Extensive and Intensive Green Roofs

Extensive systems exist primarily for their environmental benefits and are not accessible rooftop gardens. They feature shallow soil or planting media, generally 2-6 inches deep. Vegetation consists of plantings such as sods and sedums with shallow root structures that can withstand harsh environments. The systems require little or no irrigation and typically need water only during the system’s start-up period. They can weigh 15-50 pounds per square foot.

Intensive systems offer both environmental benefits and accessibility. They feature deep soil or planting media, generally 9-18 inches deep or deeper. The root structure of vegetation, which can include trees and shrubs, varies. They can incorporate architectural features, such as paving, walkways, and benches, and they require irrigation. They typically weigh 80-150 pounds per square foot.

As with conventional roofing, the membrane of vegetative roof systems must slope to provide drainage. Although the membrane systems can withstand constant water exposure, it is a good practice to move water promptly away from roof details to reduce the potential for leaks.

The Details of a Vegetative Roof

Managers need to ensure proper design and installation by engaging a professional. Each roof is different, so managers should not rely on “boxed” systems.

Before installation of the overburden, managers should schedule a flood test of the system. Water should be at least 2 inches deep at all details. If a detail fails, the installer must retest it after repairs.

Vegetative roofs offer significant benefits related to sustainability that building owners and managers seek. But to ensure system longevity, managers must specify proper materials and detailing. Otherwise, the cost to repair or replace a leaking vegetative roof system dramatically reduces these sustainability benefits.

When specifying a vegetative roofing system, managers must consider system details carefully.

Base flashing. For conventional roofs, the top edge of the base flashing must be at least 8 inches above the membrane surface. For vegetative roofs, the top edge of the flashing system should be at least 8 inches above the top of the planting system.

Penetrations. For all roof systems, managers must minimize penetrations to reduce the potential for leaks. Due to access constraints and challenges, this issue is more important in vegetative roofs.

Gravel. Managers should specify that installers use gravel at perimeters and around penetrations and details so water can flow promptly away from these areas.

Drains. Managers need to specify dual-level drains to ensure water can drain from the top surface of the overburden and the waterproofing-membrane layer.

Planters. For plants with deep root structures, such as trees and shrubs, managers should specify independent planter boxes to separate the roots from the waterproofing system.

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.

The Guide: Vegetative Roofing Systems

One sustainable storm water management technique which continues to grow in popularity in Florida is the application of green roof technologies. This involves replacing or supplementing a traditional roofing material with vegetation. The vegetation prevents the rain from quickly running down the roof surface and offsite. Water instead percolates through soil and feeds plants in the process. The vegetative roof system provides a storage component and enhances the quality of runoff through natural filtration. In general, the system is composed of a waterproofing layer, a soil layer, and a plant layer. A vegetated roof is obviously easiest to create on a minimal slope, but it can be created on surfaces with a pitch of up to 45° with large trees, shrubs, and other landscapes. Vegetative roofs require irrigation and drainage systems to maintain the garden, which can add considerable load to the roof.

In contrast to the intensive design, extensive roof systems are primarily built for the environmental benefit. They typically use only inches of soil depth and contain small, hardy plants and thick grasses which are accessed for maintenance. Additionally, loads to a building are between 15-50 lbs per square foot. The blocks are usually a heavy gauge container with about 4” of soil supporting a low-growing plant species. A pad is fastened to the bottom of the block to protect the roof surface and allow controlled drainage through the unit. The typical weight of the system is 12-18 lbs per square foot. The cost of a green roof system can vary considerably depending upon the type of installation and factors such as the overall size, plant selection, depth of growing medium, use of irrigation, and accessibility options. It is important to note that, while green roof technology does typically require a greater initial investment, the system can significantly extend the life of the roof and reduce the heating and cooling costs of a building.

As with many sustainable management practices, one must look at a life-cycle analysis to truly compare the costs. Thousands of property managers and construction professionals are instituting Vegetative ‘Green Roofs’ on their facilities across all of Florida.

RAMCON maintains a complete line of vegetative roofing solutions, as well was photovoltaic (solar) and highly reflective systems for lifetime roof savings. Learn more about vegetative roofing systems online at www.vegetativeroofing.com