Green Roof Design Model Continued - Breaking Ground Contracting

Recall we previously discussed the two primary design variables for the Breaking Ground Contracting Green and Living Roof project, those being;

• Light availability, and
• Wind Impacts.

Light and wind are the two most important design factors to consider when designing a green roof or living wall for hot and arid climates.

Without light, plants cannot complete the photosynthetic processes and will not survive.  Even slight consistent wind impacts can evaporate water from the green roof or living wall plant leaf so rapidly the plant's vascular system cannot keep up, resulting in desiccation and death.  Understanding light and wind levels are crucial to green roof design.

The Breaking Ground Contracting green roof planting area models light and wind as shown in the following illustration:

Light & Wind Model - Green Roof @ Breaking Ground Contracting

Moving into the secondary design principles we look first to adjacent vegetation determining potential plant or tree allelopathic influences on the green roof as illustrated in the following.

Light & Wind Model - Green Roof @ Breaking Ground Contracting

The Breaking Ground Contracting Green Roof is lined on the west, south and east sides by relatively mature trees, including the following genus';

• Quercus laurifolia, Laurel Oak - northwestern corner and eastern side of green roof
• Betula (River Birch) - eastern side of green roof
• Platanus occidentalis, American sycamore - southeastern and southwestern corners of green roof
• Triadica sebifera, Chinese tallow - western border of green roof

Again, Allelopathy is the little referenced yet extremely important green roof secondary design principle of the bio-chemical influences certain plants and trees have on other plants and in this instance - on green roof plants. The Breaking Ground Green Roof planting area is surrounded on three sides by tall trees. Some of these trees are deciduous and loose their leaves during the winter, others like the laurel oaks keep leaf cover most of the year.

An alleopathic tree usually exerts negative influence on adjacent vegetation via a number of different processes including;

1. Fog & dew drip
2. Leaf litter
3. Volatilization 
4. Sap drip
5. Pollen, and
6. Other biological processes

Looking at the individual adjacent trees, research data shows us;

• Quercus laurifolia, Laurel oak - although literature suggests laurel oak does not possess allelopathic qualities, care should be given to potential impacts of pollen and flower litter. The laurel oak adjacent the northwest corner of the green roof has stained the white TPO and covered the roofing material with a layer of leaf and pollen litter. Though laurel oak may not exhibit direct allelopathic influence on the green roof plants, potential for covering the plants with litter exists. Continued site inspection will be required to confirm any impacts on the green roof plantings.
• Triadica sebifera, Chinese tallow - one medium height tree exists adjacent the western border of the BGC green roof. Chinese tallow has been the subject of numerous allelopathic studies and research. Interestingly, research exists to support the theory of Chinese tallow leaf litter and fog drip may actually support germination and shoot growth on adjacent plants. In fact, Chinese tallow was shown to actually improve germination and growth rates in Little Bluestem, Schizachyrium
• Importantly, the American Sycamores, Platanus occidentalis located in the southeastern and southwestern corners of the BGC green roof have the potential to exert significant negative influence over the green roof plants. As indicated in the above list, American sycamore produces strong allelopathic effects. Data exists showing the active ingredients, scopoletin and chlorogenic acid found in the sycamore leaf may interfere with the ability of stomata on certain plant's leaves to malfunction, interrupting the vital processes of photosynthesis and either stunting plant growth or killing the plant. Close observation will be required on the effects of the American sycamore on the entire green roof planting area and in particular, the southeast and southwest corner plantings. Pruning of sycamore limbs away from the green roof may be necessary.
• Betula, river birch is not represented as allelopathic in the available research data.

Good green roof design incorporates the effects of adjacent trees and other vegetation and allelopathic effect possibilities. Recognizing and dealing with a potential allelopathic problem is much easier and more cost-effective up front. Know the basics of adjacent tree and plant allelopathism and how your green roof design integrates into a site with pre-existing trees.

Impact of Adjacent Allelopathic Trees and Anti-Allelopathism Potential on Green Roofs

Allelopathy is the little referenced yet extremely important green roof secondary design principle of the bio-chemical influences certain plants and trees have on other plants and in this instance - on green roof plants.  The Breaking Ground Green Roof planting area is surrounded on three sides by tall trees. Some of these trees are deciduous and loose their leaves during the winter, others like the laurel oaks keep leaf cover most of the year.


An alleopathic tree usually exerts negative influence on adjacent vegetation via a number of different processes including;

• Fog & dew drip
• Leaf litter
• Volatilization 
• Sap drip
• Pollen
• Other biological processes

Trees impacting the Breaking Ground Contracting green roof include;

• Triadica sebifera, Chinese tallow - western border of green roof
• Quercus laurifolia, Laurel Oak - northwestern corner and eastern side of green roof
• Platanus occidentalis, American sycamore - southeastern and southwestern corners of green roof

According to the University of Georgia, School of Forestry Resources , there are a number of significant allelopathic trees requiring attention when planting other plants nearby.  They include;

Strong Potential for Allelopathic Impacts 
Acacia spp
Acer saccharum
Ailanthus altissima
Celtis laevigata
Celtis occidentalis
Eucalyptus camaldulensis
Eucalyptus globulus
Eucalyptus spp 
Juglans cinerea
Juglans nigra
Leucaena spp
Myrica cerifera
Picea engelmannii
Platanus occidentalis 
Populus deltoides
Prosopis juliflora
Prunus cornuta
Prunus serotina leaf 
Quercus falcata leaf 
Quercus marilandica
Quercus rubra
Quercus stellata
Robinia pseudoacacia
Sassafras albidum
Ulmus americana


Moderate Potential for Allelopathic Impacts
Abies amabilis
Abies balsamea
Abies grandis
Acer circinatum
Acer negundo
Acer platanoides
Acer pseudoplatanus
Acer saccharinum
Aesculus glabra
Aesculus hippocastanum
Aesculus octandra
Arbutus menziesii
Carya illinoensis
Carya ovate
Corylus spp
Crataegus spp
Fraxinus excelsior
Ginkgo biloba
Gleditsia triacanthos
Juniperus monosperma
Juniperus scopulorum
Kalmia spp
Picea spp
Pinus banksiana
Pinus contorta
Pinus densiflora
Pinus edulis
Pinus elliotii
Pinus monophylla
Pinus ponderosa
Pinus sylvestris
Prunus pumila
Quercus alba
Quercus borealis
Quercus douglasii
Quercus gambelii
Quercus michauxii
Quercus shumardii
Rhododendron maximum
Rhus copallina 
Sorbus sitchensis
Tsuga canadensi



Slight Potential for Allelopathic Impacts
Abies concolor
Aesculus spp
Betula pendula
Carpinus spp
Casuarina spp
Cupressus macrocarpa
Fagus spp
Fraxinus spp
Larix decidua
Picea excelso
Pinus palustris
Pinus spp
Populus spp
Pseudotsuga menziesii
Quercus petraea
Quercus robur
Quercus rubra
Salix pellita
Sambucus racemosa
Sequoia sempervirens
Taxus brevifolia
Thuja plicata
Tilia americana
Tilia cordata
Tilia planifolia
Ulmus laevis
Ulmus parvifolia
Umbellularia californica

As mentioned, the BReaking Ground Contracting Green Roof will be adjoined by Chinese tallow trees, Laurel Oaks and American Sycamores.

Quercus laurifolia, Laurel oak - although literature suggests laurel oak does not possess allelopathic qualities, care should be given to potential impacts of pollen and flower litter.  The laurel oak adjacent the northwest corner of the green roof has stained the white TPO and covered the roofing material with a layer of leaf and pollen litter.  Though laurel oak may not exhibit direct allelopathic influence on the green roof plants, potential for covering the plants with litter exists.  Continued site inspection will be required to confirm any impacts on the green roof plantings.

Triadica sebifera, Chinese tallow - one medium height tree exists adjacent the western border of the BGC green roof.  Chinese tallow has been the subject of numerous allelopathic studies and research.  Interestingly, research exists to support the theory of Chinese tallow leaf litter and fog drip may actually support germination and shoot growth on adjacent plants.  In fact, Chinese tallow was shown to actually improve germination and growth rates in Little Bluestem, Schizachyrium


Importantly, the American Sycamores, Platanus occidentalis located in the southeastern and southwestern corners of the BGC green roof have the potential to exert significant negative influence over the green roof plants.   As indicated in the above list, American sycamore produces strong allelopathic effects.  Data exists showing the active ingredients, scopoletin and chlorogenic acid found in the sycamore leaf may interfere with the ability of stomata on certain plant's leaves to malfunction,  interrupting the vital processes of photosynthesis and either stunting plant growth or killing the plant.  Close observation will be required on the effects of the American sycamore on the entire green roof planting area and in particular, the southeast and southwest corner plantings.  Pruning of sycamore limbs away from the green roof may be necessary.


Finally, good green roof design incorporates the effects of adjacent trees and other vegetation and allelopathic effect possibilities.  Recognizing and dealing with a potential allelopathic problem is much easier and more cost-effective up front.  Know the basics of adjacent tree and plant allelopathism and how your green roof design integrates into a site with pre-existing trees.


One of the related positive issues of anti-allelopathism and green roof adjacent trees is a benefit derived from leaf micro-nutrient content.  Tomorrow's topic will explore the antithesis of allelopathic impacts and look at the potential biological and chemical benefits from adjacent trees.

Modeling Green Roof & Rooftop Permaculture Plant Design Performance

As previously stated, light and wind appear to be the primary design variables impacting green roofs and rooftop permaculture.


Understanding how your rooftop garden or green roof plants - vegetables, native plants or other - are impacted by light and wind is very important.


Recall from previous posts, discussions about identification and boundary delineation of  roof areas according to levels of available light and present wind loadings.  The roof under consideration was sectioned off on a five meter grid and assigned design levels of wind and light influence according to the following simplified diagram.  Light and wind can be considered the two primary design factors.

Green Roof Design Diagram

This is not to say the secondary design variables are not important, as they are.  Secondary green roof design variables include:

• Climate and Weather
• Rainfall
• Non-Rainfall Air Water Vapor and Humidity
• Dew, Types of Fog and Frost
• Smog
• Temperatures
• Adjacent vegetation
• Nearby Allopathic Plants
• Adjacent Exotic Invasive Species, and
• Local Seed Source

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.

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.


Understanding how wind and light impact the green roof can afford the designer the capability to predict plant growing success.


The following diagram suggests that the optimal growing space on green roofs and for rooftop permaculture lay in areas with moderate to strong sunlight and still to slightly moderate winds.

Green Roof Design Diagram

Finally, understanding the light and wind levels across the roof under consideration can help avoid so-called 'dead-zones'.


Dead zones are areas on the green roof subject to light and wind conditions beyond suitable for most C3 and C4 plants.


CAM and other hardy succulent plants are those most desired for roof areas identified as harsh, a result of too much light and/or too much wind.


Know your wind and light levels.


Your green roof plants will grow accordingly.

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.

Green Roof Design and Stormwater in Florida

We are starting a series of blogs on green roofs and stormwater in Florida - especially since Florida's (FDEP's) new Applicant Handbook - the design manual for site Stormwater Permitting in Florida lists green roofs as a treatment option.

Click on the above link to download the manual.  The green roof section is located beginning on page 83 or so.

We are proud the State of Florida recognizes green roofs as providing stormwater treatment.

We are concerned with some of the material in the handbook though.

The handbook approaches green roof design from a stormwater storage function first, though.

As green roof professionals we believe the appropriate design for a green roof should be from an integrated approach - habitat, carbon sequestration, beauty, insulation and stormwater - not just primarily stormwater.

We are concerned the handbook allows primarily volume credit for green roofs.  Green roof design is approached from a 'stormwater retention pond' on top of a building approach.  The more volume you store between the roof and the associated cisterns, the less you have to treat for discharge.

This approach may be appropriate for stormwater treatment but in our opinion, stormwater treatment should be a benefit of good green roof design - not green roofs should serve stormwater treatment.

Not trival, our concerns are valid because under the stormwater handbook approach one ends up designing green roofs from a 'demand side' perspective - or peak load perspective.  Stormwater treatment systems are designed based on annual rainfall loading numbers, graphs and data as presented in the applicant's handbook.

A deep, intensive green roof may be great for helping handle stormwater on a site.  However a deep, intensive green roof may not be a sustainable system.  Lets discuss why.

Florida has an overall significant rainfall average amount of between 48" to 64" per year.  To handle that peak load, stormwater systems have to be rather large.  Green Roofs designed to attenuate or hold portions of that volume are usually substantial and, deep - 4" or deeper across the roof.  Cisterns are considered a vital part of a green roof system for additional volume storage.  The more volume stormwater your green roof can store, the less you must account for in other portions of your on-site stormwater system.


An intensive or deep extensive green roof system has more soil along with the additional storage volume.  More soil means deeper roots.  Check out the attached photo.

Green Roof Plant Root Architecture

The root system here is typical of a green roof plant with 6" of soil.  Roots grow down then out.

On deep stormwater based green roofs, the plants used will develop deeper root systems.  We've discussed this principle before - it is called acclimation - plants adapt to the site.  So with deeper soils found on stormwater roofs you will have deeper plant root systems.

Now, the dry months of the year run from mid-October to mid-March.  During this portion of the year the Florida average rainfall amount will be less than the required landscape plant evapotranspiration requirements.  In other words, during the wet summer months it rains enough in Florida to provide irrigation generally for most landscapes, including green roofs yet during the winter months irrigation is usually necessary because the rainfall amounts are significantly less.

Importantly, here in Florida though we have a large annual rainfall amount - over 50" per year, most rainfall events are less than 1" - and many less than 1/2 " in total volume.  This is representative of the typical afternoon summer quickie rain shower.

Going back to the photo above, many times after a rain event the first 1" of soil may be wet after an afternoon rain but the deeper soil can be very dry.  On deeper green roofs designed for stormwater systems this can be a problem for the plants.  Additional irrigation is usually required to reach the deeper plant roots.

So by building a deep green roof capable of handling heavy rainfall events as part of a site stormwater system we are designing in a landscape feature that actually requires irrigation.

The green roof helps reduce the overall annual rainfall discharge but requires irrigation to keep the plants alive year around.

This is what happens when you design a green roof to function primarily as a stormwater system feature.

We will talk about required fertilization in the next blog - and ask - why are we adding fertilizers to stormwater green roofs?

For now - let's agree green roofs can work wonderfully as a stormwater system - however - Florida green roof design criteria should not be based on green roofs designed for stormwater systems.

Florida green roof design criteria must also take into account the following - cost-effectiveness, habitat, water-wise design, beauty and other considerations.

A stormwater designed green roof will be very heavy when saturated with water and the cost associated with critical roof support may be way too high for many projects.

A lighter weight, thin extensive green roof system may not hold as much stormwater but will be less expensive.  A lightweight green roof may be used on many residential projects that could not afford a heavy, structural stormwater based green roof.

So the point here is - great news our state stormwater handbook specifies green roof design for credit in designing site stormwater systems.  Yet we must also realize - not all green roofs function primarily as stormwater systems and so design criteria cannot be restricted to stormwater purposes.

In future blogs we will explore habitat creation, food production, landscape beauty and other important criteria affecting green roof design.

Happy Green Roofing,  Kevin