a much better-looking roof.
This video explains the ROI of a Purple-Roof system in 4 min. Please, contact us for the whitepaper!
When does the Purple-Roof Concept Make Sense for your Project?
Detention-type green roofs are incredible workhorses, excellent stormwater management tools, and can provide a clear ROI for a green roof. However, not all projects are suitable. These are some questions to consider:
Are you required to do a green roof?
If so, upgrading it to Purple-Roof most likely saves you money if you live in an urban setting. However, the low real estate prices in rural settings can make at-grade stormwater solutions more favorable in the countryside.
Is your lot almost the same as the size of the roof?
Are you occupying most of the lot with a building? 80%/20% roof/lot ratios in urban areas mean that a Purple-Roof will almost certainly be a good solution.
Can your roof handle the weight?
Live load: Since we are only holding the peak amount of rain for a few hours, the practical route is to take this out of the live load budget. In most cases, the snow load budget will be enough. For instance, in most east coast cities, there is a 40-80lbs/sqft (195-391kg/m2) snow load, so utilizing 8-12lbs/sqft (39-59kg/m2) from that budget is more than acceptable.
Can we absorb the cost of the Purple-Roof?
The opportunity cost: What if we can park four more cars or rent out 1200sqft (111m2) extra basement space to a fitness club? Multiply this over 50 years and reap the benefits.
And there is an actual cost: Upgrading a green roof to a Purple-Roof will add $5-7/sqft ($56-78/m2) to most green roof designs. But it will eliminate the tank. Tanks cost hundreds of thousands of dollars in engineering, products, and installation.
How the Purple-Roof Concept Differs from a Blue-Green Concept
Both Purple and Blue-Green roofs are high retention green roof concepts, and both concepts can detain water. So how do they differ?
A Blue-Green roof has a void space underneath the green roof where rainwater can be stored for later use. In most cases, mineral wool wicks connect the stored water with the green roof profile, allowing upward water transport through capillary action as the profile dries out. In a way, a Blue-Green roof is a green roof on stilts over a shallow pool. The pool fills up as the outflow of water is restricted, e.g., through orifice restrictions. Blue-Green roofs can also be equipped with smart outflow systems that empty the water storage in anticipation of a large storm.
Blue-Green roofs are great at effectively using rainwater, and they can look amazing - think roof gardens! However, rainwater storage comes at a structural cost as these types of roofs can be very heavy. They also require a completely flat deck which limits where they can be built. In some instances, people have also expressed concerns over having a pool of water on their roofs. We do not consider this a genuine concern provided a premium manufacturer and installer has been used.
