“The Devil is in the Detail” best describes the lessons learned regarding the design, construction and maintenance of the Green Roof for this project.
The intent of including the green roof in our project was to experience first hand the benefits and challenges of this roofing system. The proclaimed benefits of a green roof as follows:
- They return the biomass that was lost during the development of the project
- They moderate the surrounding and internal temperatures of the building
- They provide a place in urban areas that allow for growing of crops & visual appeal
- They help to manage the required on-site storm water retention regulations
- They have a high degree of aesthetic appeal
During the process of including a green roof in the design I discovered the following challenges:
- The soil science of growing plants on a roof is extremely complicated and needs to be simplified to help owners make informed decisions.
- Green Roofs have only recently been third party tested to verify that they indeed manage on-site storm water retention. EB News (Environmental Building News) Volume 17 Number 10, shared the results of the recent findings on this subject.
- They found that while all the tested roofs in the study were capable of significant thermal moderations, some held water well while others were barely better than conventional roofs at preventing or slowing runoff. When the roof was designed to include larger planting-medium retention cups, low drainage-hole area in the drainage layer, and a high proportion of perlite or soil absorption material in the planting mix it correlated with high water retention.
- This same article verify that indeed the green roofs do a good job of reducing the roof temperature of the building in lieu of a black or even white roof. This can greatly affect the possible benefits of large commercial buildings that are attempting to cool their buildings or reduce urban heat island effects.
- The embodied energy of Hydrotech’s Green Roof growing medium needs to be improved. The production of intensive energy using material included in their soil media seems to be counter productive to the sustainable doctrine.
- The recommended hot applied water proof membrane system for the Green Roof is not that much different than a high quality hot applied built-up roof system normally specified for commercial buildings. The main difference I found was that the recovery board that goes over the hot applied water proofing has a root inhibitor included in it. Two types of hot applied systems seem to be the main ones on the market, one has more recycled material included and the other has less but is more cost competitive. The less costly one also has fewer features that make the quality control of the application of the product more reliable.
We had a bit of trouble on our project, the contractors selected for the project seemed to have had a communication breakdown and the product installed was not the Hydrotech 6125, Hydroflex RB II, associated water retainage cups, (GR 15 Garden Drain), and the root barrier filter fabric that was specified. So after a couple of weeks of steady conversations with the contractor and product representative for Hydrotech we added the correct material over the originally applied material and now have over 400 mils of roofing material on our roofs in lieu of the 210 mils intended. Which for those not familiar with the unit measure of mils 400 mils is nearly ½ inch of material.
One of the best outcomes of going through the process of selection and constructing a green roof was the introduction of product called GaiaSoils. This is largely credited to my sister-in-law, Mem, sending me an earth day news article from the NY times that I am attaching for your readingpleasure. GaiaSoil is an amazing soil, it will be replacing Hydrotec’s suggested soils mixture with a soil that is made from non-toxic recycled expanded polystyrene foam, coated with organic pectin, mixed with high quality finished compost. Here is the comment from the Hydrotech’s soils scientist after reviewing the GiaiSoil specifications:
“Technically for plant survivability there is no issue with the soil. They do recognize that there may situation where this soil can be utilized but with strong cautions.
The draw back;
"American Hydrotech has looked into this soil and similar soils on the market place and they have chosen to not use or recommend the soil. The issue that we have is with wind uplift on roofs and wind erosion. When the soil is dry there is too little weight holding the system in place. I would recommend using a permanent erosion control mat to hold the soil in place to prevent the soils and insulation from blowing off the roof. The Garden Roof is a Protected Roof Membrane Assembly and there are guidelines stating the minimum weight or ballast that is required to secure a roof in place. In American Hydrotech’s Binder under the insulation section there is Dow Chemical’s Tech Note 508 that covers most of the conditions for wind uplift.
The last draw back on the soil is the moisture retention will be less than our soil or typical green roof engineered soil.”
I decided after looking through the data regarding moisture retention that the amounts differed so little and since our site had very little exposure to heavy winds the use of a more environmentally sound soil was worth the small risk we would be taking. Thus, for our situation I determined that the use of a recycled sytrofoam from New York Cities fish market was a better soil medium component than the use of Hydrotech’s recommended shale type soil mixtures that need to go under tremendous heat to create the “cat litter” type substance that they use.
Finally, I need to thank Nathan Salo for building a overflow drainage system for our green roof that will be admired by this architect for years to come.
From the wood plywood deck up is the assemble used for the green roof:
# 8 & 7 -- Hydrotech 6125 EV-FR– Monolithic Membrane is seamless, fully reinforced rubberized asphalt membrane. It consists of one coat of membrane at 90 mils into which Hydrotechs Flex Falsh F (a spunbonded polyester fabric) is embedded. A second coat of membrane is then installed a 125 mils. The total membrane thickness is 215 mils.
In our case since they installed a Carlisle Coatings and Waterproofing CCW-500 hot applied Liquid Membrane 305364 in lieu of the Hydrotech 6125 we put the MM6125 over the Carlisle product for a 400 mil application.
#5 -- Hydrotech RB II – a heavy-duty, granular-surfaced, modified asphalt sheet with an integral root inhibitor blended in. It is used when intensive applications or whenever aggressive root structures are anticipated. It is also used in sloped applications, as the granular surfacing provides additional slip resistance.
#4 -- Drainage/Water Retention Component GR15 – These are water storage troughs, or cups, on the top side of the panels that retain additional water for use by the vegetation. Diffusion holes through the panels allow air circulation and water vapor to move up into the root zone.
#3 -- Filter Fabric - This helps to prevent soil particles from washing through the system and potentially causing drainage layers and drains to become clogged.
#2 -- 6” GaiaSoil – This is the growning medium for the plants, the portion of the soil that both retains and drains water as well as provide the minerals to the plants.
Walking trellis -- Recycled content garden trellis’s that will be installed horizontally over the GaiaSoil prior to the final lay of local compost. This will assist with the erosions control and compaction of the soil as you walk on it to maintain the plants.
1 ½” Compost -- This will be continually applied to the soil over the years as the need occurs to keep the plants healthy and growing properly. Similar to what you would do for your own garden plants in your yard.
Plants will be selected with help of Gus Blumer the Landscape Architect for this project and Allyz Kraemer my friend the boreal forest biologist.
We will be providing rainwater collect water to irrigate the green roofs through hose bibs and drip lines provided on the roof.