Green roofs/living roofs

Applications for system

  • Roof sheathing, typically on low-slope roofs (3:12 or less)

Basic materials

  • Growing medium and plants
  • Root barrier membrane
  • Drainage plane membrane
  • Waterproofing membrane
  • Solid roof decking

There are many different green roofing systems, ranging from simple homemade versions to premanufactured snap-together systems with plants already established.

Green roofs/living roofs

There are three basic categories of green roofing, based on the depth of the growing medium, the types of plants grown and the amount of maintenance expected.

Extensive: These roofs use a thin layer of growing medium (usually not a soil but a mix of lightweight growing medium and/or lightweight aggregate enriched with compost), with 10-25 pounds per square foot (psf) (0.5-1.2 kPa) of weight and 5-15 cm (2-6 inches) of depth. These roofs are usually designed to have low maintenance requirements and are often not accessible. Low-growing plants with shallow root systems and an ability to tolerate drought are usually installed.

Intensive: These roofs use a thick layer of growing medium that is more like regular soil than extensive mixes, in typical depths of 15-90 cm (6-36 inches) and weights of 80-150 psf (3.8-7 kPa). These roofs can have fairly high maintenance requirements as they are often used to grow food, flowers or decorative gardens. They are usually designed to have regular foot traffic. Plantings can include grasses, flowers, vegetables, perennials, shrubs and small trees. Pathways, benches and other human elements can also be designed into the roof.

Semi-intensive: As the name indicates, this type of green roof is designed for weights, plants and uses that lie somewhere between the extensive and intensive.

Roofs can combine zones of extensive, semi-intensive and intensive use. Regardless of the type of green roof, the basic elements are quite similar:

  • A roof structure that is designed to handle the expected loads of the green roof, which can require substantially larger roof members than for other roof sheathings.
  • A low pitch is common for green roofs, though it is possible to build green roofs on steeper pitches by including retaining systems to prevent growing medium from sliding off the roof when wet and heavy. Most common is a “flat” roof, which has only a very minimal pitch (less than 1:12).
  • Solid decking is used to help support the weight; depending on the type of green roof the decking may need to be doubled.
  • A waterproof membrane is the key to the green roof. These membranes are usually made from synthetic rubber compounds. EPDM (ethylene propylene diene monomer), HDPE (high-density polyethylene), PVC (polyvinylchloride) and butyl rubber are common. The membrane is laid on the roof decking either as continuous sheets or as large sections that are glued or welded together.
  • A drainage membrane and/or a root barrier membrane (some products combine the two in one layer) are used to provide a means for excess water to leave the roof without the growing medium getting waterlogged, and to prevent tap roots from growing down and puncturing the waterproofing membrane and causing leaks. Manufactured tray systems can incorporate drainage and root barrier in a planting tray that can come pre-grown and be lain directly on the roof.
  • Growing medium can vary widely in composition and depth, depending on the intent for the roof. These range from dirt excavated from the foundation of the building to specially graded lightweight mediums.
  • Plants are chosen to meet the intent of the roof design, work in the type of growing medium provided and suit the climate and irrigation levels.
  • Some living roofs incorporate irrigation systems to ensure plants are kept watered.

Tips for successful installation

  1. Be clear on the reasons for installing a living roof. The design of the system depends on intended uses, which can range from low-maintenance, no-foot-traffic scenarios to food production or gardening areas with high foot traffic. It is difficult and in some cases structurally prohibitive to change from one type of use to another after the roof is built.
  2. Green roofs are definitely an entire roofing system, and may require more than one designer and installer to complete the whole project. Be sure that you are researching and getting quotes on the full scope of work for decking, membrane, drainage/root barrier layers, growing medium (including lifting onto the roof), plantings, parapets, railings and access points.
  3. The waterproofing membrane is the heart of the roofing system, and must be installed correctly. This process is best done by experienced professionals who warranty their work.
  4. Be sure the plants intended for the roof are suitable for the chosen growing medium and the likely conditions on the roof. Because the growing medium on a living roof is disconnected from the earth, the roof will dry out faster than the ground during periods of drought and may need to be irrigated more than anticipated.

It is difficult to measure the impacts of a green roof, especially on a small, residential scale. This type of roof uses more resources in its construction than any other roofing system, from extra structural elements to the membrane layers to the growing mediums. A green roof that produces food or has other direct environmental benefits can to some degree balance out these higher initial impacts, and that can only be decided on a case-by-case basis. If keeping environmental impacts low is a project priority, be sure to weigh up the impacts versus the benefits before choosing this type of roof.

Harvesting — High. The roofing membrane and the products used to seal and caulk it are materials containing petrochemicals that have a wide range of impacts at the harvesting stage, including habitat destruction, greenhouse gas emissions and water and air pollution.

The root barrier and/or drainage layer membranes can be sourced from recycled materials, but any virgin petrochemical content will carry the same impacts as the membrane.

Growing mediums can range from site soil to specially formulated soil and aggregate mixes. Some soil mixes contain topsoil and/or peat that may not be harvested in a sustainable manner. Be sure to verify the source of the growing medium.

The required increase in roof strength will carry greater harvesting impacts for whatever materials are used for the roof structure.

Manufacturing — High. The petrochemical elements of the green roof will have a variety of air and water pollution impacts during manufacturing, and may continue to off-gas and leach toxins in the roof assembly.

The high number of products required for a green roof raises the impacts by sheer volume, but vary based on the choices for each.

Transportation — Moderate to High. The different layers of a green roof will typically come from different sources, each transported separately. Membrane and growing medium materials are heavy, with correspondingly high transportation impacts. Installation — Moderate to High. Mechanical equipment is typically used for several stages of the construction of a green roof. Petrochemical products used to weld or caulk the membrane will off-gas. The larger the green roof, the more likely it is to have high impacts due to the sheer amount of material that needs to be moved into place.

WASTE: MODERATE TO HIGH

Compostable — Growing medium. Quantities should be negligible if proper calculations are done at the estimate stage.

Recyclable — Some drainage and/or root barrier membranes. Quantities can be moderate to high, as roll sizes do not always correspond well with actual roof size.

Landfill — Waterproofing membrane offcuts, some drainage mat/root barrier, caulking tubes. Quantities can be moderate to high, as roll sizes do not always correspond well with actual roof size.

ENERGY EFFICIENCY

The combination of evaporative cooling, thickness and shade provided by plants will help to reduce cooling loads for buildings with green roofs, in some cases quite dramatically. Some studies have shown benefits during the heating season as well, but this would only be for roof systems in which the roof insulation and green roof layers are in direct contact, and will be more pronounced on intensive roofs with taller plants that fall over and provide trapped air as an additional insulation blanket.

MATERIAL COSTS: HIGH LABOR INPUT: HIGH

The multiple layers of a green roof make it the most labor-intensive form of roof sheathing. Large, heavy items must be lifted onto the roof, including rolls of waterproofing membrane, drainage/root barrier membrane, growing medium and plants. This is typically done mechanically, by crane and soil-blower, but can be done manually, though a fairly large crew and good scaffolding setup will be needed. Premanufactured systems that combine drainage, root barrier and plants in ready-made trays can reduce the amount of labor required, and are typically used for extensive green roofs.

SKILL LEVEL REQUIRED FOR THE HOMEOWNER

Roof Decking — Easy. Homeowners able to do carpentry, including roof framing, will be able to handle the requirements for decking the roof. Waterproof Membrane — Difficult. It is recommended that the laying of waterproofing membrane be done by an experienced professional, especially if the roof has penetrations through the membrane that require sealing or welding. A homeowner may be willing to undertake this task if the roof is simple and the membrane doesn’t require special equipment to join sections together. The weight and volume of this material can present challenges and is often lifted mechanically.

Drainage/Root Barrier Layers — Easy. Homeowners able to follow manufacturer’s instructions will be able to install these layers. The weight and volume of the materials may present some challenges,

but can typically be handled manually with a good scaffolding or lift setup.

Growing Medium — Moderate. The weight and volume of this material is high, and is usually placed on the roof with mechanical assistance. Companies with soil blowing equipment can be hired, and the work overseen by a homeowner. Lifting growing medium onto a roof manually is a very large and heavy task, suitable only for small roofs or large crews organized in a “bucket brigade.”

Planting — Easy. Homeowners with gardening experience or with good instructions can handle the planting of a green roof, though research or consultation with a professional regarding plant selection and maintenance is recommended.

SOURCING/AVAILABILITY: MODERATE

The base waterproofing membrane of a green roof is the same as any conventional flat roof, and this means there are many companies that specialize in this type of roofing, and sourcing competitive quotes or even the materials to do it yourself should be easy in all developed regions.

There is a rapidly growing industry building up around green roofs, though it is mainly focused on very large commercial buildings rather than residential applications. Most major urban centers will have companies that specialize in the design and construction of green roofs, where commercial projects are driving the industry, but such services and materials may be more difficult to source in smaller centers.

Resources and courses about green roofs are widely available.

DURABILITY: HIGH

The waterproofing membrane is the key to keeping a green roof from leaking, and the multiple layers of a green roof tend to provide excellent protection for the membrane. By shielding it from UV radiation and direct exposure to the elements, most estimates for life span are in the 50-year range.

The success of the plantings on a green roof are not necessarily important to the life span of the waterproofing membrane, but are likely to be important to the homeowner or client wishing to see greenery on their green roof. Durability in the plantings will depend on the type of growing medium and plants selected, and of course the climate. Drought is the biggest issue with green roofs, as the thinner soil levels on a roof tend to retain less water than soils on the ground. A green roof may need watering or permanent irrigation to remain green year-round, and will be susceptible to all the ups and downs associated with gardening on the ground, including weeds, pests and disease among the plantings. Appropriate plants and due care and diligence in caring for them should result in a green roof that remains green for the long term.

CODE COMPLIANCE

Code prescriptions for flat roofs using waterproofing membranes exist in all jurisdictions, and a green roof is really just a variation on conventional membrane roofing. Many urban jurisdictions have been quick to write or adopt codes for green roofs specifically, and some jurisdictions are actively promoting them. Smaller jurisdictions may not have green roof provisions, but it should not prove difficult to prove compliance using existing codes for membrane roofs and the ample resources written to cover green roofs.

RAINWATER COLLECTION CAPABILITY: Low

Rainwater is held by a green roof in the porous growing medium, limiting roof run off. While this is an advantage in many ways, it limits the amount of useful rain water that can be harvested. Rainwater collected from a living roofwill be difficult to treat to a quality appropriate for indoor uses due to potential contaminants in the growing medium, but can be collected and stored for irrigation purposes.

FUTURE DEVELOPMENT

Green roofing, though it has existed in some forms for centuries, is a relatively recent development in modern building. There is a great deal of research and development occurring now, driven by the benefits of green roofing in urban centers where vast, flat roofs cause heat island effect, loss of natural habitat and storm water management issues. This is leading to a rapid development of standards, designs, products and procedures that will continue to be refined over the next decade or so. Green roofs will likely lower in price and complexity as systems are developed and become more widely accepted by codes and homeowners. Hopefully, membrane products with less chemical content and environmental impacts will be part of the development. However, issues of weight and plant maintenance will always be part of choosing a green roof.

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