Detaining water is slowing water down. By delaying peak outflow from a storm, it is possible to avoid floods, damage, and loss of life and property. This is why detention is critical to a project civil engineer and why water detention solutions are required for all projects.
Did you know that you can detain water using your roof?
Blue, Blue-Green and Purple Roofs are the detention-based solutions that will also satisfy your stormwater requirements!
How to detain water with a Blue Roof
Detaining water with a Blue roof is the cheapest but not so popular option. One of the main concerns is that all that water stored on the roof will start to leak through the membranes as many membranes cannot cope with standing water.
Blue roofs have been offered in NYC as a detention solution, but it has not taken off.
For the city, it helps only for half of the battle as it detains water, but it does not reduce the volume as all water sooner or later will leave the roof as runoff.
Despite these concerns, Blue roofs can be a great solution under and around equipment. Raise it with pavers and you have a fully walkable and dry environment that holds lots of water. However, few companies warrant the membrane with standing water and it is critical that you find a very reputable manufacturer and installer.
Building owners often express concerns as they worry that sloppy roof systems could result in potential leaks – quite understandable if you consider that you might have 15cm (6”) of water above you.
A Blue roof should only be done over a membrane that is warranted for ponding water, which limits membrane selection to higher quality, and more expensive, products. In addition, people fear the freeze/thaw cycles that create ice movement which puts putting stress on the membrane and other structures.
Thus, if you chose a superb premium waterproofing system, this is a great system! However, detention-based green roofs can do much more. For example, we would like to see detention plus a reduction in stormwater volume. Which brings us to the Blue-Green and Purple detention roofs.
How to detain water with a Blue-Green Roof
Blue-Green roofs are superb! Not only do they detain water, but Blue-Green roofs also reduce the stormwater volume using plants! The combined evaporation and transpiration processes, or evapotranspiration, adds retention to the roof. This retained water never becomes runoff, it simply leaves the roof as vapor. On top of this, Blue-Green roofs add detention.
A Blue-Green roof is built by adding a void space underneath the green roof. Essentially, it’s a green roof on stilts.
However, it adds a substantial cost to create the cavity. Also, it might be a lot of weight, so you need a strong roof. Still, some are afraid to have free-flowing standing water on the roof. Here again, it is critical to work with premium roof manufacturers and single source might be a great way to go as removing all the overburden can be quite costly.
In our Purple-version, we have improved the retention by at least 10% on an annual basis as we use needled mineral wool to wick water upward. We also overlap the crates with 2.5-5cm (1-2”) of needled mineral wool. This saves HUGE amounts of money and resources as we can reuse 0.7cm (0.3”) of water per day from the reservoir.
Yes, plants use 0.8-1.0cm (0.3-0.5”) of water per day so just imagine how much water we will save you on an annual basis if we wick it all up again. We work with a specific manufacturer that understands how to do this on top of the insulation, as this is critical because insulation cannot be submerged for more than 48 hours.
Practically speaking, a Blue-Green roof requires a flat deck as a sloped deck usually reduces storage volume by 2/3. Right now, few companies are comfortable with a ‘swimming pool’ under the green roof. Concerns are the same as for Blue roofs as well as the warranty issues.
Blue-Green roofs may have a longer adoption time before this becomes mainstream, but this technology is most definitely upcoming.
Blue-Green roofs require a superb membrane and a superb installer. There are several companies that offer creative solutions to wick up the water for the life of the project (we would recommend Urbanscape Needled Mineral wool), but we think it’s the best solution if you want to store a lot of water (over 10cm [4”]).
How to detain water with a Purple Roof
Purple-Roof+ is the simplest and least expensive solution if you would like a detention green roof. A standard Purple-Roof can be upgraded from a retention-oriented green roof to a detention + retention oriented green roof for $10-20/m2 ($1-2/sqft). This roof can then detain 2.5-5cm (1-2”) of water for anywhere between 1-10 hours.
We can improve that further. Purple-Roof+ can store another 2.5-5cm (1-2 “) and up to 5-10cm (2-4”) of water if you add another innovative layer for an additional $10-20/m2 ($1-2/sqft). That is a total of 7.5-10cm (3-4”) of temporary water storage (detention) in an upgraded semi-conventional green roof.
This is a passive system that works all the time. Every time, which satisfies the project civil engineer for their stormwater calculations. It achieves a lot for relatively little extra weight and costs.
Just as Blue and Blue-Green roofs, Purple-Roofs have the best value for flat decks, but Purple-Roofs is the only option for 5cm (2”) sloped roofs.
So, which detention-roof should I choose?
So, which roof should you choose? There is not a single perfect solution. All three roofs have pros and cons, all depending on what you want from it. Also, one project might benefit from a combination of them all!
In a nutshell, a large building with multiple levels should apply all three concepts because you want to optimize the performance of each elevation based on weight, costs, and liability.
Our mission is to go from a naked roof that is causing hydraulic harm during storms to a detention-oriented system that is green and also reduces the volume in the process.
We are convinced that detention through Blue-Green roofs are the future. Policymakers will start getting involved as the Blue-Green market becomes larger and remote-controlled. However, a debate will develop where the city will potentially dictate and control the water that is standing on your roof. But there are great tools that allow you to do this remotely, or programmed (for instance the water is held for six hours and then the flaps open)
Thus, the big questions, coming from the policymakers regarding remote controlled Blue-Green Smart Roofs will probably be these:
“Who sets the timer (release rate)?”, “How do we know the timer isn’t removed/altered?”, “How can we make sure we all don’t release the water at the same time (causing flooding again)?”, “What happens in a power outage, hackers and, yes, terrorism?”, and “If everyone has this on their roof it needs to be organized and run by the government because the impact (positive or negative) is huge if done right or wrong?”.
Detention roofs and the future
These are exciting times! It takes brains, sophistication amazing companies, to bring intellect and complexity back into the green roof game. The worst thing we all did (including us) is the commoditization of the green roof industry by saying one-size-fits-all.
Evapotranspiration rates vary hugely from city to city and a green roof truly has to be designed for each city and for each regulatory condition. Also, it must be designed for each building as weight, slope and size matters, as well as for each desired outcome from a plant, retention, and detention goal perspective.
Every time we go from black or white roofs to Blue-Green or Purple-Roofs we offer the secondary benefits of bees and butterflies, heat island effect, biodiversity, climate change, CO2 reduction…you name it.
New usually scares people. But in areas of Amsterdam flat deck roofing with a controlled release will soon be the norm. Civil engineers, building owners, and roofing companies will fight this just because this is new, but that doesn’t mean it cannot or should not be done. It is possible to make policymakers, roofers, building owners, civil engineers, tree huggers and insurance companies happy at the same time!! We sure believe so.
Contact us if you have any questions or want to know more about detention-based green roofs!