Green roofs are a viable option for reducing operating costs, especially when properly planned and maintained.
They do so by making the roof last longer and reducing the energy required to heat the building — and can even help in unexpected ways such as reduced hiring costs due to increased employee job satisfaction and retention.
Green roof systems also reduce temperature control costs for a building.
A study performed by the National Research Council of Canada compared the performance of a green roof to that of a traditional modified bitumen roof.
The study revealed that the green roof reduced the energy demand for air conditioning by more than 75 percent in the summer months.
Green roofs are more reflective than traditional black roofs, meaning they absorb less light that would eventually become heat.
In addition, plants release water into the surrounding air during photosynthesis, the process by which they convert light into energy.
The moisture released creates a cooling effect, lowering the ambient temperature of the roof.
This lowers the heat transfer from the roof to the floor below.
It also reduces the temperature of the air taken in by rooftop heating, ventilation and air conditioning (HVAC) units, which reduces the amount of energy required to cool the air as it is run through the air conditioning process.
Studies have shown that interaction with plants and the availability of natural views increases job satisfaction and reduces the number of sick days for employees.
By creating an accessible patio with seating adjacent to the green roof, employers can create a relaxing environment for employee breaks.
Employee interaction with rooftop vegetation can also help alleviate daily stresses.
In hospital or clinical settings, patients can experience improved mental and physical health through rooftop healing gardens.
A Savings Over Time
In order to maximize the financial benefits of greening a roof, building owners should place emphasis on the long-term value at the start of the project and avoid focusing solely on initial costs.
All retrofit roofs should be inspected by a roofing contractor or a technical representative of the membrane manufacturer to evaluate the condition of the membrane.
Older membranes often require replacement or re-covering to ensure the system remains watertight for the life of the green roof.
As with any roofing assembly, planners should minimize the number of membrane penetrations and transitions, as leaks are most common in these areas.
A primary benefit green roofs offer is increased longevity of the membrane, which is achieved by protecting it from the elements and ultraviolet (UV) exposure.
They reduce the heat flow transfer to the roof below, minimizing the expansion and contraction of the membrane.
Planners should avoid leaving the membrane exposed at drain covers, at parapet walls, around rooftop equipment or any other vegetation-free zones.
To maximize the life of the entire membrane, consider using stone ballast along drain covers, pavers near rooftop equipment or maintenance paths and metal flashing and coping caps on parapet walls.
At the planning phase, purchasers should also consider the long-term performance of their green roof system.
Although each project differs, green roof case studies typically calculate a break-even point of 17-20 years — around the time that a conventional roof would need replacing.
The more money that must be spent maintaining the roof, the longer it will take to reach that payoff period.
During the first two to four years, a system planted with plugs will require significantly more maintenance than one that arrives fully vegetated.
Green Roofs for Healthy Cities estimates that green roofs may cost anywhere from 25 cents to $4.10 per square foot during the initial establishment period.
The actual cost is primarily dependent on initial system design.
In climates with freezing winters, deciduous plants will drop leaves, which replenish nutrients to the soil.
When deciduous and semi-evergreens are planted using plugs, they will go dormant from late fall to early spring, leaving the soil surface exposed and susceptible to weed encroachment.
When planted with a mixture of cuttings, the evergreens are intertwined with the deciduous varieties to provide year-round coverage and protection from weeds.
Systems planted using this method — which do not have module edges exposed to the elements — have been shown to cost an average of 12 cents per square foot in maintenance labor costs.
This cost will vary depending on the size of the roof and the irrigation system or method chosen for the roof.
Caring For Green Roofs
Instead of focusing on reducing the frequency of visits, maintenance personnel should strive to minimize the overall duration of maintenance activities.
By visiting the roof frequently during the growing season, maintenance staff can pull weeds when they are young and before they have become well-rooted or have been able to seed.
Newly established weeds require less time to pull and displace less soil and neighboring plants when pulled.
Also, any clogged drains, debris or other barriers to green roof plant health may be spotted early on and before they create costly problems.
A clogged drain left untended could create pooling water — which can lead to root rot — causing plant damage or even death.
Properly engineered soil is also crucial for long-term green roof success.
Green roof soils need to shrink minimally, hold fertility very well and drain properly.
The recently approved American National Standards Institute/Single Ply Roofing Industry (ANSI/SPRI) VF-1 External Fire Design Standard for Vegetative Roofs defines a "generic fire-resistive vegetative system" as one planted in soil that is a minimum of 80 percent inorganic.
Inorganic materials used in green roofs are typically non-combustible and provide fire protection.
In addition, the inorganic materials won''t decompose over time.
Generally, green roof system providers will recommend succulent plants to conserve water and reduce required maintenance.
Plants are living beings, and even the most drought-tolerant species require water to survive.
Rainwater harvesting systems are sustainable alternatives to potable water sources in most climates.
Some municipalities have permitted rainwater from limited sources for use in landscape irrigation.
This water, which otherwise would be wasted, can be recycled to improve the health of the green roof plants and reduce the energy demand for air conditioning.
Although economic benefits of a green roof should not be overlooked, they alone are typically not enough to convince a building owner of the value.
The social and environmental benefits of green roofs are great — though much more difficult to quantify.
For companies seeking to convey their commitment to sustainable business practices while experiencing a long-term return on investment, a green roof is an ideal fit.
Amber Poncé is the business development manager of LiveRoof LLC, manufacturer of a unique hybrid green roof system with patent-pending features that allow it to offer benefits of modular and built-in-place green roof systems. The LiveRoof® brand green roof system is distributed throughout the United States and Canada by a network of licensed growers. LiveRoof LLC is a subsidiary of Hortech Inc., a Michigan-based nursery with over 25 years'' experience servicing the Midwest with premium perennial ground covers. Learn more about LiveRoof at www.liveroof.com or by calling 1-800-875-1392.