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Sustainable roofing – consider your options carefully

Roofs are an important part of the sustainability discussion. Buildings account for about 30 per cent of energy consumed and about 27 per cent of all greenhouse gases created in Canada. Roofs in cities contribute to the urban heat island, which basically makes cities between 2-5 degrees Celsius warmer than surrounding areas, due to a lack of vegetation and soil moisture. When planning a new build or the retrofit of an old roofing system, you can have an impact on this by installing a white, vegetative or high-performance roof. A building owner can offset some greenhouse gases and save energy while adding to a building’s appeal.


February 24, 2010
By Robert Colman

A generic definition of a sustainable roof is a roof that is built without taking away the resources of tomorrow. At last year’s PM Expo in Toronto, several sessions included discussions of the value and appeal of “green” roofs, but perhaps the most informative was that of Ralph Paroli, Director of the Building Envelope and Structure Program at the National Research Council of Canada’s Institute for Research in Construction (IRC). Paroli outlined the benefits of white roofs, “green” roofs (rooftop gardens or vegetative roofs) and high-performance roofs.

Start with sustainable construction

Paroli notes that sustainable construction is as important as what type of roof you decide to install. Sustainable construction encompasses both green procurement policies and waste reduction.

Design and workmanship become critical considerations right from the start of a project. Paroli stresses it’s not just the material you use, but how you use the material that makes a difference. “It really is teamwork the whole way through,” he notes.

Recycling should also be considered, says Paroli. “There are systems that can help you facilitate sorting and salvaging at the end of a roof’s life,” he notes. “For example, if you were to use coverboard to protect the integrity of the thermal insulation, when it’s time to replace the roof, you can reuse the insulation if it’s in good shape.”

And there are many ways to reuse and recycle materials. “The key here is embodied energy,” Paroli explains. “When looking at a lot of what you are doing with a roof, you want to look at the shortest trip for your material. If you can keep everything (from your previous roof) onsite and reuse it, that’s a good thing.”

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Choosing a roof – it’s not one-size-fits-all
There has been something of a craze for vegetative roofs over the past few years. For instance, Toronto has a new bylaw that states buildings over a certain size are now required to include a vegetative roof element. However, across Canada, that isn’t always the best solution.

“It is important to take account of climate and geography,” says Paroli. “For instance, is a rooftop garden the best approach for a northern community? Reflective or white roofs can reduce cooling costs, but they don’t necessarily retain heat as well in winter if not properly insulated.” Paroli stresses that with any project, you need to examine your environment and decide what works best within that environment.

The strongest message that Paroli had was seemingly the most fundamental one — insulate well. “If you have proper insulation, that can go a long way. Even if you are doing a very simple treatment with your roof, just make sure you have the proper thermal insulation, and the right amount.

“One of the problems we’ve seen over the years, for example with white roofs, is the idea that a reflective roof mitigating the urban heat island effect allows you to change your approach to insulation,” continues Paroli. “In the U.S. some people started to say, ‘now that you are reflecting some of the heat back into the atmosphere, you could use less insulation.’ That works very well in some ways but again what happens is you reduce the insulation and once it gets cold, it does affect the overall performance of the building. Make sure you don’t try to save money by reducing how much insulation you are using.”

Thermal insulation will help you save money, Paroli stresses. You just have to keep it dry and use coverboards to protect it from damage.

Durable is sustainable

Paroli regularly returned to the fundamentals of sustainability – a building has to be durable, environmentally sound and economical all at once. That means having individuals involved in the planning of the project that understand the microclimate in which you are working, and understand that they have to take a responsible approach to building – in other words, don’t save a dollar here or there and risk not being sustainable.

Paroli also noted that there are several basic elements of any roofing system that have to be properly tended to: structural support is critical, if you are to have vegetation or solar panels or wind turbines on the roof; drainage is very important; controlling access to the roof has to be properly managed; and penetration of the roof by rooftop equipment has to be carefully attended to.

On this last point he expanded.

“The number of problems that happen on a roof because of trades that are not roofing related is very high,” he explains. “You want to minimize penetration on the roof as much as possible. If you are going to have different rooftop equipment, make sure you provide good access to it – good walking paths will help maintain a more durable surface.”

Preventative maintenance is, of course, also important. “If you want your roof to function properly, the key is to schedule regular maintenance. Check seams. Small problems you can detect early on, and if you wait they could become major.”

Vegetative roofs — understand the benefits and drawbacks
There are two different types of vegetative roof – intensive and extensive. An extensive vegetative roof has a thinner soil cover and doesn’t require much management. An intensive garden is a thicker soil – a foot or more – and requires more management and might involve having an irrigation system in place.

The main consideration in any rooftop garden installation is whether or not your roof can handle the stress of the weight.

The main benefits of a vegetative roof are that it helps conserve energy, helps manage water runoff, preserves the environment, adds to the landscape and can extend the life of the roof.

But Paroli cautions that the installation cost is more expensive than a regular roofing system.

“If you think about doing the installation properly, you have to take into account that maintenance is handled properly and the garden is taken care of,” he notes. “Also, if there is a leak, if could be difficult to locate. There are different techniques to do that, and it is not as bad as it used to be, but it could be a costly repair.”

He also cautions that if you are just looking for energy savings, it is not worth the effort. “If all you are looking at is the energy side of the equation, the payback is roughly 25 years,” he notes.

Reflective roofs
Reflective roofs involve either laying down a white membrane or spraying a white paint on top of a roof, which gives roughly the same effect.

Paroli notes that, again, with the right insulation, white roofs can be a good investment. Of course, they won’t stay the same white colour over time and will lose some of their reflectivity. You also have to consider neighbours and the effect that a highly reflective roof can have on them. Otherwise, they can be a very effective way of reversing the heat island effect at a reasonable cost, and saving some investment on cooling. Storm water runoff, of course, isn’t addressed by this approach, but there are other water management approaches a building owner can use instead.

Photovoltaics and your roof
Roofs are becoming a hot destination for solar panel installations. The benefits of having grid-tied solar on your roof have been discussed here before, but how does it affect your roof?

Paroli notes that you have to make sure your roof is going to last as long as the solar panels installed on it. “A solar cell can last up to 30 years, so you should expect the roof to last as long,” he notes.

He also tackled the question of whether a white membrane or black membrane is best to have beneath the cells.

“The cells themselves are more efficient if you have a white membrane because you are keeping the roof cooler. And by it being cooler you can get more energy from it,” he explains. “On the other hand, black membranes are more able to handle the high heat, and PVs will generate heat.”

High-performance roofing
High-performance roofing can best be described as more intelligent design in roofing — using elements of each type of roofing described above to function in the best possible manner given the environment in which a building exists. Paroli offers one interesting example — putting white paint around the air intakes, so you keep that area of the roof cool instead of incurring the overall expense of putting in a new cool membrane roof.

Paroli refers to sustainable roofing as a “cradle to gradle” approach — understanding that while much will be recycled (cradle) in replacing a roof some things will end up in the garbage (grave). The point of designing a high-performance roof is very similar — to aim for heightened durability and longevity, while taking into account the environment and energy savings at the same time.

Paroli suggests that installing a new, more sustainable roof can add great environmental benefits. And he stresses that there are many ways to reach that ultimate goal. At the same time, he points out that the fundamentals are what matter most – keep the building dry and well insulated.

For more information on the Institute for Research and Construction, visit www.nrc-cnrc.gc.ca/irc.