Purple-Roof

Are Green Roofs True Stormwater Management Tools? Part 3 by Purple-Roof

An audio article by Oscar Warmerdam

______

Detention is the process of absorbing or holding water during a storm, then slowly releasing it. Unlike retention, detention works regardless of the weather. When a detention-oriented system drains, it recharges. So, a detention-oriented green roof can recharge quickly, and be ready for the next storm, or the tail end of a large storm. Even in dark and cloudy weather! Retention cannot do this, retention takes weeks to re-charge, and it better otherwise the plants would not have enough water.

Detention slows the flow of water, so that the hydrologic characteristics more closely resemble that of a forest, with peak runoff being delayed from peak rainfall.

______

Read the full article on the Purple-Roof blog:

https://www.purple-roof.com/post/green-roofs-stormwater-management-tools

_______

Don't hesitate to contact us if you have any questions at

Are Green Roofs True Stormwater Management Tools? Part 2

An audio article by Oscar Warmerdam.

________

Why retention is unreliable for stormwater management at the building scale.

Retention can be unreliable because it doesn’t work in repeat storms, large storms, or during cool and cloudy weather. Why? Water that is retained, by definition, leaves the green roof only via evapotranspiration. If there is no time for evaporation, such as during big storms or repeat storms, or if weather conditions do not permit much evaporation, then the retention-oriented green roof sits there like a wet sponge.

2nd-day storms may not be substantial enough on their own to cause trouble but may cause huge problems when combined with the precipitation from the first-day storm. Extra-large storms also often exceed the capacity of the green roof as they outflow with little or no delay. These types of extreme storms happen only a few times a year (5-7), but it is precisely these storms that cause flooding, pollution and river stream erosion.

________

Read the full article on the Purple-Roof blog:

https://www.purple-roof.com/post/green-roofs-stormwater-management-tools

________

Don't hesitate to contact us if you have any questions!

Are Green Roofs True Stormwater Management Tools? - Part 1

An audio article by Oscar Warmerdam.

Green roofs are often sold as stormwater management tools. And that is true: green roofs do manage stormwater but mostly as a volume reduction tool. This volume reduction has huge benefits on a city scale. The benefactors are the taxpayers, citizens of what is now a city with cleaner water, and cleaner air.

This is why many progressive cities have regulations in place that promote green roofs.

But, if that’s the case… If green roofs are an excellent volume reduction tool on a city scale, why don’t civil engineers put one on top of every roof they can? The answer is simple but will be a surprise to most: most green roofs do not detain water well enough. Let us explain.

_____

Read the article here:

https://www.purple-roof.com/post/green-roofs-stormwater-management-tools

_____

Don't hesitate to contact us if you have any questions at

What is Evapotranspiration Part 2:

Green roofs are tricky. On the one hand, we want evapotranspiration to be very high to get rid of stormwater as quickly as possible and prepare to absorb the next storm. On the other hand, we want the plants to conserve water to not dry out in the thin soil media between storms.

We also do not want to have to irrigate green roofs to support plants. It is a challenging task to design a plant palette that minimizes the need for additional irrigation. LEED, in particular, is giving the green roof designer a tough time balancing the desire for native plants, in a very shallow soil profile, and yet at the same time forbid irrigation.

This shows the importance of using different roof systems for different climates and abandoning the current strategy of one-size-fits-all for soil and vegetation.

Read this article on our blog:

https://www.purple-roof.com/post/what-is-evapotranspiration

Don't hesitate to contact us if you have any questions, suggestions, or comments!

What is Evapotranspiration Part I:

Detailed knowledge of evapotranspiration rates is essential for a successful green roof.

In this audio article, we dive into the topic of evapotranspiration.

If you would like to read the article, it is available here:

https://www.purple-roof.com/post/what-is-evapotranspiration

Evapotranspiration is merely a combination of the words: evaporation (evapo-) and transpiration (-transpiration). But what do these two terms mean? Let’s start with evaporation!