Due to the limited space in New York, roof gardens are an increasingly popular option for many buildings in the city. Roof gardens have a number of benefits including reducing storm water runoff, increasing air quality, decreasing the urban heat island effect, increasing insulation to the roof, creating habitat for wildlife, prolonging the longevity of the roof, and creating a more beautiful space.
New York is actively promoting building owners to create green roofs. The city recently amended the city zoning code to remove impediments to the construction of green roofs. The state is even offering a generous tax abatement for new green roofs. See our other blog post about the requirements for Property Tax Abatement for Green Roofs in NYC.
There are four major types of roof gardens, each has their advantages and disadvantages.
- Intensive Green Roofs
- Semi-Intensive Green Roofs
- Extensive Roof Garden
- Container Roof Garden
Intensive Green Roof
Roofs with more than 12 inches of soil depth are considered to be intensive green roofs. This is the highest quality option for a roof garden because it allows a wide variety of different plants, since most plants grow better when they are planted in deeper soils. However, intensive green roofs are also the most expensive option, since they require more structure to bear the weight of the soil. It is easier to do this on new buildings, rather than existing buildings, which may need to add more structure.
Depending on the soil depth, small trees could even be planted on an intensive green roof. As a general rule, trees need at least 1/3 the soil depth as the height of the tree. Therefore, a small 12 foot high tree would need a minimum of 4 feet of soil depth. Having the soil depth for a tree is ideal, since roof gardens without trees can feel a little empty. In addition, having trees also makes the space feel more private, especially if you have taller buildings surrounding your roof.
Semi-Intensive Green Roof
Roofs with between 6 and 12 inches of soil can be classified as semi-intensive green roofs. They are cheaper than intensive green roofs, but more expensive than extensive green roofs. A greater variety of plants can be grown successfully on semi-intensive green roofs than extensive green roofs. However, no trees can be planted on them.
Since many crops need at least 6 inches of soil depth, a semi-intensive green roof could be a good choice if you want to grow fruits and vegetables. Crops require a lot of sunlight, so if the rooftop is shaded by tall buildings for most of the day, an agricultural use may be an unsuccessful choice.
Extensive Green Roof
Roofs with less than 6 inches of soil depth are called extensive green roofs. Since there are many ready made modular systems on the market, these types of roofs are easier to install on existing buildings. They also do not have the weight requirements that intensive green roofs need.
However, plants on extensive green roofs are slow-growing. The soil depth does not support trees and many types of crops. The variety of plants that can be used will generally be limited to plants that have shallow roots (sedum and grasses).
Roof With Plant Containers / Container Gardens
It is also possible to create a roof garden without any soil depth. Instead, roof pavers or a wooden deck could be installed on the roof. In New York City Building Code, a container garden is defined as plants maintained in pots of 6 inches in diameter or wider.
Moveable planters and outdoor furniture could be installed to make the space livelier. These types of gardens also provide the maximum amount of flexibility. If you need more space on the roof, the planters can be easily removed. This is the most economical option.
However, this type of roof is not eligible for the green roof tax abatement, since the abatement requires at least two inches of soil covering at least 50% of the roof. It is not considered to be a true green roof, since the containers are only standing on the roof, not integrated into it.
It is also possible to have a green roof with different soil depths, combining the advantages of both systems. For example, the majority of a roof garden could have a soil depth of less than 6 inches, but also one area that is 4 feet deep to accommodate a small tree. Another option could be to have a combination of an intensive green roof and a container garden, which would give any activities on the roof more flexibility.
Roof Top Garden Options in NYC
As an architect I study building practices and New York City regulations closely, but these are complicated and quite involved. In this article we reviewed some of the basic concepts with regards to Roof Top Gardens in NYC. This post does not assume to cover every possible issue or condition, but provide a general overview of the topic.
Thank you for reading our blog post on Rooftop Gardens in NYC.
I hope this was helpful. Please leave questions and comments below. If you would like to speak with an architect you can contact us directly.
This post was written by Jorge Fontan AIA a Registered Architect and owner of New York City architecture firm Fontan Architecture. Jorge Fontan has earned 3 degrees in the study of architecture including two degrees from the City University of New York and a Masters Degree in Advanced Architectural Design from Columbia University. Jorge has a background in construction and has been practicing architecture for 20 years where he has designed renovations and new developments of various building types.