Green (vegetated) roof systems are relatively new to the United States but have been around for thousands of years.
The early forms of green roof systems were typically used throughout Europe to provide protection from the general elements (weather). They also provide thermal benefits (warmer in the winter and cooler in the summer) due to the mass of the material above.
Early forms of green roof systems did not provide a weatherproof enclosure and had other issues such as animals burrowing in the roof assembly. This would be obviously be unacceptable in today’s standards.
In the early 1970’s Germany invented a modern green roof system. The new design focused on the management of water and protection of the underlying structure from root damage.
Later developments came in the late 1980’s. The design focused on making green roof systems lighter and more affordable. This would enable green roof systems to be utilized on large low-slope (flat) roof areas.
Because of these developments there are now three common types of green roof systems:
Intensive green roof systems (also referred to as a roof garden) consists of a deeper soil base, typically a minimum of one foot of soil depth. Large trees, shrubs, grasses and other landscapes are planted within the soil.
The deep soil will hold water and most intensive green roofs have irrigation systems to water the vegetation. This means that green roof systems need a well-considered and designed water management system. The roof should positively drain to multiple stormwater drainage inlets.
Due to the imposed loads associated with intensive green roof systems the supporting structure needs to be substantial.
Extensive green roof systems consist of shallow soil bases, typically has 4 to 6-inch of soil depth. Small hardy plants and thick grasses are planted in the soil.
Extensive green roof systems are typically less maintenance intensive when compared to an intensive roofing system.
Modular Block Systems
Modular block systems are like extensive green roof systems but are made up of pre-vegetated modular trays.
Anodized aluminum containers or recycled polystyrene trays are filled with 3 to 4-inchs of engineered soil. The soil provides a suitable growing medium for low-growing plant species. The trays are typically interlocked and are provided with a protection pad and drainage mat creating an instant green roof.
Designing for Success
The design of a green roof system needs to consider the following:
The underlying structure needs to be able to support the deadload (the weight of the material itself) of the roof system and any imposed live loads (such as wind, rain, snow etc).
The weight of the soil/growing media can vary based on the mixture of soil composition such as minerals, decaying organic matter, nutrients and living organisms. The weight of the soil is significantly affected by water, i.e. is the soil dry or saturated.
To give an example dry soil weighs approximately 2,000 pounds per cubic yard. The same soil could weigh as much as 3,000 pounds per cubic yard when wet. Quite the difference!
Wind Uplift Pressures
When wind strikes a building, it is redirected up the exterior face and over the roof, creating a pressure differential. As the pressure equalizes a suction force is created which will pull on the roofing material. The greater the wind speed, the more powerful the uplift (suction) will be.
Other factors, such as the height, location and the surrounding environment will have direct impact on how a building is affected by wind uplift pressures. Design factors such as the inclusion of parapets can help to reduce the impact of wind pressures.
Alternatively, the inclusion of other building systems, such as solar panels, can have an opposite effect. You should check with your local Authority Having Jurisdiction (AHJ) as regional regulations have different building code and wind design requirements.
Selection of the Right Vegetation and Growing Media
The vegetation is the living component of a green roof system. It has visual appeal and provides a rooftop ecosystem for insects and birds etc. The choice of the vegetation and the growing media needs to be paired to ensure the success of the roofing system.
The growing media supplies nutrients for the vegetation and helps in the management and retention of stormwater. The media depth and type are directly correlated to the plant selection and the need for supplemental irrigation (if required).
When selecting the appropriate vegetation and growing media the moisture, nutrients and sun orientation need to be considered.
Detail, Detail, Detail
The success of a green roof system is dependent on its material selection, design details and quality of the installation. Typically, green roofing systems consist of seven main design components listed below.
The roof structure supports the roofing system and any dead, live or superimposed loads. Reinforced concrete or a composite roof (metal deck with concrete) infill is the common choices of roof structures for green roof systems.
There are various types of roofing membranes that can be used as part of a green roof system. The main consideration is whether a conventional roof or inverted roof membrane assembly (IRMA) should be used.
The main difference is the position of the roof membrane and insulation within the green roof system itself. Conventional roofs install insulation beneath the membrane. Inverted roof membrane assemblies position the insulation above the membrane.
The root barrier prevents the intrusion of roots into the roof deck. It is imperative that the layer is FULLY SEALED and installed ABOVE the roof membrane. It is important to ensure that the root barrier is puncture resistant and tear resistant. The root barrier material selected should have good elongation and tensile strength properties.
The free flow of irrigation and storm water towards drainage outlets are critical to all green roof systems. This is usually managed by the slope of the roof, a drainage layer and position of drainage outlets.
All green roof systems should be slightly sloped to positively drain towards drainage outlets. The number of outlets should be enough to cope with the amount of water expected from annual rainfall and roof irrigation.
The drainage layer is basically an air gap between the growing media and underlying roofing components. Its primary function is to allow irrigation and storm water to flow towards drainage outlets unobstructed via the air gap. It is important that the drainage layer is above the waterproofing layer, i.e. roofing membrane.
In some instances, the drainage layer is deigned to hold water for longer periods. This is sometimes referred to a retention layer. The retention areas supply water and nutrients to the growing vegetation on the green roofing system.
The filtration layer separates the growing media from the drainage layer. It is a critical layer for any vegetated roof assembly. It is designed to let water pass from the growing media through to the drainage layer but prevent the passage of solid substances, such as soil or stones. It is imperative that the filtration layer is FULLY SEALED.
The filtration layer should be a non-woven material and porous. The pore sizes need to be precise in order to prevent passage of soil but allow the migration of water. This is an important consideration for the design of green roofing systems.
Vegetation and Growing Media
As described above both components should be designed to complement each other. Succulents are high survival plants in arid conditions. Grasses, flowering perennials, shrubs and trees are high-performance plants in cooler conditions. The amount of water plants and trees absorb should be considered during the design of a green roofs water management system.
Drainage and Edging Restraint
It is imperative that the roof drainage remains clog free. The best solution is to use edging restraints to separate vegetated and vegetation-free zones. It should be constructed of a solid and rigid material that has perforations at the drainage course and a solid face at the growing layer.
Pros of Green Roof Systems
The construction of green roof systems is relatively new to the United States however the system is continually gaining popularity given the associated benefits. Check with your local AHJ as many cities are now offering financial incentives.
The associated PROS are as follows:
Estimated Useful Life
A well designed, installed and maintained green roof system can expect to have an estimated useful life of 30 to 50+ years. This is largely due to the underlying membrane being protected against general weathering and solar degradation.
Energy Savings and Reduction in the Heat Island Effect
Buildings with greens roofs help regulate temperature. When compared to the ambient air temperature, the temperature of roof soils and substrates is lower in the summer and higher in the winter. This means the soil thermal properties help to keep the building cool in the summer and warmer in the winter. This is particularly true for intensive roofing systems, due to the increase in soil depths.
In the summer, vegetated roofs help to counteract summer cooling needs by reducing the solar heat gain of the building. The vegetated roofs reflect solar radiation, absorb radiation by photosynthesis and evapotranspiration through the vegetation and growing medium. Such mitigation of solar radiation results in reduced energy needs for cooling the building in the summer.
In the winter, there is a reduction in energy needed to heat buildings with green roof systems. This is due to the mass and thermal resistance of the growing medium.
Basically, buildings with green roofing systems save on energy bills and are great for the environment.
Captures Noise Pollution
Green roofing systems are great for mitigating noise pollution. Due to the mass of the soil, soundwaves are absorbed thereby reducing noise levels. This is great for buildings in high population urban environments.
Visual Aesthetics and Better Air Quality
Dependent upon the type of green roof system employed (intensive or extensive) the growing media can support a wide range of vegetation. This creates rural landscapes within urban settings.
Green roof systems are considered to improve the visual aesthetics of buildings from an exterior standpoint. It is great for the building users looking out onto the roof areas from windows.
Green roofing systems also help to improve the immediate environment. Plants and trees absorb carbon dioxide, supply oxygen, help absorb pollutants and cleanse the air.
Reduces Stormwater Runoff
Most modern-day roofing materials are impervious, i.e. do not allow the passage of water. They are designed to shed water to the nearest drainage point as soon as possible. This can often result in stormwater goods being overwhelmed and flash flooding occurring.
The inherent design properties of vegetated roofs help mitigate the effects of flooding. Surface water runoff is limited by holding water in the soil. The water held can re-enter the atmosphere through condensation and transpiration. This reduces the overall demand on stormwater management systems.
Helps Urban wildlife
Green roofs help to create habitats for insects, butterflies, bees and birds, especially in urban environments.
Cons of Green Roof Systems
The associated CONS are as follows:
Increased Initial Cost
The cost to construct a green roofing system is much more than other traditional roof systems. There is the need for a more robust and expensive roof structure capable of withstanding heavy loads. The various layers and components that make up green roofing system mean more material, labor and time involved in construction.
It is important to note that although the initial cost maybe more than an alternative roofing system the life cycle of the roof must be considered. A green roofing system may require a larger upfront cost but will last longer and save money on energy bills for heating and cooling.
Difficulty in Locating Leaks
Due to the various components of a green roofing system and depth of the growing media it can be difficult and expensive to locate leaks within the roof assembly. A leak detection system can be installed prior to the installation of the green roof assembly however this adds to the cost of the installation.
Green roofs typically need active maintenance to ensure vegetation-free zones and drainage points remain clear from debris and vegetation. Fertilization and irrigation maybe regularly needed. Green roof plants need to be frequently inspected for insect problems and fungal diseases. They may also require trimming and pruning from time to time.
Common Deficiencies with Green Roof Systems
The majority of the common deficiencies encountered with green roofing systems typically relate to poor design details and poor workmanship within each roofing component.
The Underlying Structure
The imposed loads of green roofing systems can vary dependent upon the depth of the growing media (depth of soil) and water retention characteristics. Failure to provide a structure that is not only capable of supporting the deadload (soil and growing media) and any imposed live loads (wind, rain, snow etc) will result in catastrophic failure (structural collapse).
As green roofing systems are designed to transition seamlessly into the surrounding environment it is important that measures are put in place to prevent excessive overloading, such as point loading from a vehicle wheel. Such measures could be the introduction of perimeter bollards etc.
The membrane within a green roofing system forms the underlying barrier against water intrusion. It is critical that the membrane is fully sealed, and penetrations are limited. Detailing around drainage points is particularly important. It is imperative that the roof membrane is protected from roots as they will typically cause damage as they seek a water source.
The deficiencies associated with the root barrier membrane within a green roofing system are typically associated with poor detailing. The root barrier membrane should be fully sealed, parapet to parapet, and above the roof membrane. Particular attention is needed around drainage points. The right root barrier material should be selected based upon its puncture and tear resistance.
The drainage layer is one of the most critical components within a green roofing system. This is because it is located below the growing media (soil) and provides an air layer to ensure clear drainage.
The retention of too much water will result in the growing media (soil) being constantly saturated. However, not enough water will likely result in the vegetation dying.
Common deficiencies encountered with drainage layers are:
- Incorrect installation of drainage cups (installed upside down) when using a combination drainage and retention layer.
- Failure to bring the drainage layer up at penetrations and at the surrounding parapets.
Filtration layers retain the growing media (soil) from the drainage layer and are a critical component of a green roofing system. Common deficiencies encountered are:
- Failure to fully seal the system, parapet to parapet.
- Failure to seal at edges.
- Incorrect pore size (must be small enough to prevent the passage of growing media).
- Insufficient flow rates (must allow the free passage of stormwater).
- Most filtration layers are not UV resistant so they much be protected.
Vegetation and Growing Media
The vegetation and growing media are the final layers that support and provide the living components of the roofing system. Both components must be matched to ensure a successful green roofing system. The water retention, nutrient, depth and characteristics of the growing media needs to be considered during the design of the roofing system. Typical defects include:
- The incorrect choice of vegetation and growing media for the building location. This can result in vegetation dying.
- Failure to design for effective root transfer throughout the green roofing areas.
Drainage and Edging Restraint
Designing for clear drainage is an essential aspect of any green roofing system. The system components described above help to maintain clear drainage channels. Inevitably vegetation is attracted to moisture, so it is important that:
- Drainage points are covered.
- Perforations are provided at the drainage layer
- Drainage points are frequently inspected and cleaned to avoid problems.