Using a Detention Roof to Create a Green Roof ROI

by Anna Zakrisson on Friday, January 10, 2020 updated Wednesday, June 23, 2021

Go straight to Purple-Roof Green Roof Modeler

Purple-Roof DETAILS

Managing stormwater generally means creating detention. Retention trims a little off the top of the volume, sometimes, but it’s not what most civil engineers call true stormwater management. Detention is because it’s more predictable and reliable. So, if green roofs can become Detention Roofs, they should cross over from novelty to true options for stormwater.

What is a Detention Roof?

Water detention deals with the water that becomes runoff. Detention is slowing runoff. The water flow is restricted to generate predictable release rates. This slowing down shaves off the peak of the storm. Hence, sewers don’t overfill and the city doesn’t flood.
Some roofs achieve this by storing water underneath the vegetation layer. These roofs are often referred to as blue-green roofs. Blue roofs store water, but have no vegetation and so miss out on all the secondary benefits of a vegetated roof such as heat island mitigation, pollution capture, biodiversity increase, and much more.

Read more about different types of detention-based roofs in our article “Blue Roofs, Blue-Green Roofs, and Purple-Roofs all DETAIN water

But What About the Detention Roof ROI?

But you might counter with that “green roofs are still relatively expensive.”
It’s true. A vegetated is more expensive than a rain garden. But a green roof can be a bargain if it means you can free up urban space for revenue-generating uses like additional parking spaces, or even a Café.

A Detention Roof is a much more efficient use of urban space. This improved use of real estate generates a clear ROI for the green roof.

Here is a short video explaining the Purple-Roof detention roof concept further:

Contact Purple-Roof for more information on how to get that return of investment from your green roof using the Purple-Roof detention roof concept!

Video transcript: Detention Roof ROI:

You have a building with 10,000 square feet of retail,
You need to manage your stormwater, so you install a rain garden on the side, but that won’t handle enough…
…so, you add a tank in the garage. The tank is the size of 3 parking spaces.

You’re already doing a green roof for some other reason, maybe LEED or maybe retention.
What if you used a detention roof, and that allowed you to reduce the tank from 3 parking spaces to 1 parking space
…and reduce the rain garden from 1,200 square feet to 600 square feet

Maybe you just added enough parking and square footage for an extra unit, or a Café, or both.
That would probably pay for the green roof in ONE YEAR!

This is really great! We can get retention, detention, and bees and butterflies all from that one investment!
In fact, you’ve got more options than you might think. In addition, there are different types of detention green roofs; sometimes, a blue-green roof is the best fit. Or if you’re not interested in green at all, a Blue-Roof might be the best fit.

Go to to see a range of options for a green roof, as well as blue-green roof stormwater management.
And check back for more interesting blog & video content.

Further reading:

What is a Live Roof ROI? - the Key is Detention!

Read more about stormwater runoff and how detention roofs can prevent combined sewer overflows:
What is a Combined Sewer Overflow?

Find out how Purple-Roof temporarily detains water through friction:
How friction-based detention roofs work

Purple-Roof is not the only type of detention roof. There are other types on the market, each with different merits:
Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water

This is the Blue-Green smart detention roof from Metropolder: PolderDak

Read more about stormwater retention and detention in Living Architecture Monitor: Untangling Stormwater Retention and Detention by Brad Garner.