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LinkLine: Spring 2009

Specifying Vegetated Roofs

by Scott Mize, CCS, CSI

“Green” is all the rage right now.  As fashionable as it is to be “green”, the sustainability movement to will have to demonstrate that “green” building systems and products are smart and cost-effective choices as well as environmentally-friendly ones.  Any approach to sustainability without those basic merits risks becoming just another design feature, or worse, a passing fad. 

Few systems are more important to the practical integrity of a building than the roof.  Also, the roof represents a small fraction of construction cost, but is the source half of post-construction litigation.  Therefore, it is paramount that the roof be long-lasting and leak-free.  At the same time, the huge unused surface area of the roof offers opportunities for more environmentally-friendly design.

One strategy for putting the roof to use in the service of sustainability is to turn it into a meadow, lawn or garden.  Referred to herein as a “vegetated” roof, this approach has many advantages. 

BSD’s new Section 07 5050 – Vegetated Roof Components is your gateway to selecting the best type of vegetated roof for your building and your guide to selecting those components of a vegetated roof system that occur “above the membrane”.

Why specify a vegetated roof? 

First, LEED credits are available.  A vegetated roof helps address the Sustainable Sites (SS) “Site Development”, “Stormwater Management” and “Heat-Island Effect-Roof” credits.

Second, a properly-installed vegetated roof protects the roof membrane from attack by the ultraviolet component of sunlight and air pollution precipitated into runoff by rain.  Real-world experience shows that this protection can greatly extend the service life of the roof. 

Third, the vegetated roof will add to the roof’s insulating value.

Finally, some vegetated roofs can provide a usable amenity space for the building’s occupants. 

Vegetated Roof Types

The simplest and least expensive type of vegetated roof is the modular type.  The modular vegetated roof consists of standardized plastic trays with factory-installed root barriers, drain fabric, soil or growth medium,  Trays are pre-planted with drought-resistant grasses, succulents and perennials.   The trays come in various standard sizes up to 2 feet by 4 feet and depths of up to 8 inches.  The trays are placed on the roof membrane or waterproofing and have “feet” that lift the bottom of the tray off the roof surface to allow excess water to drain away. The roofing or waterproofing manufacturer may require the modular system to have a layer of sacrificial membrane or other protection sheeting beneath it.  Irrigation, if desired, is by drip system or by watering.

The advantages of the modular system are obvious:  Simplicity, standardization, low first cost, no direct impact on the structure or roofing other than dead load.  Should the plants die, the entire tray is easily replaced.  The disadvantages are less obvious, yet real:   Since the trays don’t retain water, it may be necessary to supplement rainfall with irrigation.  The drought-resistant plants typically used are more like meadow grasses and weeds than typical landscape plants, so they may not be as attractive to the eyes as a more intensive landscape installation.  Finally, the plants and trays have to be protected from foot traffic, so they are not generally suitable as walking surfaces. 

The next level of complexity is the extensive system.  This term refers to a system with six inches or less of soil where the protection layer, root barrier, drainage layer (if provided) and soil/growth medium are placed directly on the roofing or waterproofing membrane before plants are installed.

Extensive systems have many of the advantages of the modular type, though to a lesser degree:  Simplicity, relatively low first cost and low dead load – especially if a lightweight soil mixture is used.   

The disadvantages of the extensive system are similar to the modular system as well.  Because the soil is shallow, the plants used are limited to the same general type of drought-resistant grasses and groundcovers used in the trays.  With a water-retention layer or supplementary irrigation, a greater variety of plants can be used. However, this system remains limited to plants with shallow root systems.  Given the shallow depth of soil and type of plant, extensive systems must be protected from all but the limited foot traffic required for routine maintenance.

Finally, there is the intensive system.  Intensive vegetated roof systems have protection layers, root barriers, one or more drainage layers and more than six inches of soil/growth medium placed directly on the roofing or waterproofing membrane.  This deeper system can accommodate an additional layer of insulation over the membrane, which in turn requires at least two drainage layers (one above and one below the insulation).

The intensive system is the most expensive; the most complicated and imposes the greatest load on the building structure.  However, the intensive system has the soil depth and water-retention capacity to be a true “garden roof”.  A supplemental irrigation system may or may not be required.   A great variety of climate-appropriate plants can be used and, provided the soil is deep enough, shrubs and even small trees can be accommodated.  Finally, the intensive system can support foot traffic.  With the addition of gravel or pavers, the entire vegetated roof becomes accessible to the building occupants.

Vegetated Roof Systems and SpecLink

Selection of the type of vegetated roof depends on a number of factors: building type, roof deck type, project budget, owner expectations, whether the owner plans to seek LEED certification, etc.  It is especially important that the vegetated roof selected be appropriate to the roofing or waterproofing membrane used and be acceptable to the manufacturer of the roofing or waterproofing.  Finally, the type of vegetated roof will depend on the climate of the area where the project is located.   For example, it doesn’t make much sense to install a densely-landscaped intensive system as an amenity on a project where the weather is too hot or too cold for human comfort for a large part of the year.

Once the design professional selects the type of vegetated roof, it is important to involve the structural engineer and landscape architect in the design of the entire roof system.  The engineer will confirm that the structure can support the additional weight of saturated soil and plants.  The landscape architect will ensure that the type and arrangement of the plants is appropriate for the location and determine whether or not supplemental irrigation is required.

In SpecLink, Section 07 5050 – Vegetated Roofing Components lists the three types.  Once the user has specified a deck type and roofing or waterproofing membrane elsewhere in the project manual, the user then selects the appropriate type inside Section 07 5050.  SpecLink’s linking feature then assists the user by “turning on” required components, excluding incompatible ones and highlighting optional choices.  Once options are selected, the linking feature further refines the selection.

The roof deck and roofing are specified elsewhere in Division 7.  Since the plant mix must be customized in the case of each roof, the plants are specified elsewhere (Division 32) as well.

The design professional should seriously consider using vegetated roof system components supplied by or approved in writing by the roofing or waterproofing manufacturer.  “Single source” specifying of roofing or waterproofing membrane and vegetated roofing components will eliminate a number of potential pitfalls.

First, components manufactured and supplied by a single entity are very likely to be compatible and to have been successfully used together in a vegetated roof application.

Second, contractors acceptable to the “single source” entity are likely to be affiliated with or trained and certified by the manufacturer.  Affiliated contractors are will have prior experience installing the type of vegetated roof specified and be knowledgeable about how the components work together.

Third, a single source is much more likely to warrant the installation and do so without terms, conditions and exclusions that are to the owner’s disadvantage. 

Given the great importance of the roof and the relative ease with which good design and proven components can ensure the roof’s integrity, it is risky to try and save money by picking and choosing from untested designs, unproven components and inexperienced contractors.  

This risk-management issue is illustrated by the small number of roofing and waterproofing manufacturers who will warrant a vegetated roof over their products.  Most, if not all, insist on a “single-source” installation before they will issue a full-system warranty.   Also, while there are many companies who produce vegetated trays, mats, blankets, etc., several major roofing companies have chosen to make their own vegetated roof components or affiliate with just one manufacturer of vegetated roof components.

Finally, the warranties provided by manufacturers typically cover the roofing or waterproofing membrane and all the manufactured components (slip sheets, drain boards, insulation, etc.) of the vegetated roof system.  Warranties are available for the planted material as well, but most warranties exclude the excavation and replacement of soil and plants required to expose the membrane should the membrane need repair during the warranty period.