Engineering A Green Roof Rooftop Gardens & Green Roofs PowerHouse Growers

Engineering A Green Roof Rooftop Gardens & Green Roofs PowerHouse Growers

Green roofs can provide a building with a number of benefits . This type of integrated vegetation is becoming popular due to the clear sustainability and advantages that green roofs offer to the building in question.

Today more new buildings are being constructed with green roofs than ever before. In engineering a green roof, existing buildings can now be retrofitted thanks to the vegetation technology of many advanced engineering and architecture firms.

Below is a broad overview of the factors in engineering a green roof. This will also address some concerns that developers and other professionals have in regards to the maintenance and longevity of a green roof. At PowerHouse Growers we advocate for sustainable practices that mean just that: they’re meant to last.

Dissecting A Green Roof

Green roofs have evolved remarkably since the days of moss-covered, thatched roofs of cottage residences lining the English countryside. Modern green roof technology consists of eight separate layers all designed to maintain the integrity of the roof, the vegetation, and their primary functions.

Eight layers comprise a green roof. Image via GreenGarage.ca

Green Roof Layers

  1. The roof deck – The actual roof layer itself before the green roof technology is added
  2. A protection board Acts like a buffer to prevent plant roots from penetrating
  3. A waterproof membrane – On new construction a rubber material like EPDM is used. TPO is also used which could be considered a better option as its eco-friendly and recyclable.
  4. The insulation – This is an optional layer and depends entirely on the climate of the building’s location. Insulation prevents the vegetation from extracting the building’s heat during the winter or cold air during the summer.
  5. Drainage & storage – An important component that acts as another root barrier and prevents, just as in container gardening, root rot as well as manages the amount of water retained by the roof.
  6. Protective fabric – This is a tough synthetic layer of fabric that protects the waterproofing material and may also act as a filter in some circumstances when it has a capillary design.
  7. Grow medium —  This is typically a light weight engineered soil, expanded clay pebble, perlite, or volcanic rock. The grow medium must be water retentive to ensure healthy plant growth but not become heavy when wet.
  8. The plants — Vegetation is chosen specifically for the particular climate of the green roof’s location as well as the roof’s exposure to the sun. Plants can also be chosen depending on the desired depth of the green roof’s design. Succulents and sedum are most common in drier climates, while native grasses, and even some vegetables can be chosen for damper climates. The depth of the soil or grow medium is also an important factor in selecting plants due to their unique rooting needs.

An extensive, residential green roof. Image via Prentiss Architects.

Considerations Before Building A Green Roof

There are important considerations to take into account before engineering a green roof and selecting which type to construct. These consideration factors include:

  • Maintenance. Can your building support a high maintenance green roof or will you need as little maintenance as possible?
  • Load-bearing capabilities. Can the engineering of the roof support all eight layers of a green roof?
  • Access. Will visitors utilize the green roof space or is it merely for environmental benefits?
  • Pitch and slope. Is the roof flat or is it sloped? How much of a slope?
  • Financial investment. Green roofs are of course more expensive than regular roofs. The developer must consider the long-term costs and benefits of the various types of green roofs.
  • Fire and accident prevention. Dry vegetation can ignite fires therefore regular irrigation is required in dry climates.
  • Exposure to the elements. Excessive wind can damage certain types of plants therefore vegetation should be selected with this in mind.

Intensive green roof at the United Bank of Switzerland constructed in 1990. Image via Zinco-greenroof.com

Types of Green Roofs

There are three types of green roofs that can be engineered either on existing buildings or for new construction. Each have their own special requirements and are an important deciding factor when planning the design of a green roof.

Extensive Green Roof

An extensive green roof’s primary function is to create environmental benefits. Its depth is typically between three and 6 inches of a lightweight grow medium supporting ground-cover plants. Its low maintenance requirements make it ideal for flat roofs atop office and apartment buildings. It’s also a good retrofit option for existing low-sloped roofs.

An intensive green roof is one that must be supported by a heavy load-bearing structure as it usually contains not only vegetation but also walkways and other park-scape elements for visitors. This fully landscaped living roof typically has a depth of eight to 12 inches of grow medium and may support various types of vegetation including trees. This type of green roof requires year-round maintenance and is definitely a higher-budget option.

This is a hybrid option of both extensive and intensive green roofs. It employs both social and environmental benefits. Depending on its size it can be a feasible budget option and may not require as much maintenance as a fully intensive green roof.

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