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Advances in Shallow & Green Roof Design – Making do with less

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In the seminar “Building in Nature’s Image,” during Build Boston 2010, Jeff Licht of Botanicals Nursery and the University of Massachusetts, Boston,showed how examples from nature, or biomimicry, can be used to harness and conserve energy within buildings. Jeff Licht showed how to break away from the formulaic rooftop layer of sod that many designers opt for as a “green” roof, and how to instead create a “roofscape truly inspired by nature.” In his practice, Licht looks carefully at an area’s local ecosystem to find examples of biological features – such as osprey nests, drumlins, rock outcrops, pine barrens or subalpine meadows – to inspire alternatives to the more-common ingredients of planted roof installations, such as metal gratings, plastics and trucked-in soils. In his roofscapes, you might find corn, wood pellets and biochar substituting for imported soil and minerals, with natural plant species to better mimic local ecosystems, and mini-hills and swales that provide natural drainage patterns. These natural-looking garden roofscapes have proven hardy even in tough locales where previous green-roof installations have failed. As President of Botanicals Nursery LLC, he has applied native plant research to the design of specialized lightweight media to grow these plants, enhance storm water management and generate carbon sequestration benefits accruing from the use of unique, nearly inert organics. Excerpts from his talk at Build Boston…

Much of my professional work consists of testing ideas which if found useful make their way to the design table and from there to the construction of vegetative spaces on roofs, balconies, decks and other appurtenances. Because I work in a cold temperate climate in the northern part of the Northeast United States it colours my approach to research; yet; I believe there are some universal truths found under and amongst the concepts and claims I will make in this brief paper. They are based on my engagement with “green roof” professionals, scholars and students. I will deal only with built in place roofs which are shallow or somewhat shallow horizontal surfaces. There are three concepts which are important to those engaged in or contemplating the “green roof” territory of the “sustainable building” realm. The first is defining useful parameters of the green part of the equation – the origin of adapted plants, physiological attributes and the type of growth media which will adequately support them on a roof. Next, we should understand the role which growth media plays beyond horticultural value including contributions to functional eco services of the roof environment – the term eco services is used here to denote a combination of ecological benefits which accrue from the combination of a roof assembly system, growth media and plants. Lastly, I want to briefly discuss a “next” step in the evolution of “green roof” assembly systems past the current crop of fabrics, plastics, metals and ceramics. The cost of extracting, factory production and shipping represents a significant financial and environmental decision supported by designers, general contractors, their installers and clients. We should consider ways to deconstruct and rethink current strategies to streamline the design and construction of roof projects from a more recyclable perspective.

Envisioning a multiple subtext to think putside the green roof box

In New England, we create design/build opportunities based on a unique regional perspective which reflects climatological factors we share with maritime Eastern Canada though not with the rest of the United States. While there are many potential avenues of investigation and application, I want to stress what for me is an important connection between climate, recycling and rethinking of horizontal roof vegetation projects. I believe an effort to deconstruct and then reconstruct a new biology-based and recycled-based approach will provide a useful impetus for ways to influence future design/build decisions here in New England and elsewhere. Let us begin with a look at rethinking the assembly concept. First, it is well understood that containment of plants and growth media on a roof is constrained by several features. One of these is the style or approach to containment and this is generally defined either as built-in-place (BIP) or non BIP systems. This latter group consists of either trays or modules, both of which are more similar to each other than to the straight BIP. For both BIP and non BIP, the following are necessities:

  1. need for a non-vegetative space adjacent to the vegetation;
  2. some vertical barrier, however modest, to serve as a backstop to a roof assembly;
  3. a decision as to whether one should build a “green roof” in place or bring pre-vegetated containers (trays or modules) to the roof and place them there; or
  4. a decision whether or not to house the built in place assembly within a permanent containment structure or one which is impermanent.

For the purposes of this paper I will concentrate on what I believe are greater benefits for the BIP than for the non BIP systems – based on both my own experience and that of many professionals with whom I share “green roof” work. Especially where the latter (non-BIP) are so utterly dependent on extracted minerals, many exotic plants, highly produced plastics, metals and fabrics, compared with a built in place which need not depend on these amenities. While they are structurally different, both BIP and non-BIP are layered with protective fabrics underneath and soil barrier fabrics on top (sometimes in the middle for non-BIP) which are themselves covered over with a filter fabric which prevents fine growth media from migrating down through the containment onto the waterproofing and into storm drains or scuppers.

The multi-tiered water containment and fabric components of the system assumes that this kind of dual (or sometimes triple) fabric make up a singular mode of media and water retention. This is a framework which I have re-conceptualized where other materials are used to create a different method and result. The use of new materials represents a reworking of the bottom of the system framed in terms of conscious mimicry of naturally occurring terrestrial landscapes in New England. In turn, this mimicry reflects the simplicity and functionality of the landscape which can be emulated to some extent with recycled materials which emphasizes the same dynamic processes exploring ways to propagate and grow regionally appropriate plants.

Alternative system materials

The reinvention process reflects not only replacing typical “green roof” components – drainage, filter fabric, protective fabric parts – from the bottom up but a replacement process which carries a low or no-production cost. Discussions with materials scientists in Wisconsin led me to look at OCC (old corrugated cardboard) as the means for replacing several parts of the typical assembly system. The actual OCC material such as cartons from offices, dumps and other waste sites (which have not already been compacted and resold to the paper industry for reincarnation as new boxes and cartons) were used. Clearly there are other post-consumer and post-industrial products which could be used but the OCC I chose is available in many parts of the world in sizes and shapes which can be fashioned quickly and easily to create containment. One of the problems with defining alternatives to either typical BIP (but certainly trays or modules) is not being able to get away easily from the container theme. What I have found as I began to experiment with OCC (and we are now starting to refine the process at UMass Boston) was that the availability of “waste” was not limitless but quite easy to locate and remove from a stream of waste before compaction, resale and reuse for new cardboard products.

Due to the substrate depth limitations imposed on cold climate “green roofs” where I work, I required a minimum OCC depth of 4.5cm but for those of you in warmer temperate climates such as Latin and South America, Africa, South and Eastern Asia and much of Australia, this depth can be adjusted. Once you have the OCC, how does it get used? I elected to use this either with or without a protective layer underneath as shown in the photo of a model roof. Then my students and I sought to manipulate carbohydrate, protein, bacteria and other additives to adjust OCC dematerialization.

So far the results have been quite positive and from a financial (cost) point of view, much more appealing. Knowing we are eliminating most of the extraction and production costs normally associated with providing filtration and water retention represents a substantial savings – perhaps 30% ($5 US) of the overall, retail roof assembly costs.

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