Green Roofs

Green Roofs

Goal

Our goal is to provide professional services (preliminary design, comprehensive design, and construction management) for simple roof gardens and sedum GRs that absorb stormwater and make our cities cooler, more livable places. Simple sedum roof projects evolve into modular and built-in-place rooftop farms, native species meadows, and complex amenity plazas. Over the last decade, we have watched countless cities pass legislation promoting GRs as a tool for storm water management. Switzerland and Austria have passed specific GR standards and Codes of Practice. California, however, has been reluctant to act despite persistent heat waves and dwindling urban green space.

Options

  • Ground cover (moss, creeping thyme, clover, silver carpet, aloe)
  • Succulents (natives, maintenance-free, low-growing, and colorful flowers)
  • Colorful, pollinator-friendly flowers with a pleasant aroma
  • Fire-proof vegetation
  • Edible herbs (rosemary, thyme, balm, mint, basil)
  • Local healthy food grown on rooftop farms (fruits and vegetables). Flat roofs of shared housing communities allowing neighbors to connect.
  • Harvest honey with a next-generation beehive (easy to operate, beekeepers aren’t stung, and bees aren’t smoked or killed).

Benefits

Installing a green (garden) roof (GR) or a rooftop farm can extend the life of your roof by 300% when well maintained. The host of other benefits include energy savings; biodiverse ecology with new habitats, visual appeal, tranquility, and pleasant aromas; flash flood prevention with growing media absorbing rainwater (collects 80% of rainwater compared to 20% on standard roofs); local healthy food source (urban agro); and absorbed urban noise pollution. Energy savings result from temperature reductions of up to 18 °F from increased insulation, shading of plants, and evapotranspiration. The cooler microclimate allows the building to employ smaller mechanical systems (chiller, air distribution system, etc.) which have large cost savings.

The residual benefits are reducing the urban heat island effect and reducing air pollution. Studies show a 20% reduction of air pollutants between two buildings covered with plants. They also improve the FAR (floor area ratio), fast-track and waive fees for plan check (reviews) and permit issuance, and improve amenity roofs.

They manage storm water runoff from heavy rains, as they can be used for both storm water runoff retention (storage) and detention (delayed release). This ensures minimal negative downstream impact of runoff, particularly reduction of combined sewer overflows (CSOs). A combined sewer system (CSS) collects rainwater runoff, domestic sewage, and industrial wastewater into one pipe, where it is sent to a wastewater treatment plant to process.

Pricing

EPA roofing cost estimates:

  • $10/SF to install an extensive GR
  • $25/SF to install an intensive GR
  • $0.75 — $1.50/SF for annual maintenance costs
  • $5/SF to install a shingle (asphalt) roof
  • $7/SF to install a tile roof

Installation pricing can vary widely depending on the roof’s access challenges, brand, warranty, residential vs. commercial, etc. The most important variable for pricing is the size, as significant economies of scale exist in sedum, extensive (<4” media) roofs.

The owner will usually want to see an adequate ROI unless it is only to meet regulatory requirements. If the designer fails to prove this can cause them to be value-engineered out. Fortunately, particularly in high-rise cities the benefits have been quantified, and the results point to monetary and health benefits.

Keep in mind, plants are a tax-deductible business expense. When you sell a house, you pay capital gains tax on the difference between the sale price and your adjusted basis. The IRS lists landscaping, installing a sprinkler system, or building a walkway or driveway as outdoor improvements that adjust your basis.

Intensive GR

  • Depth: ≥ 10″
  • Load-bearing capacity: 200 – 1,000 kg/m2
  • Supports: trees and shrubs
  • High initial investment
  • Requires more maintenance
  • Might require significant reinforcements, such as shear wall braces, to support the additional weight (people and plants in the case of amenity roofs, rooftop farms, and community gardens).

Semi-Intensive GR

  • Depth: 6″ –  10″
  • Supports: grasses, herbaceous perennials, and shrubs

Extensive GR

  • Depth: 3″ – 6″
  • Load-bearing capacity: 50 – 200 kg/m2
  • Supports: ground cover, succulents, herbs, and grasses
  • Low establishment cost but not accessible

Construction

  • Waterproofing membrane: ethylene propylene diene monomer (EPDM) or butyl rubber (IIR).
  • Root barrier: more than 2.5 mm thick, high-density polyethylene (HDPE) or high-density polypropylene (PP).
  • Tough protection mat: weighs 500 – 900 g/m2, water & nutrient reservoirs, recycled non-rotting PP or polyester.
  • Drainage layer/system: gravel, dimpled plastic sheets, ribbed fabric laminates.
  • Filter layer (fabric): resists clogging from fine soil, made of geotextile such as fleece (e.g. coco liner, sphagnum moss).
  • Growing medium (plant substrate, green roll): long rock mineral wool fibers (various mineral mixtures) specially needled to form a compact and stable felt; ensures excellent water retention.
  • Soil: optimum water retention capacity, light weight, pH: 6.5 – 8.0, lava (pumice, expanded clay, Rockwool, etc).
  • Vegetation (flora, plants, greening, sedum-mix blanket)

Case Studies

  • Urban Strong in Brooklyn, NY: designed a 6.75-acre (6 blocks) GR at the Javits Center Manhattan Building, one of the largest of its kind in the US. 
  • 7M gal/yr of storm water absorbed
  • 26% less energy used (6 °F cooler)
  • 25 species of bird + 300,000 bees
  • $3M/yr saved

Recover in Boston, MA: installation and maintenance.

American Hydrotech in Chicago, IL: supplies components, systems, and approaches.

Good Earth Plants in San Diego, CA: design, installation, and maintenance.

California Academy of Sciences in San Francisco: offers a living roof with rolling hills and fields covering 87% of the 2.5-acre rooftop. Inside is a planetarium, aquarium, and natural history museum, including the 4-story Osher rainforest. Edged by solar panels, the seven hills of the roof are lined with 50,000 porous, biodegradable vegetation trays made of tree sap and coconut husks. An estimated 1.7 million plants fill the trays and their roots interlock to create an extraordinary oasis.

Biosolar

Biosolar roofs combine vegetation around rooftop solar. The two function in symbiosis, with the plants keeping the panels cool by creating a cool microclimate via evapotranspiration, and condensation forms and drips off the panels irrigating the plants. PV systems work best at temperatures below 90 °F, and if temperatures exceed 110 °F, the solar power output decreases by 10% – 25%. Solar panels achieve increased efficiency with low-power fans (cooling with forced convection) and if they’re manufactured with white, non-reflective cells.

Installation

A super sack on a boom lift installs soil well, if no blower trucks are available. Two main types of soil are aggregated and organic. Organic soils have more nutrients, but they need replenishing. For the best soil quality, use sandy clay soils so the soil can breathe, not clay or loam soils that can dry out.

Maintenance

Sedums (low-growing succulents) provide aesthetics in multistory buildings. Maintenance once or twice a year should be sufficient. This entails weeding and feeding, keeping the plant edges tidy at L-shaped flashing, and adjusting watering.

Policy

GRs contribute to community well-being at all levels. Toronto was the first city in North America to have the first GR by-law and it has the highest rates of adoption. 40% of stormwater is now managed in Toronto, and other US cities (Washington DC, New York, Massachusetts, and Chicago) are adopting GRs. We also admire San Francisco’s GR policy.

 The Public School Green Rooftop Program (H.R. 1863) allocates $500 million in federal funding to support the installation and maintenance of GRs on K-12 public schools in underserved neighborhoods across the US. Whether local or statewide, we should continue to establish local ordinances and programs that support GR technology.