Commercial Vegetative Roofs in Dallas, Texas at NIS Construction Inc. & General Contractors
The intent of this guide is to provide information regarding the state of the art of vegetative roof design and construction.
Vegetative roofs, also known as green roofs, are thin layers of living vegetation installed on top of conventional flat or sloping roofs. We have chosen to use the word "vegetative" rather than the word "green" in this guide because a non-vegetative roof could be considered to be environmentally "green" without being vegetative. For example, due to it being white and therefore mitigating heat gain within the building and reducing heat island contribution, a white non-vegetative roof might be considered as being "green" or environmentally friendly. In other words, "green" has too broad of a connotation to be clear for use in this guide, and we recommend that the industry adopt the nomenclature "vegetative," rather than the overly broad "green."
Vegetative roofs are divided into two categories: 1) extensive vegetative roofs, which are 6 inches or shallower and are frequently designed to satisfy specific engineering and performance goals, and 2) intensive vegetative roofs, which may become quite deep and merge into more familiar on-structure plaza landscapes with promenades, lawn, large perennial plants, and trees. With respect to the vegetative overburden, this guide addresses only the more shallow extensive vegetative roofs.
Figure 1. Four Seasons Hotel, in Texas. Designed by NIS Construction Inc & General Contractors, installed in 2004.
The challenge in designing extensive vegetative roofs is to replicate many of the benefits of vegetative open space, while keeping them light and affordable. Thus, the new generation of vegetative roofs relies on a marriage of the sciences of horticulture, waterproofing, and engineering.
The most common 4 extensive vegetative roof cover in temperate climates is a single un-irrigated 3- to 4-inch layer of lightweight growth media vegetative with succulent plants and herbs. In most climates, a properly designed 3-inch deep vegetative roof cover will provide a durable, low maintenance system that can realize the many benefits that vegetative roofs have to offer. Some manufacturers consider a landscape up to 8 inches deep to be extensive systems.
All well-designed extensive vegetative roofs include subsystems responsible for:
- Drainage: Vegetative roof drainage design must both maintain optimum growing conditions in the growth medium and manage heavy rainfall without sustaining damage due to erosion or ponding of water.
- Plant nourishment and support: The engineered medium must be carefully designed to provide for excellent plant growth, no wind scouring, and proper water holding capacity.
- Protection of underlying waterproofing systems: Vegetative roof assemblies must protect the underlying waterproofing system from human activities (including the impact of maintenance) and biological attack, and solar degradation. A capillary break immediately above the membrane is required for most membranes.
- Waterproofing systems: Waterproofing is critical for protecting the structure from water intrusion.
- Insulation systems: Insulation is critical for saving energy.
Figure 2. Generic Extensive Green Roof on a Concrete Deck
Figure 3. Generic Extensive Green Roof on a Steel Deck
A wide range of methods can achieve these functions. For instance, drainage layers may consist of plastic sheets, fabric or synthetic mats, or granular mineral layers. Similarly, the physical properties and performance characteristics of growing media (engineered soils) and plant materials may vary with the climate, plant community, or engineering requirements. Figure 2 shows a generic cut-away of a common type of vegetative roof assembly that utilizes a lower granular drainage layer in combination with an upper growth medium or substrate.
The selection of a particular approach may depend on performance-related considerations, such as runoff control, drought-tolerance, biodiversity, appearance, or accessibility to the public. While many pre-engineered systems are currently available, it is frequently necessary to customize these systems to satisfy specific performance objectives.
There are many potential benefits associated with extensive vegetative roofs. These include:
- Controlling storm water runoff
- Improving water quality
- Mitigating urban heat-island effects
- Prolonging the service life of roofing materials
- Conserving energy
- Reducing sound reflection and transmission
- Improving the aesthetic environment in both work and home settings
- Mitigation of wildlife
As a result vegetative roofs may be appropriate as an addition to many types of buildings, including commercial, industrial, institutional, and residential settings. On the other hand, the additional cost, possible water usage to irrigate the plants, and required ongoing maintenance may make them less appropriate.
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