Best Methods to Test Green Roof Retention

by Dr. Anna Zakrisson on Thursday, July 30, 2020

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How is Green Roof Retention Tested?

Testing green roof retention – improving testing standards

Green roof retention has been poorly tested in the past. There are standards available that traditionally are used to test retention. The problem is that that is not what they do. Their original purpose was to establish the maximum weight of a saturated roof.

This video explains how green roof retention is tested according to current standards and how we could improve these methods to gain more accurate numbers for real-world green roof retention.


Testing green roof retention video script

Today we’re going to look at TESTING green roof retention.
Joep: So, retention is water held on the roof that can only escape as VAPOR through evapotranspiration. The capillary micro-pores retain water in the soil, and absorbent materials like needled mineral wool, plus any water retained in any storage cups underneath.
So, Dick, can you explain how we measure retention?
Dick: There is NO protocol.
Joep: Wait, you were supposed to talk about ASTM…
Dick: Yes, there’s a test ASTM E-2399 is often USED to measure retention, but that’s not ACTUALLY what the test does. The test was written to determine the maximum weight of the green roof.
Joep: OK, so maybe let’s look at misconceptions about the ASTM test.
Dick: Great idea. I’d love to clear this up once and for all!

Here is how the ASTM test is done.
Here we have two identical green roof profiles. First, let’s see ASTM E-2399 run on one.
First, we establish the weight of the dry components. Obviously, this is not completely dry, because the plants are alive.
Then we submerge the profile for 24 hours to fill up every possible pore space with water.
Then drain it for 2 hours, and weigh to see how much water it is holding.
Joep: That looks pretty thorough. But how much is it like rain?
Dick: Here’s what happens when it rains.

So which one is going to give you the most accurate weight of the green roof if it becomes completely saturated?
Joep: ASTM.
Which method is more like what actually happens?
Joep: The second one. It’s like rain.
Joep: This is very interesting! Every engineer asks for the ASTM test for retention value. And that goes directly on the permit drawings.
Yeah, for now. And people use to use things like rotary dial phones. And fax machines. And the Macarena.

A company called Green Roof Diagnostics is doing some groundbreaking research about how to calculate retention values. And the ASTM test for retention doesn’t do well for this purpose. And you get more bang for your buck with detention anyway. So stay tuned for more on detention!

Further Reading on Green Roof Retention

Check out these articles if you would like to learn more about green roof retention!

What is Green Roof Retention?

What is Green Roof Retention and Why Does it Matter?

Evapotranspiration Rates Determine Green Roof Retention

Evapotranspiration - the Green Roof Engine