green roofs

Environmental Impact Rating

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.

Chart of Embodied energy & carbon

Energy Efficiency: n/a

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

Labour 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. Pre-manufactured 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.

health warning

Working at heights to install roofing has inherent dangers. Proper setup and safety precautions should always be taken when working on a roof.

Skill level required for homeowners

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 lifespan are in the fifty-year range.

The success of the plantings on a green roof are not necessarily important to the lifespan 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 CAPABILITIES: 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 roof will 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.

Resources for further research

Youngman, Angela. Green Roofs: A Guide to Their Design and Installation. Ramsbury, UK: Crowood, 2011. Print.

Hanson, Beth, and Sarah Schmidt. Green Roofs and Rooftop Gardens. Brooklyn, NY: Brooklyn Botanic Garden, 2012. Print.

Dunnett, Nigel. Small Green Roofs: Low-Tech Options for Greener Living. Portland, OR: Timber Press, 2011. Print.

Dakin, Karla, Lisa Lee Benjamin, and Mindy Pantiel. The Professional Design Guide to Green Roofs. Portland, OR: Timber Press, 2013. Print.

Snodgrass, Edmund C., and Linda McIntyre. The Green Roof Manual: A Professional Guide to Design, Installation, and Maintenance. Portland, OR: Timber Press, 2010. Print.

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.