Friday, July 8, 2011

Brown Green Roofs, Green Roof Forensics

Any green roof designer has had plants die from time to time.  Roofs are one of the most harshest places to grow across and good green roof design involves much more than basic landscaping.

Here in the tropics an understanding of environmental factors is crucial to ensuring green roof survival, beginning with a solid knowledge of plant selection. Importantly, long-tern survival depends not only on appropriate underlying green roof support systems, but soil media and 'Right Plant Right Place' issues also.

The tropics, for instance possess finicky climates with wild temperature swings, horrendous humidity, extreme temperatures, desiccating winds, salt spray, stealth-like killer fungi, hurricanes and more.

This doesn't mean green roofs do not work in the tropics, they do!

Unfortunately, the attached photos depict what can go wrong with a green roof here.
Struggling Green Roof

Sedum is not generally considered a plant suitable for the tropics.  Note I say generally as I am sure there may be exceptions.  Southern Blight fungus and hot, sticky humidity overwhelm sedum during the hot summer months.  The same applies to many Delosperma also.  

Rainfall quantity distribution here is unusual.  In many places receiving 150 cm rainfall each year the majority of that amount may come in only a series of heavy downpours.  Green roof system drainage must be designed to handle extreme water flows, otherwise failure may occur.  Apparently some of the soil media here has ended up in the swimming pool below.

The example shown here did not possess additional irrigation.  Instead as the plants withered, a rotary sprinkler was pulled up to the roof in a futile attempt to save the plants.  Im not sure how long the sprinkler ran, but it did run long enough to erode grooves in the soil media.
Irrigation erosion from sprinkler

A green roof system without irrigation is a environmentally friendly approach, and can work.   However other factors, including appropriate plant selection must be considered.

Soil media must reflect site specific drainage requirements.  Soil media high in fines may blind geosynthetic fabrics which comprise portions of the green roof drainage system, restricting vertical permeability and causing soil media to wash off the roof.

In this case it appears a lack of vertical drainage has caused rainfall to rush across the sloped roof soil media surface, scouring the grit and producing serious erosion.  It is entirely possible that during a tropical storm, such as TS Fay in 2008 where 600mm+ rainfall dropped in some places, the soil media here would end up in the gutters or on the ground unless appropriately stabilized.  Moreover, damp water retention pads and even the smallest of pooled water depressions can lead to hoards of pesky mosquitos.

Green Roof design involves much more than planting inexpensive fertilizer pumped up plants on a roof platform.

Appropriate green roof design takes into account wind, light levels, available daylight intensities, temperatures, invasive aspects, native plant considerations, nutrients, air quality including NOx levels, pests, adjacent potential allelopathic influences, air water vapor and much more.

Investing in a Green Roof Professional's time up front will save significant costs in the long-term.

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