Green roofs, also known as living roofs, have become increasingly popular in recent years as an eco-friendly and sustainable building solution. A green roof refers to a roof that is partially or completely covered with vegetation and a growing medium planted over a waterproofing membrane. While green roofs provide many benefits, such as absorbing rainwater, providing insulation, creating wildlife habitat, and reducing air pollution, they do have some potential disadvantages that must be considered.
Costs
One of the biggest potential disadvantages of green roofs is that they can be more expensive to install and maintain than conventional rooftops. The cost to install a green roof ranges greatly, but is generally higher than installing a conventional roof. According to a report by the National Research Council of Canada, the installation costs of an extensive green roof can be double that of a conventional roof. The costs can be higher for more complex intensive green roof systems as well.
The higher initial installation costs are due to several factors. Green roofs require much more planning and expertise to install properly than conventional roofs. They also require additional layers and components such as a root barrier, drainage system, growing medium, and vegetation. Thicker growing mediums and more complex irrigation and drainage systems also raise costs.
Once installed, green roofs also have higher maintenance costs than conventional roofs. Regular maintenance is required to keep vegetation healthy, such as weeding, fertilizing, and irrigation during dry periods. Access and safety equipment may be needed to allow workers to maintain roof systems safely.
Leaks and Water Penetration
If not properly installed and maintained, green roofs run a greater risk of developing leaks, moisture issues, and water penetration than conventional roofs. This is because green roofs are more complex systems with more layers and components that must work together properly.
Issues can arise if the waterproofing membranes under the growing medium are not complete and continuous. Seams and joints in the membranes must be installed correctly with proper overlap and sealing. Fastener penetrations from the roof decking also need proper seals. If the waterproofing layer fails, leaks can occur causing internal damage.
Poor drainage issues are another potential cause of leaks or moisture build up. If the drainage layer or outlets become clogged with vegetation and growing medium, it can lead to excess moisture in the roof assembly. Proper slope, drainage mats, drainage channels and outlets must be incorporated into the green roof design and maintained over time.
Structural Loads
The additional weight of green roof components, especially when wet, requires that the existing roof structure can support the excess loading. For a typical extensive green roof, the saturated weight can range from 12 to 50 pounds per square foot depending on the depth of the growing medium. For intensive green roof systems, the loads may be higher.
The structural capacity of the roof deck and supports must be assessed to ensure they are adequate. Retrofitting an existing conventional roof with a green roof system often requires reinforcing the structural frame, leading to higher costs.
If the roof structure cannot support the weight of a saturated green roof, it risks potential collapse. Proper structural engineering analysis is required to determine the appropriate green roof assembly for the building.
Weather Considerations
Green roofs have additional weather considerations compared to conventional rooftops. The components must be able to withstand freeze-thaw cycles, high winds, extreme heat and cold temperatures, and other weather factors. Plants used on green roofs need to be hardy varieties that can thrive in roof conditions.
Wind scour can remove lightweight growing medium from the roof if it is not deep enough or contained properly at the roof perimeter. Inadequate medium depth over the waterproofing membrane could lead to damage of the waterproofing from solar radiation.
Retaining walls, grids, erosion control fabrics, and perimeter containment boards help reduce the risks of wind scour. However, these measures add complexity and costs to green roof installation and maintenance.
Fire Safety
Green roofs can require additional fire safety considerations compared to conventional rooftops. The vegetation and growing medium provide potential fuel load in the event of a fire.
Incorporating fire breaks into the roof design helps segment the fuel load into smaller sections. Automatic sprinkler systems may also be required on intensive green roof designs or where local codes dictate. Particular plant selection is also important, avoiding certain dried grasses and herbs that burn quickly.
Access routes and perimeter walls/parapets need to accommodate external firefighting activities if needed. Firefighter loads and access points require special structural consideration.
Accessibility and Safety
Safely accessing green roofs for regular maintenance activities can be more challenging than reaching conventional rooftops. Installation of walkways, rails, harnesses, and other safety equipment may be required to traverse a green roof without damaging the vegetation or integrity of the waterproofing membrane.
Stability, slip resistance, and trip hazards need to be evaluated for maintenance routes. Access points to the roof also need to be adequate for tools and equipment needed for upkeep. Hand rails may need extension to surface above the top of the planted growing medium.
If regular access is difficult to accommodate on an existing structure, it can limit the feasibility of retrofitting a conventional roof with a green roof system.
Conclusion
In summary, green roofs offer many potential benefits but also come with disadvantages to consider. The main drawbacks of green roofs include the higher upfront installation and ongoing maintenance costs compared to conventional rooftops. There are also more risks concerning leaks, excess weight loading, weather resistance, fire safety, and accessibility.
With proper design, quality installation, regular upkeep, and readiness to address issues as they arise, many of the potential disadvantages of green roofs can be minimized. Each building needs to be evaluated individually to determine if a green roof makes sense based on budget, structure, location, and user needs. When successfully executed, green roofs provide sustainability, aesthetic, and performance benefits in spite of their complexities.
