The introduction spends a number of pages looking at different countries and discussing the policies and green roof advances in each of these places. There are many technical details in the book, enough to easily create small scale projects using only the information inside.
Green roofs are becoming an increasingly common sight, on innovative new buildings, on buildings with pre-existing design challenges and very frequently on commercial buildings with flat roofs. Many roofs are prepared as a habitat for local vegetation, and are not intended to grow any species other than those found naturally in the local flora. Still others have wetlands built into them, and become a familiar place for birds to land and nest upon.
Planting on roofs and walls is becoming a discipline of its own in the realm of ecology, as it creates a complex system of habitat for many creatures wherever it is constructed.
This book is beautiful and has a colour photograph on almost every page while most have two or three. For anyone new to the topic, discussions about retrofits required for supporting different types of roofs and networks, planting and watering systems, run-off questions and the integration of living roofs and walls into new buildings are all within these pages. The book was originally listed as a gardening book, but is now listed as architecture. It is a mix of both those disciplines, as well as a strong connection to urban ecology, and a discipline in its own right.
There are now many identifiable hybrid types of roof greening, as buildings are being created to support elaborate green roofs while others are merely adapted, some being extensively greened while others only contain a few green areas. Those green roofs involving high-input garden technique have been covered by several books, while this book is more interested in introducing these low-input roofs and their accompanying living walls.
Bioengineering is a term which describes constructing walls with the intention of integrating planting. Chapter 2, titled Why Build Green Roofs, explores in-depth the environmental benefits of green roofs, or bioengineering in action, including such positive spin-offs as increased biodiversity, which is also represented in graphs. Run-off and roof types are also represented, as are the benefits of green roofs regarding air pollution and carbon sequestration.
It is clear green roofs radically diminish the urban heat island effect as well, provided the buildings are sufficiently irrigated with a source such as recycled rainwater irrigation. The authors cite one particularly interesting study involving a hypothetical green roof coverage of 50% for Toronto's "downtown heat island," a study which found irrigated green roofs reduced the heat effect by a whopping 2 degrees, while unirrigated roofs were discovered to be of far less benefit but still created cooling. Information about recycling and irrigating roofs dispels the suggestion that green roofs and rooftop gardens become water intensive. Other analysis included noise reduction as a positive compliment, and the discovery that solar panels increase an uptick in effectiveness when placed on green roofs, as "the output of (photovoltaic) cells is higher at lower temperatures." The analysis also found that there has been insufficient effort in truly promoting green roofs at a city scale, despite multiple and even cost-offsetting benefits.
Chapter 3, titled Constructing Green Roofs, explores the almost limitless possibilities for construction, and forever dispels the idea that a flat roof is a requirement. The challenges of wind, types of irrigation, and concerns of fire risks are also discussed. Adding one of the better myth-busting facts, while thatched roofs are for instance a fire risk, a green roof is actually the opposite of a fire risk. There are no known green roofs fires in Germany, where there are millions of square feet of green roofs.
Green roof structural requirements are discussed and include underlying structural supports, a green roof deck, a vapour control barrier, and a "waterproofing layer or membrane." Additional covering protects this membrane brilliantly. Green roof materials are explored as methods of planting, growing medium, soils, soil depths and types of systems. The materials section Components of a Green Roof includes several excellent case studies with interesting photos and excellent designs explaining plant combinations for various purposes.
The second half of the book is dedicated to facade greening, the fine art of planting on walls. This was the section that interested me the most. This section begins with a discussion of climbers, something which humans have used for ages but only recently studied. Climber vines often require very little encouragement where there is brick, but a much more dense facade can be made using lightweight steel trellis panels and coconut fiber, as one example, which allows a mix of plants that establish quickly and penetrate the panels well. Different types of trellis such as grids and steel cables are designed for different plants, for instance bougainvillia has adorned buildings for millenia and loves a widely spaced grid. Steel ropes as well as glass fiber are materials which are durable enough to support plants on walls. The authors write, "for heights greater than two stories, choosing supports for climbers and their installation moves into an engineering rather than horticultural territory." However, the book does move into this territory, and I noticed that the earlier version of the book was listed as a gardening book, while this newer edition is listed online as on the topic of architecture. The book encourages plant facade builders to carefully consider design factors, aesthetics, and whether the plants need horizontal as well as vertical supports. As well the book adresess the condition of the wall receiving load bearing fixings, and various tensioning methods.
The authors then proceed to a number of incredibly beautiful case studies, where their limitations and challenges are analyzed in detail. One of the keys to success in creating a living wall is a constant source of water and nutrients to the plants. As the book remarks, "some extremely varied and visually attractive plant communities can be found growing on cliffs where there is a sufficiently constant supply of nutrient-laden water flowing over and down the rock surface. Often these plant communities are highly distinctive, frequently offering refugia for locally rare or highly specialized species. In the temperate zone, some of the best known are the 'coastal bluff' communities of cliff and canyon walls in the Cascade range in north-west North America." Natural hydroponics are occurring when nutrient-rich water trickles down cliff walls from decaying material above. Recreating this through an irrigation system is key to success. The book reinforces to us that green roof runoff is more than merely "a useful resource" and is nutrient dense water that belongs in an integrated plan. Water from roof run off, for example, can be used to irrigate wall plantings to a very positive effect.
The rest of the book is an extensive list of some of the best and most appropriate species for wall and roof gardening. It is an extensive plant directory including plant height and drought tolerance among other details and makes the rest of the book extra indispensable. I wished the book had a bit more detail about food that can be grown on roofs and walls, but the emphasis was a bit more on creating habitat, on reducing urban heat issues and on aesthetics, something the authors acknowledge at the introduction. I recommend this book strongly to anyone interested in green roofs and walls, not as a read but as a permanent fixture to their library.
Dunnett, N., & Kingsbury, N. (2010). Planting Green roofs and living walls. Timber Press.
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