Reinventing the traditional vegetated roof for detention

Within two days in August 2018, Toronto received almost 14 per cent of its annual rainfall, causing great damage and more than $80 million in insurance claims. Photo ©
Within two days in August 2018, Toronto received almost 14 per cent of its annual rainfall, causing great damage and more than $80 million in insurance claims.
Photo ©

Engineered growing medium

Typical vegetated roof media are higher in mineral aggregates and lower in organic matters, allowing for good drainage and rainfall retention (for more information, read “Review on the roles and effects of growing media on plant performance in green roofs in world climates” by F. Kazemi and R. Mohorko, published in 2017 in Urban Forestry and Urban Greening, and “Moisture Measurements as Performance Criteria for Extensive Living Roof Substrates” by E. Fassman and R.J. Simcock in the Journal of Environmental Engineering, 2011).

Water-retention layer

Recycled synthetic fleece or natural rock mineral wool increase the water holding capacity of a vegetated system.


The three-dimensional composite layer provides drainage and a water reservoir, designed to permit excess rain to flow through to the roof drains. The drainage layer can be manufactured from looped nylon filaments or fabricated from modular cups. The latter also offers some retention capability.

Root barrier

A flexible and impermeable sheet made of low-density polyethylene (LDPE) protects the roofing membrane from root penetration.

While friction detention technology can be added to almost any structure designed with vegetated roofs, the most suitable buildings have large footprint, space constraints, and require cisterns. Such buildings have larger podiums as opposed to a small tower roof and could use the space made available by reducing the water storage tank.

Determination of suitable projects is made by running calculations based on:

  • roof plan with dimensions showing drain locations in relation to building’s parapet and roof slope to each drain on either side of the cricket;
  • site development plan; and
  • regulatory site requirements.

Good retention but limited by weather

Vegetated roofs retain water well when they are mostly dry. Retention is good at managing small rainfall events as they hold a certain quantity of water and never allow it to flow into the roof drain. This water only leaves as vapour, slowly, taking about one to two weeks for 25 mm (1 in.) of water to fully dry out through evaporation and transpiration, collectively referred to as evapotranspiration (consult B. Garner’s Green Roof Diagnostics data accessible via Purple-Roof Green Roof Modeler, 2019). Evaporation is the process of the sun and wind converting liquid water to vapour. Transpiration is the process of plants drawing water from the soil and releasing the water through pores in their leaves. Transpiration is faster than evaporation, which is why green infrastructure can efficiently convert water to vapour. However, the rate of transpiration and evaporation are weather dependent, so little can be done to speed them up. The effectiveness of a traditional vegetated roof in managing stormwater through retention depends on many factors, mostly the amount and frequency of rainfall.

When saturated, a traditional vegetated roof requires ideal weather conditions to dry and to have the capacity to absorb the next rain event (read “Green roof performance towards management of runoff water quantity and quality: A review” by Berndtsson J. Czemiel for Ecological Engineering, 2010). This may take weeks. Also, when it rains back-to-back for several consecutive days or when monsoons last months, traditional vegetated roofs provide little SWM benefit. Further, there are also the local climatic minimums and maximums—all cities are subject to one-in-100-storm events that building professionals must plan for. Large storms are, of course, the most destructive, and this is where detention technology is implemented in the form of cisterns, ponds, or rain gardens as more reliable tools to meet municipal stormwater regulations.

Unpredictable, extreme downpours continue to flood communities

In most climates, approximately half the rainfall volume occurs in small, and less intense rain events, such as brief gentle showers or misting rain. The other half occurs in larger or intense storms (read “Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof” by C. Butler and C.M. Orians for Ecological Engineering, 2011).

For example, in 2018, Toronto received a total of 885 mm (35 in.) of rainfall. Of that/total, 75 mm (3 in.) poured down on August 7 in an intense storm causing power outages, disrupting public transport, and flooding parts of the city (details at M. Welsh’s “Toronto keeps flooding when it rains hard” article in the Toronto Star and M. Mann’s “The age of the flood,” article in Toronto Life).

Less than 10 days later, another volatile storm brought 50 mm (2 in.) of intense rainfall, flooding Toronto’s Union Station and causing commuter chaos downtown.

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