Confusing Green Roof Terminology

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A friend of mine recently responded to our Purple-Roof blog article that defined Retention and Detention on living roofs (green roofs/vegetated roofs). It was clear that there had been a misunderstanding. I quickly realized that we defined the words Detention and Retention differently, and this, of course, warranted an article on the topic.

Living roof retention and detention

What started the discussion was that he said that every living roof detains. I disagreed. He continued to state that he had 18 hours delay on a 10cm (4”) extensive living roof. I claimed this was impossible.

It turns out we are both right. We were simply dealing with a misuse of definitions.

Why is this worthy of a blog? Well, our industry membership uses a particular set of industry vernacular, and then another industry filled with civil engineers uses another.

Civils are extremely sensitive to the correct application and meaning of the words ‘Detention’ and ‘Retention,’ and it hurts our industry’s credibility in the eye of the civil if we use them in the wrong context. You may ask yourself why we should care what the civil thinks?

The civil engineer is the gatekeeper to the success of the living roof industry. They decide what tools they deploy to manage stormwater, and when most if not all project civil engineers exclude living roofs as a primary stormwater management tool, we should take note.

Living roof terminology debate

We had a long and stimulating debate. It is important to know that my friend is a designer of long-lasting living roofs, and we are also designers of long-lasting living roofs that also comply with the needs of the civil engineer. Overlapping BUT different objectives.

One of the things he wondered why we always take the worst-case scenario that the living roof is fully saturated because most often, it is not. He is right; most days, it is not fully saturated.

Statistically, he is correct. BUT when we use the Purple-Roof green roof retention modeler on our site, you see that most rain events come in clumps (multiple days together). If a traditional living roof can retain only about 50% of the annual rain, it is because the OTHER 50% is mostly made up of what we call 2nd day or subsequent rain events.

These traditional living roofs do no re-charge their retention capacity quick enough for the next storm since they do not work 100% of the time: Only 50% of the annual rain events are retained.

If all rain events were nicely spaced out as a slow 2.5cm (1”) rain event applied every ten days, or so, we would retain almost 100% of the annual rainfall. But rain doesn’t fall nicely distributed. Rain falls in groups of 2-5-day rain events, and therefore a lot of the time the living roof is fully or almost fully saturated.

What is living roof detention?

When the roof is partially dry (or partially wet 😊) and it starts raining again, it is not correct to say in the civils’ eye that we are delaying the storm through detention, it is merely ‘filling up the profile to maximum retention”. Yes, that is a delay, but calling it detention is wrong and confusing. Detention is what happens AFTER a living roof is fully saturated.

My friend is also correct that every living roof he ever designed can detain water. I disagreed wholeheartedly at first, BUT I argued from the perspective of the civil engineer, and this is where we can agree and disagree. He is correct that every living roof detains in the purest sense of the word because if it only does so for 1 second than to be fair, he is right.

A civil engineer would completely disregard that time frame though. To us, and the civil a one-second delay is not worth mentioning, and it only confused the clients. Also, since the civil engineer is the gatekeeper to evaluate a stormwater management tool, we rather live by her/his rules.

It should be noted that traditional living roof with very long ‘distances to the drain, let’s say 76m (250ft) or so, will have a long time delay due to water that needs to travel underneath the living roof for that 76m (250ft) distance. But for most roofs, there is one drain per 186m2 (2000sqft). That limits the distance to the drain to about 6-8m (20-25ft). If you toss a bucket of water on the upper side of the roof, it will only take 30-40 seconds for that water to find the drain.

For a living roof to detain enough water, delaying the outflow by 2-3 minutes is not enough. We need to delay it for 1, 2, 3… maybe even 8 hours. On one roof we needed to manage a 15.7cm (6.2”) 24-hour storm, with a peak of 21.6cm (8.5”) per hour, and detain about 10.2cm (4”) of water across the whole plane of the roof for 8.5 hours to comply with the outflow rate of 1.89L/second/929m2 (½ gallon/second/10.000sqft).

So, we kept up to an 18 wheeler/16.2m (53foot) long trailer worth of water volume on the roof and released it over about 8.5 hours.

A traditional living roof would have done this in 2-3 minutes.

Lastly, he mentioned data that shows that his traditional 10.2cm (4”) extensive living roof from 1985 had an 18-hour documented time delay.

I said this is impossible from a wet-wet scenario. However, when we dove into the details, it turns out his statement was correct. The last drop of water did come off his roof 18 hours later. It’s a fact. Aha! He is talking about the tail length of the storm. Interesting. So again, it’s a matter of definitions.

A civil engineer is not that interested in the tail length of a storm. Managing the peak intensity of the storm, lowering the peak outflow rate, and delaying the peak of the storm (moving the centroid of the storm) is, however, interesting.

That is the critical bit of information to the civil, and to us, the civil engineer is the gatekeeper, and we provide the information that she needs, not what we want to hear.

As a side note: a very long tail of 18 hours is good…and bad. If it starts raining after 10 hours once more, the water in the tail now worsens the flooding of the roof. So, we want a time delay, but not too long; otherwise, it interferes with the next storm potentially.

Coming back to why a civil engineer only cares about fully saturated starting conditions vs. annual averages. That is simple: because the civil needs to manage flood conditions. She is not interested in ideal conditions; she is interested in the worst-case scenario. She is only interested in fully saturated conditions because those are the conditions that cause flooding. Anything that does not cause flooding is not relevant to the civil. Civils do not care about averages they care about handling the storm each time.

The best way a civil once explained it to me when I was talking still about traditional living roofs that work on average 80% of the time, my civil friend said: ‘ would you jump out of a plane with a parachute that works 80% of the time???’

Silence on my part.

“Oscar, I need a solution that works 100% of the time”.

“The living roof needs to detain the storm every time, or it is not useful to me.”

This is what we solved; this is how the Purple-Roof concept was born.

It works 100% of the time, every time.

Finally, it should also be noted that there are many other detention-based concepts for rooftops. We believe that diversity is key for a thriving future vegetated roof industry.
Read more about the other concepts in these two articles:
Top 3 Systems for Detaining Water on a Roof: Blue, Blue-Green, Purple and The Green Roof Industry - Diversity Benefits Us All!

& Using a Detention Roof to Create a Green Roof ROI

If you have any questions, comments, or feedback, don't hesitate to contact us at Purple-Roof!