The garden roof assembly or “green roof system” has been available in the United States for more than 70 years. Construction consists of two equally important phased applications: the waterproofing application and the garden assembly. The ultimate success of a rooftop garden depends largely on the proper design and installation.

Waterproofing

A rooftop garden with a paver-stone deck area utilizes Envirospec’s PAVE-EL Paver Stone Pedestal System, which elevates, levels and uniformly spaces paver stones while protecting the roofing substrate.

There are three waterproofing design elements that are essential to the success of the system:

1. Support of added weight to the building. There has to be an inherent compatibility of the rooftop garden with the structure on which it is applied.

2. Integrity of the waterproofing membrane and system.

3. Location and size of the roof drains.

The waterproofing application in rooftop garden construction typically mirrors that of plaza deck waterproofing. In this context, the waterproofing membrane is applied at the bottom of the system, in a protected or inverted state. In this configuration the inverted membrane is protected from garden components, tools and, to some degree, plant roots.

Membrane Considerations

The membrane system must be durable enough to resist mechanical damage from gardening tools and the penetration of plant roots, and it has to last, without repair or replacement, for the life of the building. It may be advantageous to consider the use of below-grade waterproofing materials, which are typically installed in inverted plaza deck construction, as opposed to roofing materials that are applied in warm roof (insulation over deck) configurations, for these applications.

The membrane material must possess, as its primary characteristic, the ability to prevent moisture from entering the facility. This key element should take precedence in the system design over plant type and layout. Rooftop gardens require a membrane that:

• resists root penetration
  
• resists ultraviolet rays
  
• can withstand severe temperature changes and atmospheric conditions
  
• is flexible enough to meet building movement at construction joints and intersections with vertical elements
  
• resists surface wear during construction
  
• can withstand attacks by insects and microorganisms, subsoil animals and soil chemicals
  
• can stay in place over an indefinite life span without deterioration
  
• prevents water penetration
  
• can remain submerged in wet conditions for substantial periods of time.

There are materials and systems on the U.S. market that meet these requirements and have performed in these configurations for 30 or more years. The best way to assess material performance is to visit similar projects and talk with building occupants about the waterproofing performance of the materials.

Protection Board

The protection board is applied over the waterproofing membrane and serves as the protective barrier during garden construction. The protection board also protects the membrane from damage by garden tools and mechanical equipment. This material should be hard, strong and durable. After the initial installation, it remains in place and becomes an integral part of the completed system. There are several types of protection boards available, including asphalt/felt boards, polyurethane film and polyester. The boards come in sizes from 1/4 inch to 1/8 inch.

Insulation

The inclusion of insulation in rooftop garden design is often dictated by local and national building codes. The main purpose of the insulation is to meet R-value requirements. (R-value is the measure of thermal resistance of a particular material at a particular thickness.) As in conventional roof systems, insulation provides thermal properties that reduce heat loss from the building in winter months and subverts heat from entering the building in the summer, cooling the building.

In addition, the plants and grass can act as reflective surfacing, reducing cooling costs in the summer. They also function as insulating layers that restrict heat transfer, reducing heating costs in the winter. Other garden components, particularly soil, provide positive thermal properties, though the exact amount is debatable. The depth of the soil and other garden components cannot be accurately measured for R-value because of the different amounts of moisture that they can contain.

It must be pointed out that the R-value of the insulation in an inverted system can be significantly reduced over a 20-year period because of the continual presence of moisture in the system. Studies indicate that 2-inch insulation can lose up to 40 percent of its thermal value over a 20-year period when it is continually exposed to moisture. This fact must be accounted for during the design stage.

The important properties of insulation in rooftop garden construction are slightly different than conventional roof systems. The most important property of the insulation is that it is durable enough to resist heavy weight throughout the surface. Weights such as deep (saturated) soils, toppings and in some instances poured concrete, are typical burdens applied over insulation in rooftop garden constructions. The insulation must possess the capacity to withstand destruction from these loads. The insulation must also be moisture-resistant, from a structural, not a thermal perspective. It must be light in weight and easy to cut and install. Rigid extruded polystyrene foam (Styrofoam) is often applied in these applications.

The Drainage Layer

This living roof in Germany is similar to what Ford Motor Co.’s Dearborn Truck Plant roof will resemble once it's fully grown. The sedum changes color and texture with the seasons. Photo courtesy of Ford Motor Co.

There are four essential components to the roof garden. The first is the drainage layer. Positive drainage is a critical requirement of rooftop garden design. Proper drainage is critical at two points within the total system: at the substrate level (roof deck) to collect water from the building to the sewer system, and within the garden construction to collect water to the interior roof drains.

Substrate Drains

Next come the substrate drains. Rooftop gardens play an integral role in slowing down water run-off from the roof area to a city’s storm-sewer system. This is advantageous in older urban areas where the storm sewer is not up to the capacity of growth in an area. It has been established that the soil in the garden can retain 15 to 20 percent of rainfall for a two to three-month period. Retaining water also aids in the growth and flourishing of the garden plantings.

However, it is often misconceived that roof drainage is not required on rooftop garden systems. This theory is not only incorrect, it is potentially dangerous. The water needs to be removed from the system to comply with the structural constraints of the building. The weight of a 1-inch depth of ponded water on a 100-square-foot area can exceed 135 pounds; the weight of 2 inches of ponded water in a 200-square-foot area can exceed 1,082 pounds. This additional dead-load weight, coupled with the excessive live-load weight, could result in substantial structural damage.

Local building codes and proper roofing practices prohibit the deletion of any roof drains on a roof structure. For new structures, the roof drain pattern must meet the requirements of the local code authority and the American National Standards Institute. The location and size of the roof drains installed is based on the average rainfall calculations for the geographic area of the structure. Positive deck slopes to the drains can enhance proper drainage. In new construction, slope requirements by code are a minimum of 1/4 inch per foot.

The Drainage Medium

The third component is the drainage medium. The drainage medium is applied above the waterproofing system and consists of a rot-proof material that allows the water to flow through to the substrate drains. This is a critical component, which must be compatible with the filter fabric and the planting medium. Successful roof drainage systems reduce the blockage of water to the substrate drains and assist in the elimination of excess water on the system.

Early rooftop garden construction consisted of a drainage bed of 1 to 2 inches of broken drain rock or pea pebble. Typically no filter fabric was applied above this bed. Filter fabric was installed over these types of drainage beds in the 1960s, allowing moisture to flow through the bed without the loss of soil, clogging drains or the stone being completely washed away. The effectiveness of this procedure can be traced to the success rate of rooftop garden systems installed up to 70 years ago that are still performing today. The drawback of this method is that it is labor intensive and adds weight to the structural deck. New drainage systems have been introduced in recent years, with more products being offered as the market segment of garden roofs expands.

Filter Fabric

The filter fabric, introduced to rooftop gardens in the 1960s, is installed to prevent the loss of soil, mulch or plant debris while allowing for the flow of moisture to the drains and keeping the drains unclogged. The filter fabric also keeps the planting medium in place. A durable filter fabric can serve as a root barrier protecting the roof membrane.

As for the planting medium and top dressing, there are various forms of soil composition that are suited for rooftop gardens. The success of the plantings often depends on the types and depths of the planting medium. This type of composition is above the scope of this column.

Conclusion

As the installation rate of rooftop gardens in the United States increases, it will become important that roofing and waterproofing designers become familiar with the components and installation procedures. The major emphasis should be placed on the waterproofing membrane system. If the waterproofing system fails, removal is required to the substrate, which entails great time and expense. The garden assembly can be applied in a cost-effective manner if the designer keeps the design “simple.” The success ratio of rooftop garden assemblies in the United States that are more than 50 years old certifies that these systems can be both aesthetically pleasing and waterproof.