4.7.2 Design

During the initial planning stages, the designer should consider the following questions:
  • What is the appropriate type and design of vegetated roof based on its intended function?
  • Is the load bearing capacity of the building able to support the intended vegetated roof? What is that capacity? Is the size of the roof sufficient?
  • Can the vegetated roof be maintained easily and affordably?
  • What stormwater benefits will accrue from the design?

This section identifies the essential design considerations for intensive and extensive vegetated roofs and makes recommendations based upon climate and environmental conditions. The design elements, including the roof deck, roof structural support, fire protection, protective layer, waterproof layer, drainage layer, substrate, vegetation, and leak detection systems (optional), are described below.

Roof Deck
The roof deck can be made of steel, concrete, plywood, or any other material sufficiently strong to support the load of the vegetated roof. The slope of the roof deck beneath the vegetated roof should be slightly steeper than conventional roofs because minor ponding will not evaporate as quickly under a vegetated roof assembly.

As discussed above in Section 4.7.1: Applications and Limitations, vegetated roof slopes between 5 and 20 degrees are most suitable and can provide natural drainage by gravity. Roofs with slopes steeper than 10 degrees require an analysis of engineered slope stability and those greater than 20 degrees require a structural reinforcement system and additional assemblies to hold the soil substrate and drainage aggregate in place (WSU-PSP, 2012).

Roof Structural Support
It will be important to ensure that the additional weight of the vegetated roof is distributed evenly across the roof deck and support structure below. Working closely with a structural engineer throughout the design of the vegetated roof is essential. Consider the weight of saturated soils, weight of snow in the winter, as well asa maintenance regime to mechanically remove snow buildup to prevent roof damage and collapse.

Fire Protection
Flammable materials in the construction of the vegetated roof should be avoided because of the dry heat that is known to occur in eastern Washington. Designers should maintain a clear stone or gravel border around parapet walls, roof top windows, chimneys, and other openings where fire may spread. Specifying fire-resistant vegetation can also minimize the total amount available fire fuel. Factory Mutual provides fire ratings, research, and testing related to reducing property-related hazards. Factory Mutual’s knowledge center can be accessed at www.fmglobal.com.

Protective Layer – Root penetration layer

Maintaining a continuous separation between the roof membrane and vegetative root zone will reduce the potential for root damage. The material should be raised above the substrate at the edges and around vertical projections, like vents.

Waterproof Layer

More organic construction materials, such as oil-based bitumen and asphalting felt and fabrics decompose and require more frequent maintenance, leaving roofs susceptible to leaks. They are also the most common form of roofing materials. Various mechanically-produced materials are available for waterproofing the roof, such as rolled sheets or inorganic single-ply membrane or fluid-applied membranes. Ensuring a complete seal on these membranes, especially at the joints, is critical.

Drainage Layer

Drainage layers store and channelize stormwater infiltrated through the substrate and offer additional space for plant roots. Materials used may be granular stone, porous mats, lightweight plastic or polystyrene drainage modules. Selection of materials will depend upon weight requirements as well as the objectives of stormwater system design.

Vegetated roofs provide their greatest contribution to stormwater management for low- to moderate-intensity storms. Heavy storms saturate the soil more quickly, thereby reducing retention potential on a shorter timeline. The drainage layer should seek to balance the objectives of storage and drainage.

Substrate
Vegetated roof soil, or substrate, must support the chemical, biological, and physical requirements of the plants, which is especially challenging due to the system’s disconnection from the ground. Substrate varies in depth and composition for structural, planting, and stormwater management purposes. Depending on the soil composition and weight, additional roof support may be required. Weight, water storage, and nutrient holding capacity are the primary factors to be considered when selecting substrate and drainage material (WSUPSP 2012).

The substrates of vegetated roofs perform the majority of water retention. The amount of water retained is primarily a factor of substrate depth although studies suggest that substrates deeper than 6 inches do not necessarily provide more retention capability (Retzlaff, 2006). Substrate depths of 2 to 3 inches support a wider range of succulent species, grasses, and herbaceous plants. Depths of 4-8 inches will enable a wide range of drought-tolerant perennials and grasses and some tough small shrubs. Substrate depths of 12-20 inches will enable many perennials and shrubs to be grown, whereas trees require 32-52 inches.

Vegetation

The main difference between a plant palette in an on-theground landscape amenity and one on a vegetated roof is root depth. Vegetated roofs need shallow rooted species that are adapted to thin soil profiles, high temperatures, and periods of drought. Additionally, diverse palettes, as opposed to monocultures, tend to result in better overall plant survival. Select plants that:
  • Cover and anchor the substrate surface relatively quickly.
  • Form a self-repairing mat.
  • Take up and transpire the available / retained water.
  • Survive the extreme climatic conditions (cold hardy, drought-tolerant, wind-tolerant).

Eastern Washington has many good native and highly-adapted plant choices that are appropriate to vegetated roof settings. These plant choices are tolerant of the extreme climatic conditions that exist. For extensive roof designs, designers should consider selecting naturally-occurring plant species that survive with little to no input. Meadow-like bunchgrass mixes and desert shrub-steppe plants may also be appropriate in some settings of eastern Washington.

Leak Detection Layer (Optional)
Electronic leak detection systems are an optional technology designed to precisely locate a leak if one occurs after construction. Using a leak detection system reduces the likelihood that the significant portions of the vegetated roof materials will have to be removed in the event of a leak (WSU-PSP, 2012).


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Fig4-7-3_Green-Roof
Figure 4.7.3 - Basic Vegetated Roof Components
Basic components of a vegetated roof.  Source: CleanWater Services