Living Roof Design

History

Living roofs can be built in two different ways. Built-in-place living roofs involve installing a growing medium and vegetation directly on the roof surface. This approach allows for more customization and flexibility in terms of the shape, size, and vegetation, but it takes more time and money to install. These types of roofs are better for larger and more complex structures where a specific plant selection or design is important. Modular living roofs are pre-made panels put together on the roof. They are faster and easier to install, can save money, and are a better fit for buildings with limited access to the roof. However, it may be more limited in terms of customization options, and may not be suitable for large roofs with complex shapes or slopes.

Although anthropologists suggest that grass roofs and living roofs have existed since prehistoric times, the earliest recorded rooftop gardens were located on the storied ziggurats of ancient Mesopotamia. Built by civilizations like the Sumerians, Akkadians, Babylonians, and Assyrians between approximately 2200 BCE and 500 BCE, these massive, tiered structures demonstrate the age-old concept of green roofs. In northern Scandinavia, sod roofs have been a traditional architectural feature for centuries, providing insulation and protection from the elements. The modern green roof movement began in Germany during the 1960s, focusing on environmental benefits and sustainable urban development. Since then, green roofs have become increasingly popular, with Germany now boasting around 10% of its rooftops “greened.” The concept has spread globally, contributing to urban biodiversity, enhanced aesthetics, and energy savings.

Living roofs are increasingly recognized as an important contributor to community well-being, with Toronto leading the way as the first city in North America to introduce a living roof by-law and boasting the highest rate of adoption. This initiative has allowed Toronto to manage 40% of its stormwater, and other US cities, including Washington DC, New York, Massachusetts, and Chicago, are following suit. San Francisco’s Better Roofs Ordinance mandates that new buildings incorporate either 15% solar panels or 30% living roofs. Additionally, the Public School Green Rooftop Program (H.R. 1863) provides $500 million in federal funding to support living roof installation and maintenance on K-12 public schools in underserved US neighborhoods. In Europe, Switzerland and Austria have established specific standards and codes of practice for living roofs.

Benefits

  • Extend roof lifespan by 300%, reducing landfill waste.
  • Enhance building aesthetics and create amenity spaces.
  • Lower indoor temperatures by up to 18°F, reducing HVAC needs.
  • Reduce cooling demand by 25% and heat loss by 26%.
  • Urban agriculture provides aromas, healthy food, habitats, and biodiversity.
  • Manage rainwater with growing media absorbing 70%-80%, compared to 20% on standard roofs.
  • Prevent flooding by retaining the first 5mm of rainfall and delaying stormwater runoff.
  • Reduce combined sewer overflows (CSOs) by managing rainwater.
  • Absorb urban noise and cut air pollution up to 20% (particles cut by 6% and SO2 by 37%).
  • Pair with solar panels for up to 15% better performance.
  • Alleviate urban heat islands; e.g., as tested in Minneapolis: an asphalt shingle roof is 158°F, a white PVC roof is 101°F, and a living roof is 90°F.
  • Boost Floor Area Ratio (FAR) and earn up to 15 LEED credits based on design and integration.

Intensive

Also known as a “roof garden,” this type of living roof has a deeper soil layer, accommodating a diverse range of plant types, including trees and shrubs. However, it requires more maintenance and irrigation than other types of living roofs. This type of living roof offers exceptional insulation, improves air quality, mitigates stormwater runoff, and creates usable space for gardens or recreational spaces.

  • Vegetation: Trees, shrubs, perennials, annuals, crops
  • Soil Depth: 10 to 24 inches
  • Load Bearing Capacity: 100 to 150 lbs/sf

Semi-intensive

This type of living roof has a moderate soil layer and a mix of a diverse plant selection, encompassing perennials, groundcovers, and select shrubs.

  • Vegetation: Perennials, groundcovers, some shrubs
  • Soil Depth: 6 to 10 inches
  • Load Bearing Capacity: 50 to 100 lbs/sf

Extensive

This type of living roof has a shallow soil layer and is planted with low-maintenance, drought-tolerant plants including sedums, grasses, and herbs. It boasts the lowest maintenance and irrigation demands among all living roof types.

  • Vegetation: Sedums, grasses, herbs
  • Soil Depth: 3 to 6 inches
  • Load Bearing Capacity: 10 to 50 lbs/sf

Layers

Vegetation

  • The vegetation layer of a living roof encompasses a diverse range of plant species including trees, shrubs, grasses, flowers (i.e. wildflowers), and succulents (i.e. sedum-mix blanket).

Growing Medium

  • A green roll comprised of mineral wool fibers serves as the nurturing ground for the greenery, offering the necessary nutrients and moisture required for plant growth. With an ideal water retention capacity, lightweight composition, and a pH level of 6.5 – 8.0, it typically comprises materials such as lava, pumice, expanded clay, and Rockwool.

Filter

  • Geotextile fabrics, including fleece, coco liner, and sphagnum moss, serve as a barrier between the growing medium and the drainage layer. Their purpose is to prevent the migration of fine soil that can clog the drainage system.

Drainage

  • Gravel, dimpled plastic sheets, and ribbed fabric laminates make up the drainage layer responsible for diverting excess water away from the growing medium and vegetation, thus preventing potential roof damage.

Water and Nutrient Reservoirs

  • Utilizing recycled, non-degradable materials such as PP or polyester fabrics, this layer provides additional water and nutrient storage to support plant growth.
  • While the drainage and water and nutrient reservoirs layers may serve similar purposes, they are typically separate layers.

Tough Protection Mat

  • Typically weighing between 500 to 900 g/m2, this layer made of HDPE or HDPET provides additional protection for the roof membrane and insulation layers.

Root Barrier

  • Measuring over 2.5 mm in thickness and made from HDPE or HDPP, this layer prevents roots from penetrating the roof.

Waterproofing Membrane

  • Typically made of synthetic materials like PVC, EPDM, or butyl rubber (IIR), this layer provides a waterproof barrier to protect the roof from water damage and leaks.

Roof Insulation

  • Typically rigid foam board or spray foam that provides additional thermal insulation to the building.

Roof Structure

  • This layer may include additional supports or reinforcement to ensure the roof can safely bear the weight of the living roof.

Options

Ground Cover

  • Moss, creeping thyme, clover, silver carpet, aloe.
  • Shallow roots, drought-tolerant, and good coverage.

Sedums

  • Agave, aloe, sempervivum.
  • Low maintenance, low growing, shallow roots, can tolerate harsh environments (high temperatures and low rainfall) by storing water in their leaves, stems, or both.

Colorful Flowers

  • Pollinator-friendly flowers such as coneflowers, black-eyed susan, daylilies, lavender.
  • Adds aesthetic charm, pleasant scent, and vibrant colors while attracting pollinators.

Herbs

  • Creeping rosemary, thyme, balm, mint, basil, parsley.
  • Versatile in its uses, this plant is both edible for culinary purposes and renowned for its pleasant aromas in aromatherapy. Additionally, it attracts beneficial bees and butterflies.

Herbaceous Grasses

  • Big bluestem, little bluestem, switchgrass, purple lovegrass, prairie dropseed, muhly grass, feather reed grass, blue oat grass.
  • Deep roots for erosion control, insulates the building, reduces ambient temperatures, creates habitats for birds and insects, and promotes biodiversity.

Rooftop Farms

  • Tomatoes, lettuce, peppers, berries.
  • Enables growing produce in urban areas where space is limited, but requires more maintenance than other options.

Fire-resistant vegetation

  • Reduce the risk of fires and provide a natural buffer zone.

Next-generation beehive

  • Innovative, user-friendly beehives that promote local bee populations and honey production without the need for beekeepers or smoking bees.
  • Examples: Flow Hive, BEEcosystem, B-Box Hive

Installation

When installing soil, consider using a boom lift with a super sack as a versatile alternative to blower trucks. This option not only allows for precise control and accurate soil placement but also provides easy access to confined and elevated spaces.

The quality of the soil is paramount for optimal plant growth and is determined by various factors, including its texture, structure, organic matter content, pH levels, and nutrient concentrations. The ideal soil type varies depending on the specific plant requirements, as different plants have distinct needs. For instance, succulents thrive in well-draining soil, while ferns prefer consistently moist soil.

Two primary soil types exist: organic and aggregated. Organic soils contain decaying plant and animal matter, enriching the soil with nutrients but requires frequent replenishment. In contrast, aggregated soils like loam soil is roughly equal parts sand, silt, and clay particles. Sandy soil facilitates effective drainage, whereas clay soil, with its greater proportion of mineral particles, has good moisture retention.

Sedums, a type of low-growing succulent, require well-draining soil, minimal weeding, occasional watering (every 2-4 weeks during the growing season (spring and summer) and less frequently during the dormant season (fall and winter)), and can thrive in nutrient-poor soils, since they are not heavy feeders. However, adding a slow-release fertilizer once or twice a year can help promote healthy growth and flowering.

Maintaining your living roof by weeding, watering, and inspecting drains is crucial. It’s important to eliminate any dead or damaged foliage and ensure tidy plant edges and well-defined borders in planting areas by trimming overgrown or leggy stems. The utilization of L-shaped flashing proves to be an effective method for achieving clean and well-defined edges for your sedums.

Construction Costs

  • Intensive: $40 – $100 per sf
  • Semi-Intensive: $25 – $40 per sf
  • Extensive: $10 – $25 per sf
  • Annual Maintenance: $0.75 – $1.50 per sf
  • Tile (e.g. terracotta, Spanish-style, clay, barrel): $7 per sf
  • Asphalt shingles (e.g. standard or 3-tab, dimensional or architectural, luxury or premium, impact-resistant, solar-reflective): $5 per sf

Our installation pricing is determined by several factors, including roof size, accessibility, choice of brand, warranty options, and whether the project pertains to a residential or commercial building. Furthermore, it’s worth noting that incorporating greenery into your property can offer potential tax benefits, as outdoor improvements such as landscaping, sprinkler system installation, or the construction of walkways and driveways can adjust your property’s tax basis, potentially reducing your capital gains tax when you decide to sell.

Design Costs

  • For residential projects, the design fee typically ranges from 10% to 15% of the construction cost.
  • For commercial projects, the design fee generally falls between 8% and 12% of the construction cost.
  • Project administration and site review typically account for 6% to 12% of the construction cost.

Case Studies

Urban Strong

  • Location: Brooklyn, NY
  • Premier Project: Javits Convention Center located in Manhattan, NYC
  • Size of Living Roof (sf): 292,000
  • Year Installed: 2018
  • Website: urbanstrong.com

Recover Green Roofs

  • Location: Somerville, MA
  • Premier Project: Boston Medical Center Rooftop Farm located in Boston, MA
  • Size of Living Roof (sf): 5,000
  • Year Installed: SP 2017
  • Website: recovergreenroofs.com

American Hydrotech

  • Location: Chicago, IL
  • Premier Project: California Academy of Sciences located in San Francisco, CA
  • Size of Living Roof (sf): 142,860
  • Year Installed: 2007
  • Website: hydrotechusa.com

Good Earth Plants

  • Location: San Diego, CA
  • Premier Project: FAANG Company Moss Wall located in Los Angeles, CA
  • Website: goodearthplants.com

Green Rise Technologies

  • Location: Readyville, TN
  • Premier Project: University of Virginia Hospital located in Charlottesville, VA
  • Size of Living Roof (sf): 191,000
  • Website: greenrisetech.com

Biosolar

Biosolar roofs represent an innovative technology that combines vegetation with solar panels, creating a mutually beneficial relationship. The plants leverage evapotranspiration to create a cool microclimate that reduces the temperature of the panels by providing shade and evaporative cooling through transpiration. While the condensation that forms on the panels provides moisture for the plants.

Studies have demonstrated that biosolar roofs can boost the efficiency of solar panels by up to 15% under certain conditions. This prevents overheating, which can impair the performance of solar panels. In addition, the plants help to minimize the accumulation of dust and debris on the panels, which can also impact efficiency. Typically, photovoltaic (PV) systems function optimally at temperatures below 90°F. However, temperatures exceeding 110°F can result in a 10%-25% reduction in solar power output.

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