A Growing Industry: Contrasting Types of Vegetative Roof Systems
Surrey City Library's vegetative roof system, Surrey, BC, Canada.
With cities like Los Angeles and Toronto requiring green roofs as a percentage of the total roof space, architects and building owners are looking for ways to expand their knowledge on available vegetative roof options. Today, manufacturers offer single-source warranties and systems that can often last as long as the structure itself. This results in reduced cost and improved environmental benefits for building owners.
Increased property value, economic incentives and amenity space are all positive effects of a vegetative roofing system. Green roofs also improve air quality by removing harmful compounds like carbon dioxide from the air and returning oxygen to the atmosphere as a byproduct of the photosynthesis process of the plants.
Sound insulation and reduced noise pollution are additional benefits. Hard surfaces tend to reflect sound, rather than absorbing it and the combination of soil, plants and trapped layers of air can act as an insulation barrier to both lower and higher frequencies of machinery, traffic and airplanes.
Green roofs are typically comprised of a base layer of metal or concrete decking, insulation and roofing membrane, along with a root barrier, drainage and engineered soil called growing media for the roof’s desired vegetation.
Types of Green Roofs
There are two main types of vegetative roof systems. The first is a modular system, which comes in a pre-vegetated container and includes already-grown vegetation. With this option, the building owner has an instant vegetative roof upon completion of the installation.
The more extensive and common vegetative roof system is the built-in-place (BIP) system. BIP systems are typically installed in a variety of layers depending on the design of the roof and its purpose. Growing media is hoisted onto the roof and distributed before additional vegetation is planted. These systems can be further segmented into extensive, semi-intensive and intensive options.
The extensive BIP systems are used to satisfy an engineering or performance goal and are the most common type of green roof. Media and vegetation are typically less than 6 inches tall and designed for lower-maintenance plants such as succulents or herbaceous perennials. Extensive systems can be used on visible or non-visible roof surfaces and are particularly beneficial for roofs with limited load-bearing capacity.
Pregrown modular system
Stormwater management is a prominent driver of extensive BIP systems, with a 50%-90% water retention rate during a typical rainfall. Precipitation is captured and stored in the growing media and special water retention layers. The stored water is then used by the vegetation and returned to the atmosphere through transpiration, or evaporates from the media or plant foliage.
For a deeper media depth, semi-intensive vegetative roofs allow for greater design options and visual impact. Typically developed using a more complex mixture of succulents for ground cover and perennials, grasses and native forbs, these plants add diversity and texture to visible roofs. Semi-intensive roofs are great for small amenity spaces and softening a building’s appearance with enhanced aesthetics.
Where extensive and semi-intensive BIP vegetative roofs offer performance solutions, intensive BIP roofs are all about aesthetics and creative designs. This type of roof is best for media and vegetation 8 inches and taller and play host to rooftop gardens and a wide variety of amenity space. There are endless design possibilities with intensive roofs, and the only restrictions are structural loads with existing roofs and budget parameters.
Although vegetative roofs are best planned at the design phase, they can be installed on nearly any existing liquid-applied or single-ply system, such as TPO, EPDM, modified bitumen and PVC. Building owners who want to maximize their investment should consider manufacturers that offer a single-source warranty. These types of packages provide coverage for roof leaks, material and the vegetation for a specified period of time.
Vegetative roofing systems can bring any commercial building a variety of benefits, including increased stormwater retention and improved aesthetics. However, to ensure system longevity, it’s crucial to consider which type of green roof will perform best considering factors such as climate and underlying roof systems.
About the Author
Thomas M. Hanzely is Firestone Building Products—National Sales Manager for the Skyscape Vegetative Roof Program.