Green Roof Xericscape Primary Design Variables & Rooftop Polygons

Interestingly, according to the USEPA, Americans use approximately 1.5 billion gallons of water every day on landscaping.  Contrast this over-looked wealth and waste to the realization two hundred million hours each day are spent by families across less developed nations without adequate water infrastructure in securing daily domestic water supplies, some carrying heavy jugs of muddy water on their backs great distances.

Here in the U.S. we use twice as much water for landscaping than the number of gallons of gasoline we burn in our automobiles daily.  With the present U.S. population estimated to be about 311,000,000 persons, landscape water use is on the average about 5 gallons per person per day, or about 19 liters per person per day.  Yet fortunately many governmental agencies are presently encouraging use of native species and wildflowers acclimated to reduced watering or nature based irrigation.

So, if sustainable development practices call for conservation of water in the landscape then irrigation in green roofs should be no hidden exception.  

Because roof ecosystems are subject to significantly harsher biophysical conditions than most ground level landscapes, industry response sometimes has typically been one of adding irrigation and fertilizers to hopefully mitigate additional heat, dryness and desiccating wind stressors typically impacting green roof plants.  Moreover, because the green roof industry here in the US is still relatively young there is a lack of detailed design data to assist in planning and installing nature irrigated green roofs.

Fortunately water conservation practices in green roof design can be simple and cost-effective.   Though one can delve deeply into design theory, effective nature irrigated green roof design theory can truly be best understood with spending time outdoors in and around the project site, looking up and paying attention to what is already there.  We shall see that though we can model design variables in an attempt to analytically predict ‘what works best on a green roof’ sometimes a walk through the town, looking up to see what plants grow naturally in gutters, in the cracks of mortar and across roofs, provides the most useful design information.

To fully understand the design criteria needed for a nature irrigated, native species and biodiversity focused green roof we must first divide the rooftop area into simple polygons that are representative of existing environmental factors.  We want to know where on the roof areas are exposed to harsh, desiccating winds and we want to know where on the rooftop the sunlight becomes either lacking or unbearable.

Understanding these rooftop design variables is made easier through the use of rooftop polygons and there are two abiotic categories of green rood design variables to be used in calculating roof polygons;

• Primary Variables, and
• Secondary Variables.

Primary Variables include those variables that may vary based on individual roof polygon, including;

• Light, and
• Wind.

Secondary (yet important) variables include those design inputs remaining generally consistent across the roof and not considered in the roof polygon calculation.  They are;

• Heat Zones
• Cold Zones
• Precipitation and Water Vapor Profiles
• Smog
• Allopathic Plants Nearby, and
• other variables.

Though not used to determine the roof polygon boundaries, the secondary variables play an important role within a design model considerations and we will discuss those interactions in detail in future sections. 

Importantly, certain light wavelengths are required for photosynthesis.  Without light photosynthesis does not occur and plants do not grow.  Light is the first primary design variable to be considered when creating roof ecosystem polygons.  Understanding where on the roof photosynthetically reactive radiation and light volume measured by Daily Light Intervals or DLI is critical to good green roof design.

Wind is the second of the primary design factors used to determine roof ecosystem polygons.  

Though ambient outside air temperatures, precipitation, air quality and other design variables are generally consistent across the roof, light availability and wind effects can change depending upon where one stands on the same roof.  Both light and wind are critical factors in designing a green roof for dry and arid climates.  Without adequate light plants will not live and consistently buffeted by desiccating winds plants may cease to transpire and quickly die.

Over the next couple of weeks we will be exploring xeric green roof design variables.  First, as an introduction to developing rooftop polygons an Youtube powerpoint presentation is included here for review.

The presentation was used as an introduction for native plants on green roofs but has a solid explanation of rooftop polygon development included.  After reviewing the powerpoint you will have an understanding of rooftop polygon development, and how the two primary design variables can be incorporated into the green roof model.

Further discussion of why light and wind are the two primary green roof design variables will be included in subsequent articles.  Enjoy the powerpoint worksheet.  TOmorrow's post will discuss in length why light and wind are te two primary design variables for green roof plantings.