2024 Track 1 Day 1: Liu


Engineering Green Roofs for Stormwater Management


Traditionally, stormwater is detained in a underground storage tank and released slowly through a controlled orifice to meet municipalities’ quantity control target. However, there are many scenarios that make an underground storage tank impossible (e.g. high water table) or financially unattractive (e.g. competing demand for parking spaces).

There is increasing interest in rooftop stormwater detention technologies such as blue roofs and detention green roofs.

Detention green roofs are specifically designed to detain and release rainwater in a controlled manner and can replace underground storage tank to meet municipalities’ quantity control targets.

Blue-green roofs detain rainwater in a reservoir under the green roof and control the release rate through control flow drains. They require 0% roof slope to be efficient and are best suited on dead flat plaza decks.

Friction-detention green roofs regulate the release rate by slowing down runoff through a friction drainage mat in a controlled manner. Since detention happens at the drainage level across the entire roof, it is efficient on low-slope roofs.

We will illustrate how to design detention green roofs through modeling to meet quantity control targets using different inputs such as design storm, maximum release rate, retention and detention storage volumes. We will discuss how to vary the green roof parameters such as reservoir cell, retention layer, and growing medium thickness to meet the municipal requirements. We will demonstrate via a case study the resulting cost savings and illustrate the importance of a multi-disciplinary design approach.


Karen Liu

Karen Liu, Next Level Stormwater Management​

Dr. Karen Liu is the Green Roof Specialist with Next Level Stormwater Management. She is a member of the CSA A123 Technical Committee responsible for numerous roofing standards and was a key participant in the research consortium that developed the national wind testing standard for vegetated roofing CSA A123.24.

Karen serves on Green Roofs for Healthy Cities’ Technical Committee, reviews green roof standards, and develops training programs for green roofing professionals. She also serves as a lay councillor for the Ontario Association of Landscape Architects.

Karen is an experienced researcher and educator and conducted green roof research at both National Research Council in Ottawa and British Columbia Institute of Technology in Vancouver. As a former global product manager with a major German green roof company, she has extensive practical experience across North America, Europe and Asia.