A farm inspired terrace garden in Greenwich Village, NYC

What should I know before building a roof garden or terrace garden in a luxury building?

Residential roof garden design is a collaborative process. Begin by consulting with a landscape designer, reviewing your building’s alteration agreement and understanding any restrictions to the residential outdoor space you own. Creating a beautiful rooftop garden is a comprehensive enterprise — depending upon the complexity of your project it may include designers, structural engineers and architects, various contractors such as roofers, carpenters, horticulturalists, electricians and plumbers. There are countless benefits to constructing a roof garden!

Designing a luxury rooftop terrace

The landscape design plan for your roof garden or terrace garden should consider your needs and aspirations, analyze the challenges of the site and create programming based upon who the users are and their lifestyle.  As with all garden plans  — it should address the site from an aesthetic, architectural, functional, horticultural and sustainable perspective.

Understand your roof's weight restrictions.  

Interpret the weight restrictions for your roof or terrace, as this will impact what elements you choose and the means by which you install them into your outdoor landscape. The loading capacity is how much weight your roof structure can support.  Each roof has a maximum pounds per square foot that determines the amount of weight bearing load it can support. **

**How do I find out about the weight restriction for my dream roof garden?

The management company for your building, an architect, developer or building engineer of record may be able to inform you on what the weight-bearing load is for the roof. It is commonly found on the building plans in a loading schedule. If that information is unavailable, you could enlist the services of an engineer and/or a roofing contractor that can do a probe to determine this information. Once you have that information you can then determine what is possible in terms of limitations for your rooftop garden design.

Safety considerations for rooftop gardens

Railing heights on a roof garden should be at 42” above grade in NYC. Other cities and towns may have varying local laws.

The building roof surface needs to be protected otherwise it can become damaged along with the waterproofing seal. Stone or porcelain pavers can be set on pedestals above the existing roof creating a protective surface for the roof and a recreational “deck” for residential use. Trex (a composite material) or wood pavers (which is considered combustible) can also be used, but in NYC, local laws only allow up to 20% of roof surface to be covered by a combustible surface. 

Umbrellas on windy, exposed terraces or roofs should be attached to secured to the building structure.

Pay attention to fire codes. Here in New York City electric and gas grills with dedicated lines are allowed on a terrace. However there are specific fire codes as to placement of these items within the space. Propane is not allowed

Planters need to be kept a minimum distance from parapets.

What is the difference between an intensive roof garden and an extensive roof garden?

Roof gardens have been around for hundreds of years. Intensive green roofs or terraces are an amenity primarily found on luxury condo or coop buildings. They have deep soil, irrigation systems, a greater diversity of plants and habitats. With little or no protection from the sun, roof gardens are generally created for entertainment and recreational activities and to create an outdoor living space for people living in a building. 

An extensive green roof has planting medium that are a few inches deep, have little or no irrigation, incur a stressful condition for plants, is suitable to cover large areas, — maintain drought-tolerant sedums (rooftop succulent plants) or perennials which are shallow-rooted and use little water. Plant diversity on these roofs is kept low to simplify care and to be sure all plants have similar moisture requirements. During dry times these rooftops may turn brown, only to revive with rain. They are attractive, functional and promote efficiency.

Garden installation and rooftop garden construction.

Access to the rooftop garden — the logistics of bringing materials onto your extensive roof space, small rooftop garden or terrace or balcony should be thought through with great detail. What are the limiting dimensions of all the many doorways, elevators, and passages, stairwells that lead from the front door of your building through a freight elevator to your outdoor space? Common spaces and interiors are to be protected throughout the building phase.  Ideally material can be delivered conveniently, but if needed there are situations where material can be hoisted via crane onto a roof deck.

Minimizing weight for your roof garden

• Porcelain pavers are considerably lighter than the weight of concrete or stone pavers.

• Use lightweight soil or a soilless mix of planting medium.

• Lightweight planters fabricated from 1/8” aluminum or fiberglass.  Wood, stone and ceramic planters are typically heavy!  

• Choose non-woody plant material such as grasses, succulents, other perennials and annuals.  If trees are a possibility select those with small root balls.

What kind of rooftop planter boxes should be purchased for a terrace garden?

We recommend using low weight planters for a roof deck or terrace. Planters should be fabricated from fiberglass (or similar constructions) and aluminum. Terracotta and plastic planters are not a wise investment as they crack from freeze- thaw, and do not hold up to the elements well. Wood is an organic material and will decay if it is filled with soil.

Containers (planters or pots) should have excellent drainage with the correct planting medium used and amount of drainage holes provided.  The container should be raised above the surface with some “dunnage” raising it an inch or two to allow for water to wick out and protect the surface upon which it sits, allowing airflow between the bottom of container and surface.

What are the best plants for a rooftop garden? 

Many residential rooftop gardens and terrace gardens exist in full sun; therefore one should consider plants that typically grow in full sun. Likewise if a taller building shades your terrace, consider plants that are part sun. As always pay attention to the amount and quality of the light that the plant receives.  Plants that typically grow in full sun and get desiccated by high winds can work well on a roof - think of plants that survive on a mountaintop, a beach or the open plains. Herein lies the gardening axiom… “Right plant, right place.”

Pergolas, trellises and umbrellas on a rooftop

Any structure on a roof or terrace needs to be attached to the building for the safety and liability of you, the building and the surrounding community. These would need to be approved by the building management and reviewed by a structural engineer and architect for its ability to withstand wind loading, its load (weight) and connection details.  Building codes and zoning need to be considered.  A roofer will need to waterproof after installation.  

Securing furniture on a terrace in New York City.

Wind is a critical element that must to be factored into the design of a city roof garden.  To prevent furniture from blowing away in heavy winds, many buildings require property owners to secure it to concrete pavers or store it when not in use. In addition to winter storms, regular seasonal wind gusts can be extreme on a city roof deck.

Maintaining your rooftop garden. 

Terrace gardens and roof gardens all have a certain level of rooftop landscaping maintenance, as would any traditional, well-cared garden. To be truly successful, choosing the appropriate plants, fertilizing if necessary, mulching to reduce evaporation during spring-summer-fall and doing a fuller layer of mulch for a winter freeze thaw cycles is standard.  Practicing good garden cultural practices, or cultural controls minimizes plant failure, diseases and pests. Regular pruning and dividing perennials are recommended as plants can become compacted in their planters. Pavers and site furnishings need to be cleaned regularly, drains and surfaces cleaned.

Watering a terrace garden 

The most reliable watering method involves setting up an automatic drip irrigation system.  Assuming you will be successful using watering cans an/or a hose is unpractical. Installing an irrigation system - a drip irrigation system that can be adjusted through the seasons is the wisest choice. (This assumes there exists an outdoor faucet.) The system should be installed with a back flow device and drained in the late fall before possibility of freezing lines.

Soil on a roof garden

Planting medium should be a lightweight soil or soilless mix.

The growing medium for a rooftop gardening is not the same material used for a houseplant or a traditional townhouse garden or small suburban garden. Traditional soil is heavy and packs tight after repeated rains, reducing water retention and aeration for plant roots. Soil in a terrace planter needs to be lightweight as keeping the weight minimal is of greatest importance. Peat moss, perlite, vermiculite, pine bark may be some or all of the ingredients within the mixture to keep it lightweight. Containers should be lined with landscape fabric and/or screens to prevent erosion of the growing medium and the clogging of the roof drains. 

Fertilizing roof garden plantings

Dependent upon the plant material chosen, the plants will have different needs. For those plants that do like to be fed (ie: roses) or appreciate organic matter in the soil, remember that planters are constantly being irrigated; therefore soil is regularly leaching out.  Therefore fertilizer and organic matter (compost) should be added on a regular basis to compensate accordingly.

The best rooftop gardens and city terraces consider ALL these factors.

All imagery ©ToddHaimanLandscapeDesign2020