A Guide to Greening Roof Decks
Within the last ten to twenty years green roofs, more specifically extensive green roofs, have moved primarily from a European garden archetype to a more ubiquitous one here in the states. Several cities over the years have passed ordinances, laws and/or provided incentives for green roofs within their city limits. Chicago and Washington DC are two U.S. cities that were at the vanguard of this movement. Most recently the New York City Department of Buildings passed Local Laws 92 and 94 both in 2019 that require all new buildings and existing buildings undergoing certain major roof renovations to have a solar photovoltaic system, a green roof system, or a combination of the two. Although the number of green roofs may be increasing dramatically, what hasn’t changed is the level of design thought, detailing and coordination that goes into the implementation of even the simplest green roof.
Before design even begins, the most critical component of the green roof process is convincing the building owner that a green roof will be beneficial to them. If there are specific codes in place the difficult part is complete. If there are no codes requiring a green roof, the owner will need to be convinced to set aside significant funds for something that might seem superfluous to some; therefore, the selling of the idea may be the most critical aspect at least initially.
Potential benefits of green roofs:
Reduction of heat island effect
Potential reduction of water use for plantings
Reduction in heating and cooling costs
Increased life of waterproofing membrane
Public relations may be the one benefit that is becoming more important to owners as the issue of sustainability, climate change and the environment in general is at the forefront of many people’s minds. Where in the past users may have felt that a green roof or garden would be a nice amenity to have, times have changed such that some may now demand these sustainable features. This may be particularly true for tenants in residential projects but in fact this demand may also become more common at the workplace.
The biggest obstacle for owners is cost. It is true that a green roof will be more expensive than an exposed EPDM system. It is also true that there are several other alternatives that could be installed that are less expensive or have fewer maintenance demands; however, sometimes in rare circumstances a green roof might be the less expensive option. This may be particularly true if it is an extensive green roof. Towers|Golde worked on a project that included existing brick pavement with mortar joints over structure, it was determined through the cost estimating process that the green roof would be less expensive than replacing the original paving. The life cycle costs must also be considered when discussing pros and cons. While the upfront costs may be more, over the lifespan of the roof membrane it may not be.
The discussion with the owner should not be limited to only the benefits of green roofs. There are other aspects that owners need to be aware. The success of a newly installed extensive green roof relies very heavily on proper maintenance and weeding. This is an often-overlooked aspect which differs significantly from gardens on grade. Mulching and fertilization needs are different for these types of plantings and a contractor who is unfamiliar with green roofs may end up compromising the green roof by not implementing specific maintenance measures.
Also, the aesthetics could be an issue for some owners particularly if the planting is predominately sedums. Sedum can turn shades of brown that some might not expect. Owners may be expecting something green all year so this needs to be addressed up front.
While the success or failure of the green roof may ultimately hinge on the landscape architect several other disciplines are usually necessary in order to design and detail the required elements. The architect, structural engineer, mechanical engineer and the electrical engineer all have integral roles in the development of a green roof. It is useful to determine scope split among the disciplines early in the design process to avoid confusion and duplication of work. Normally all the waterproofing elements of the roof are detailed by the architect or engineer; however, the selection of the waterproofing - both type and manufacturer - relate directly to the green roof system. Many waterproofing manufacturers limit what is placed over the membrane. If it is not an “approved” product the warranty may be void. As waterproofing manufacturers become savvier and want a piece of the economic pie, they have started to offer full systems from the waterproofing membrane all the way up to and including the plants. While this may be convenient it might not be what the designer wants nor may it necessarily be best for the green roof design. Because of this the architect and landscape architect must be on the same page regarding the manufacturer and their specific warranty requirements.
No matter what system or individual elements of the green roof are used, prepare to have a significant back and forth between all design disciplines as well as the manufacturer. The level of coordination involved and ultimately the amount of time spent is much higher on a green roof than it is for a garden on grade. Design fees should be anticipated and adjusted accordingly.
GREEN ROOF COMPONENTS
In addition to the roofing components (which are not covered here) the next layer of a green roof system is the drainage. This allows water to filter through the soil to the roof deck where it eventually finds its way to the roof drains. These come in different sizes and configurations but most commonly it is a dimpled plastic sheet that can be filled with lightweight drainage aggregate. It provides an uninterrupted layer that water can pass through. You can even pour concrete footings and bases directly on to this layer and it will not interrupt the drainage capacity occurring below the drainage board.
The substrate used for green roofs is not your typical regular garden soil. This is due to several reasons primarily having to do with weight restrictions. Typical soil is heavy particularly when it is saturated and could reach over 120 lbs per cubic foot depending on the soil type. Another issue with regular soil is that it also settles and degrades over time because of the potential high fertility and organic content. This could require supplementation or replacement which can be costly.
Growing medium used for green roofs is engineered to contain a high mineral content that will not break down over time. This makes the medium highly porous. In order to support plant growth, the engineered soil needs to include 10-20% organic content. This medium typically weighs 50-90 lbs a cubic foot saturated.
This growing medium can be installed using different methods. The medium can be installed as bags laid on top of the drainage board and the plants punched into the fabric of the bag. It can be installed as trays that have the plants pre-grown inside the trays. Both bags and trays are useful ways to plant if the roof is only accessible by elevator. The soil can also of course be loose. This loose soil can either be blown on the roof utilizing special vehicles or it can be craned to the roof in large bags which are then dumped into the plant bed. Blowing the soil is a messy business and can only be done on lower roofs.
The growing medium is where a lot of the cost issues arise. Currently we are seeing $175.00 a cubic yard installed cost. Compare that to a $60.00 cubic yard cost for regular soil and it is easy to see why some green roofs can be costly. Due to the expense of growing medium it is critical whether a new building/roof or an existing one that roof deck pitch is considered when estimating the amount of growing medium required. If a relatively level plant bed is preferred, then the depth of the medium will vary due to the pitch of the roof deck and topping slab.
Different methods of planting are used for green roofs. They can be pre-planted in trays which provides an instant effect. There are also pre-grown mats and tiles that can be rolled out onto the surface like sod. This too provides somewhat of an instant effect but not as lush as the trays provide. Probably the least expensive option is cuttings. Because Sedums, which are the predominate plant used on extensive green roofs, root so easily they can be installed by just spreading cuttings onto the surface. Care must be taken with the timing of delivery and installation as well as watering. Plant selection with cuttings will be more limited than individual potted plants.
Individual plants can also be installed. Sizes ranging from a “plug” to containerized stock can be used. Plugs although inexpensive are sometimes susceptible to rodents and geese who often pull the plants out of the ground. It should be noted that if larger plants are used it will be necessary to knock-off or wash away some of the soil around the root ball. The larger the container the more soil that can add to roof weight and/or compromise and clog the growing medium.
Planting decisions impact greatly the amount of coverage that can be expected over the years. The better coverage the less potential for weeds. The less weed blanket or exposed medium is seen the nicer the garden is going to look initially.
As suggested above pre-planted trays and mats provide full instant coverage. Elements that effect coverage is depth of medium, irrigation (temporary or permanent) and finally spacing of the plants.
Depending on the type of roof garden that is designed permanent irrigation may not be required. Extensive green roofs can usually omit permanent irrigation; however, woody shrubs, trees and even larger perennials and grasses may need supplemental irrigation throughout the growing season.
When planting on green roofs mulch is usually omitted. This is due in part because it can blow off in windy conditions. It also will degrade over time, clog and compromise the porosity of the growing medium. A straw blanket can be used to control weeds in the short term with container plants planted through the blanket.