Green Roof Retention and Detention

Retention and detention are, unfortunately, poorly understood concepts within the green roof industry.

Learning more about stormwater retention and detention has required us to step well outside our comfort zone, have conversations with people outside the green roof industry, especially with hydrologists - many of whom are unfamiliar with green roofs, but who understand the hydrologoic concepts thoroughly.

Purple-Roof is a concept - not a product - and the core of this concept is to design green roofs in a more scientifically rigorous way . It just so happens that the key improvement needed in green roof was to incorporate stormwater detention, in addition to retention, and other improvements will follow, most likely related to water quality. But that core scientific rigor will not be realized without greater understanding.

This video series attempts to convey what we have learned about the principle differences between stormwater retention and detention, for general understanding by architects, landscape architects, and anyone else who is interested in a detention roof or green roof hydrology.


Introduction to the Retention and Detention Video Series

Episode 1: What is Stormwater Retention

Learn the definitions of stormwater retention and stormwater detention. The differences can best be described by how the water leaves the roof: retained water leaves as vapor through evapotranspiration, and detained water flows out gravitationally at a later time. Retention is demonstrated by pouring water into a cup, and onto a sponge. Both the cup and the sponge retain water, and neither detains water.

Episode 2: What is Stormwater Detention

This video explores stormwater detention in general and conceptually using examples of streams and sewer pipes. In both examples we see that water volume is much less of a problem than intensity of flow. In the stream example, high intensity of stormwater runoff creates erosion. In the sewer pipe example, high intensity of stormwater runoff creates a combined sewer overflow (CSO) that discharges sewage into water bodies. In both instances, reduction of intensity - via detention - solves these problems. As seen in Episode 1, stormwater retention alone does not reliably reduce runoff intensity, so retention is not able to solve either problem.

Episode 3: Introducing Green Roof Detention

Continuing where Episode 2 left off, this video explores how green roofs can provide detention. This is demonstrated by filling the retention capacity of a Traditional Green Roof, a Sponge Roof, and a Purple-Roof. Neither the Traditional nor the Sponge Roof concepts provide any detention. Even at a tiny sample size of 2 square feet, the Purple-Roof concept demonstrates measurable delay, which increases significantly with scale. A simpler conceptual demonstration of the same detention roof phenonmenon is included at the end using beakers, which are similar to traditional stormwater detention tanks.

Episode 4: How is Green Roof Retention Tested

The green roof industry has traditionally focused on stormwater retention, but the ASTM test standard used to measure retention is not accurate. ASTM E2399 over-estimates retention significantly, because the test conditions do not replicate rainfall conditions on a roof. Addendum to this video: After filming, we became familiar with ASTM F1815, which is common used in golf course green design, but which is virtually unheard of in the green roof industry; ASTM F1815 is a much better predictor of actual retention. Green Roof Diagnostics is currently writing standards about how to apply ASTM F1815 to green roofs.

Episode 5: Conceptualizing Green Roof Retention

Green roof retention is fairly easily conceptualized from a horticultural perspective. Plants require a certain amount of water to survive - much less to thrive. If the plants are happy, their substrate is probably retaining enough water. If the plants are struggling or dead, their substrate is probably not retaining enough water. This is a very easy way to think about minimum retention volumes. But once we try to estimate maximum retention volumes, we need more sophisticated tools, such as modeling.

Episode 6: What's All The Buzz About Retention

Retention is a great tool for statistical volume redution: i.e. retention is excellent at lowering annual stormwater runoff volume for a city. The Purple-Roof Green Roof Retention Model is introduced, which estimates per-event and annual stormwater retention volume reduction. However, retention is ineffective during second-day storms and extra large storms, so retention does not reliably reduce peak stormwater flow rates. Stormwater detention reliably reduces peak flow rates.

Episode 7: Why Does Green Roof Detention Matter?

Using green roofs to provide detention generates clear return on investment (ROI), especially in urban areas. This is because nearly all jurisdictions require stormwater management to reliably reduce peak flow rates during design storms... which if you watched the prior videos, is only reliably provided via stormwater detention. If green roofs are used to store water on the roof and slowly drain out (i.e. detain), this frees up valuable real estate for other uses, such as parking, fitness center, child care, coffee shops, and more. Basically: a detention green roof can reduce or eliminate a stormwater tank, which is extremely valuable in most cities.

Episode 8: What is a Detention-Oriented Green Roof?

This video explores the green roof as a watershed. Stormwater detention within a watershed is usually influenced by distance, slope, and friction. On a rooftop, distance and slope are not usually variables that can be adjusted, but the Purple-Roof concept is able to adjust a key variable: friction. Incorporation of friction into a thin-profile green roof is tricky, but this key change allows the green roof to function similar to a natural watershed. Core to the Purple-Roof concept is converting bare roofs to detention roofs that replicate these natural phenomena.