Pros and Cons of Vegetative Roofs

The decision to invest in green products and technologies requires understanding both the up front benefits and the longer-term impact. In other words, do energy savings and a smaller environmental impact during manufacturing and installation outweigh the effects of possibly having to replace the product after only a few years?

In specifying a vegetative roofing system, that balance is important because the impact of replacing a system is significant. Crews must completely remove overburden materials to replace or even repair the membrane system.

To successfully specify a vegetative roof and maximize its sustainability, maintenance and engineering managers need to understand the system’s many components, as well as the potential benefits and drawbacks.

BENEFITS OF VEGETATION
Vegetative roofing refers to plant materials installed over a weatherproofing system. These roofs provide many benefits to the building and environment:

Storm-water control. The planting system, including soil media, reduces the overall volume of run-off, the peak volume-rate requirements for drainage systems, and contaminants in run-off water — all of which reduce the demand on storm-water drainage and treatment systems.

Air quality. Increasing the amount of plantings in any area allows for natural air treatment, reducing airborne contaminants.

Energy savings. The planting system provides a buffer between ambient temperature and roof insulation, reducing the fluctuation in high and low daily temperatures, as well as the rate of temperature change. Both of these benefits result in reduced load on the building’s mechanical heating and cooling systems.

Service life. Assuming a reliable installation, vegetative roof-membrane systems have increased service life over conventional membranes because they are protected from ultraviolet (UV) rays. Vegetation covers the membrane and protects it from thermal-shock stresses.

Aesthetics and wellness. If a vegetative roof is visible and accessible, occupants and visitors can benefit from this exposure. Some research indicates access to nature scenes can increase productivity, as well as general health.

DISADVANTAGES
Although vegetative roofs offer many benefits, managers also must consider the disadvantages of such systems:

Maintenance. While conventional roofing membranes require periodic maintenance to repair membrane degradation, vegetative roofing requires periodic landscaping maintenance to remove unwanted weeds and other plants so they do not overtake and detract from the plantings.

Increased costs. Depending on the selected planting method — modular vs. built-in-place — or overburden system, a vegetative roof can cost up to twice as much as a conventional system.

Membrane access. If leaks occur, the planting or overburden systems cover the membrane system. Inspectors must remove this top layer to find and repair the cause of leaks. As with any system, this process is challenging because the interior leak might not align with the membrane breach.

Unwanted wildlife and insects. The planting systems provide suitable habitat for birds, insects, and other wildlife, which managers might not want. Managers might have to specify additional protection to prevent unwanted wildlife and insects from inhabiting the system.