Does a Detention Roof Make Sense for Your Project?

by Oscar Warmerdam on Wednesday, March 4, 2020

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How to determine if Detention Roof is a good fit or not

Detention roofs are incredible workhorses and are excellent stormwater management tools, provide all the other benefits of a green roof, and can provide a clear ROI for a green roof. However, not all projects are suitable.
In this article, we discuss the situations where a detention roof, e.g. a Purple-Roof, is a great solution and where you have a lot to gain from installing one.

We also discuss where other solutions might be more suitable. In the end, we want every project to be a success and we don’t you to waste your time with technology that is not appropriate for your specific project situation.

The first step in finding out if Purple-Roof Detention on the roof has any merit it is to ask the following questions:

  1. Are you required to do a green roof?
    If so, upgrading it to Blue-Green or Purple-Roof might save you money if you live in an urban setting.
  2. Is your lot almost the same as the size of the roof? Are you occupying most of the lot with a building? 80% roof/lot ratios mean that either a Blue-Green or Purple-Roof (Detention Roof) is most likely going to be a good solution.
  3. Can most, if not all the roof, become a green roof, or do you have any equipment, swimming pools, etc. located on the roof?
    Is a Detention Roof the best solution for me?

Nonetheless, before proceeding, you should also consider the following questions:

  1. Why do I need to detain stormwater?
  2. What are my options to detain stormwater?
  3. How big is the Design storm and how small is the allowable outflow rate? What size is the problem /challenge?
  4. Can we handle the weight of the Detained water on the roof?
  5. Can we absorb the cost of the Detention-type Blue-Green or Purple-Roof?

1. Why do I need to detain stormwater?

Regulatory requirements (often different than permit code) ask you to fully detain all the rain on your property during a large rain event.

What is a design storm?

A Design storm is a rain set rain pattern established by the local municipality that has a peak rain intensity and total accumulated volume over time, usually 24 hours. This is the storm that you have to comply with.

The Design storm looks like an upside-down T, with most of the rain falling in the center of the time frame. These kinds of storms are the ones causing flooding as they suddenly appear and are gone before you know it, which is also why they are causing so much trouble, it’s not the volume of rain that is the issue. It’s the intensity.

Intensity is the key term to remember: The water that falls onto hardscaped city surfaces needs to be kept in place temporarily and released slowly for it to be harmless to the surroundings.

It needs to behave like it would have done before mankind developed modern cityscapes. The local municipality would ask you to create conditions that are similar to what they call “pre-existing conditions” where surface runoff would encounter a certain amount of downhill friction that would slow down the velocity and delay the water as it meanders towards the creek. Hard road, roof, and parking lot surfaces have no friction to speak of, and any water that falls on it needs to be detained.

2. What are my options to detain stormwater?

A detention device is a void space where water can be quickly collected and slowly released. Detention devices can look very different and can be above ground bioretention cell or a rain garden, an underground tank, Blue-Green Roofs, or in our case a Purple-Roof.

There are many more detention options out there, but these are the same functionality wise.
The objective is simple: Gather the storm quickly and release it slowly.

3. How big is the Design storm and how small is the allowable outflow rate? What size is the problem /challenge?

Things to consider are firstly what Design Storm does the property have to comply with? That means how big, and what shape does the Design storm have where your project is located? Does the property need to meet the 25- or perhaps the 100-year Design storm?

Secondly, where is the Detention volume releasing into? Is it releasing the stormwater at the allowable rate into the sewer, or may infiltrate it into the ground? This doesn’t matter as long as you understand that everything has an allowable outflow rate.
A sewer has a very specific allowable outflow rate as set by the city, and also if you let the water infiltrate into the ground, the inflow infiltration rate of the soil is the restricting factor.

The exact sewage outflow depends on your local jurisdiction. Someone at the regulatory department sets an allowable outflow rate that is usually expressed in liters/second/hectare (cubic feet/per second/acre).
This rate might differ on both sides of the street, or per district of your city. It all depends on the existing sewage capacity where you connect to. About 35L/s/ha (0.5 CFS/s/acre) is a common outflow rate.
Take for example a 3700m2 (40.000sqft) building. During a 10-year storm in some typical New York City that may very well accumulate 81L/m2 (2 gallons/sqft) so 302m3 (80.000 gallons) accumulate in a matter of 45-60 minutes of time.

That is about 10 full-size ocean containers…

That is the inbound amount. So you have to for every project there is a set INBOUND (Design storm) and a set OUTFLOW (allowable outflow rate) and the difference between the two is the storage volume needed of the bioretention cell, the tank or the Blue-Green or Purple-Roof.

Where different types of Detention solutions make sense:

In the suburbs a Bioretention cell makes the most sense:

If you have space, a bioretention cell makes the most sense. It’s usually the cheapest solution but it’s not as cheap as you think. $330-390/m2 ($30-35/sqft) is a reasonable actual installation cost. And they are requiring landscape maintenance, plant replacement, and usually large-scale servicing and/or replacement every 7-10 years. However, a bioretention cell ads biodiversity, landscape, cooling, and aesthetics.
From a water perspective, it evaporates about 25-30% of the annual rainfall, and the water that is released is filtered and usually considered a water quality improvement.

But, if you don’t have a garden, you need to design something that goes underneath the building. Then, a bioretention cell makes no sense.

In the city a Detention tank makes the most sense:

If you don’t have a garden, then you need to place the water in the basement under the building. You need to deal with construction delays, rock bottoms that require blasting or hammering, or soft bottoms that require pillars to support tanks.
All of this is very costly, plus imagine what you could have done with all that lost space; you could have rented it to a brewery, built a fitness center, created parking spaces, or added a supermarket.

Lost revenue is truly something to consider.

Further drawbacks of tanks are the following: if and when it leaks you will never find out, meaning that for 100 years contaminated rainwater can leach into acquirers. Also, tanks do not reduce volume, they simply hold it for a bit and release 100% of the storm, so there is no benefit to the city except for time delays. It is an old gray infrastructure that brings no secondary benefits whatsoever.

Alternatives to a Detention Tank? Detention Roof!

If you don’t have space in the garden or in the basement, leaving the water on the roof is the smartest place to keep it. As a matter of fact, if you are dealing with a situation where green roofs are mandatory, upgrading the green roof to a Detention green roof (such as Blue-Green or Purple) makes the most sense.

A Detention Roof is a storage device with the predictable allowable outflow rate of a tank. We have many years of data to prove this.

4. Detention Roof: can the roof handle the weight?

  • Dead load: Can your building handle and extra 59-88kg/m2 (12-18lbs/sqft)?
    Usually, they can if they are made of concrete, or if they design it with an extra 59-88kg/m2 (12-18lbs/sqft). This is the super conservative route.
  • Live load: Since we are only holding the peak amount of the rain for a few hours, the practical route is to take this out of the live load budget. In most cases, the snow load budget will be enough. For instance, in most east coast cities there is 18-36kg (40-80lbs)/snow load so utilizing 59-88kg/m2 (12-18lbs/sqft) from that budget is more than acceptable.

5. Can we absorb the cost of the Detention type Blue-Green or Purple-Roof?

Well, there is opportunity cost: What if we can park 4 more cars, or rent out 111m2 (1200sqft) extra basement space to a fitness club? Multiply this over 40 years and reap the benefits.
And there is actual cost: Upgrading a green roof to a Blue-Green or Purple-Roof will add $56-78/m2 ($5-7/sqft) to most green roof designs. But it will eliminate the tank.
Tanks cost hundreds of thousands of dollars in engineering, products, and installation.

More often than not, you will conclude the same as we do. Detention on the roof makes the most sense, for the pocketbook, for the environment, your karma, and the bees and butterflies.