An example of the components of an effective blue roof, Photo Credit: Philadelphia Water.
When we think about roofs, one goal is to drain the water from them as quickly as possible.
Standing water adds weight, which can weaken roof structure over time,especially on flat roofs. ‘Pooling’ leads to a deterioration in the roofing material where water accumulates: the standing water can result in moss, algae, and plant growth, and damage to or weakening of the roof membrane. Water trapped on roofs leads to leaks.
The need to move water off a roof quickly is one reason why we stress the need for routine inspections and cleaning – lichen and moss retain water. On sloped roofs, the growth of plant material damages the roof surface and provides water an intrusion point. Clogged gutters lead to water backup under the edge of the roof, seeping behind gutters and rotting fascia boards.
Even with your roof and gutters performing as they should, heavy rains can result in more water than your gutters can handle, leading to overflow.
But what if your roof was designed to hold water, releasing it over time or as needed?
The Blue Roof
What is a blue roof?
It’s a roof designed explicitly to provide initial temporary water storage and then gradual release of stored water, typically rainfall. … Water is stored in blue roof systems until it either evaporates or is released downstream after the storm event has passed. – Wikipedia
Why get a case of the blues?
The landslide factor
The primary purpose of a blue roof is controlling water flow. With our annual rainfall levels, flooding can and does occur. In extremely wet years, the accumulation of ground water on hillsides and slopes can lead to landslides. There is only so much water the ground can hold before it gives way.
For homes (your home) sitting on…or close to…a steep slope, that presents a risk. Blue roofs mitigate that risk.
Blue roofs can play a major role in reducing flooding, landslides, and excessive water runoff (runoff that could be stored until it can be absorbed as ground water).
The pollution factor
Did you know that water flowing from storm drains often empties directly into Commencement Bay and the larger Salish Sea, carrying with it chemicals toxic to Orca, salmon, and other marine critters? In fact, the single largest source of pollution that contaminates the Salish Sea is polluted storm water runoff. – Citizens for a Healthy Bay
Blue roofs allow storm water runoff to be controlled. Rather than entering the Salish Sea, the collected storm water, if released slowly enough after the ground’s saturation level has dropped, becomes groundwater. This allows for a natural filtration process, protecting our environment while increasing the groundwater table.
An added environmental benefit is the ability to collect water temporarily stored by a blue roof for later use. Water released can be diverted to rainwater-harvesting barrels. While we don’t have a water shortage issue here in the PNW, collecting water used for irrigation over ground water is a sound environmental practice that saves groundwater reserves and reduces your water bill.
Blue roof design
On a blue roof, a system of detention ponds collects storm water, temporarily stores it, and then releases it slowly when the rain has stopped. It acts as a temporary sponge, mimicking the hydrology of the site before the building was constructed. – PHP Systems/Designs
The release of water can be handled through both active and passive designs.
- Passive systems – These are usually based on physical obstructions. They can be a series of drainage channels that wind their way to ground level forcing storm water to take a longer time to reach the ground or a large storage basins with small drains which limit the runoff slowing it’s release. Passive systems are designed to hold and release water through their design.
- Active systems – In an active system, water release is controlled by mechanical means. This could be as simple as a pressure valve which opens at a set pressure point to as complicated as a computer system which releases water at specific times or programmed pressure settings based on groundwater saturation levels at the building site. They can be further designed to control how the water is released and where it is released with a series of dammed drainage channels that can be opened and closed remotely – similar to switching trains on tracks.
Active or passive?
While both are effective, choosing an active vs passive blue roof system depends on the building design, the site, and your goals.
If your home or office building is sitting near a cliff or steep slope, or in an area with a high groundwater concentration, it may require water to be released on a more controlled time-table or to control where and how much water is released at a given time.
If the site is less susceptible to erosion and slides, and your goal is rainwater collection for irrigation of a garden, passive systems will work well.
Systems can further be designed using a mix of passive and active controls.
The Massachusetts Clean Water Toolkit has a great article on various blue roof designs, systems, and applications.
Cost of a blue roof
As with the cost of any roofing system, the size and design will dictate the cost. A small roof with a basic passive system will be less expensive than a larger one using active means to regulate water release. Where you are located can affect the overall cost.
Based on the research we are finding, you can expect to pay roughly $1 per square foot for a blue roof system.
Yes, they will require maintenance like any traditional roof. Passive systems will need regular cleaning and maintenance to ensure water can flow as designed and drains are not blocked. Think of a passive blue roof as a more complicated gutter system. Now think about what happens when you don’t keep them clean?
Worse, if water fails to drain as designed, the weight of that collected water could top the point the roof system was designed to handle.
Active systems require similar maintenance but add in making sure all control systems – from pressure regulators and monitors to computer systems – are functioning properly.
Getting a blue roof.
Unlike a traditional roof, more is involved with blue roofs. Water weighs about 8.3 pounds per gallon, and a blue roof could easily hold 200 gallons or more. That is a lot of weight. Your roof will need to be designed to handle the volume. That will likely require an architect.
There will be site considerations that could require an ecological or groundwater survey to determine:
- Where can you safely release the stored water?
- At what rate?
Then there’s the permitting and building codes.
If you are thinking about upgrading to a blue roof, we’d recommend starting with your city building department.
Conventional roofs vs blue roofs vs green roofs vs conventional roofs.
Blue roofs aren’t the best choice for all rooftops. They aren’t an option for many existing structures. Nonetheless, you can get some of the same benefits by creating a system using larger gutters and rainwater barrels to collect and control storm water runoff. Once collected, water can be released or used for irrigation purposes. Working with a landscape designer can further maximize the benefits.
Going green or feeling blue?
A green roof is more sustainability-focused, with water collection and control being a secondary benefit. Green roofs provide insulation benefits, create habitats, and help clean and purify our air. They can create a functional space for outdoor use.
With a blue roof, the goal is to increase water storage capacity and control its release. A blue roof will be considerably less expensive, but a green roof provides more benefits. Which way you go, including a mix of blue and green, depends on a number of variables.
With blue roofs, and to a degree, green roofs, the idea is to collect and store water. These are specialized roofing systems.
If your roof is a more traditional design, it’s not a good idea that it retains water. The goal is to move it off the roof as efficiently as possible through drainage systems on a flat roof to pitch and gutters on a traditional sloped roof. Water that backs up leads to potential long-term damage, water intrusion, and a shorter overall lifespan of your roof.