Pitched vs. flat

Despite the cold, climate still allows a wide choice for roofing

Roofs are sometimes referred to as the fifth side of a building, suggesting their role as part of an integrated, complete envelope system that includes waterproofing, exterior walls, and cladding.

"Whether you use pitched or low slope, sometimes called flat, and what materials are used, depends on the type of building and its purpose, and how the roof might contribute to its architectural qualities," says Trevor Sziva, marketing director at the Alberta Roofing Contractors Association.

With more complex buildings that have various levels immediately below the roof, Sziva says, "It might not be practical to use a slope roof."

A steep slope or pitched roof on a building of this kind would involve a rafter system with many angles. In such cases, if one wanted to include a steep slope for aesthetic effect, it is usually more practical to have one at the front, with a flat roof behind. Flat roofs also have an advantage when it comes to large spans.

"A rafter or truss system supporting a large span is complex, expensive and not very practical," Sziva says. "There are a few large buildings with a slope. When you do see a slope on a high rise, it's often got a mansard and the rest of the roof is low slope, hidden behind. Low slope has the benefit of simplicity of support, and it's typically lighter and less expensive."

Although warm, temperate summers and harsh, cold winters are common across Alberta, roofs do vary to match some of the climate variations within the province. Apart from the north part of the province getting slightly colder winters than the south, the foothills and mountain areas in the west also present a climate variant. These alpine-type areas are often windier and get more snow in winter and rain in summer than the central and eastern parts of the province.

Architectural standing-seam metal roofs can be a good choice for single-family and small to medium multi-unit residential complexes. This type of roof consists of metal panels running vertically on a pitched roof deck, each panel having two seams that stand vertically and are crimped together to seal the joint and keep out the elements.

THE ASPHALT EDGE

"[Standing-seam metal roofs] have the advantage of being less susceptible to snow and ice than shingles," Sziva points out. They are also very durable, lasting 50 to 100 years.

Asphalt shingle is the biggest seller in the province. "But if you want a pitched roof that lasts 50 to 100 years, then you choose either concrete tile, slate, or metal," Sziva says.

"All are suitable for the climate here, including shingles, but what you decide on depends on what you are looking for in terms of cost, appearance, and durability."

When cedar shakes are used, Sziva recommends that it is best to use a top grade for buildings in Alberta because of the province's climate.

Asphalt shingles come with either a fibreglass or organic (paper) base, but the latter is vulnerable to moisture, he says. Recent years have seen a trend to fibreglass, although the extent of the trend varies between regions. For instance, compare the difference between the degrees that Alberta and British Columbia have embraced fibreglass, and it's hard to believe that climate wasn't a factor. Estimates suggest that in B.C. fibreglass has now captured about 80 per cent of the asphalt shingles market, compared to Alberta, where fibreglass appears to have about 60 per cent of the market. Paper-based asphalt shingles, unlike fibreglass ones, are vulnerable to moisture and B.C.'s climate is in most areas significantly wetter than Alberta's-hence more use of organic in Alberta than B.C.

Some experts say organic materials for roofs should be mostly eliminated.

"Why put organic on a roof when everything else is high-tech?" asks Roger Miller, a project manager at Williams Engineering Canada Inc. "Organics get wet and then fail."

In many parts of the world and for much of the 20th century, the most common type of flat roof was a built-up-roof (BUR). It uses layers of felt-or some similar organic material-and asphalt, and can be applied in any season. But as insulation practices in Alberta began to improve after the oil crisis of the 1970s, says Sziva, BUR roofs on newer, better insulated buildings that were no longer being heated from within became brittle in winter with a tendency to crack. That led to leakage.

The solution to this was SBS-modified bitumen. This involved blending a rubber polymer-styrene butadene styrene-with the asphalt, to prevent brittleness. It's a two-ply system that uses a base and a cap sheet and is usually applied either with torching or adhesives.

Each application method has its advantages and drawbacks. Adhesives pose no fire hazard but don't work properly in cold weather. Torching does potentially involve a fire hazard, but will work in any weather. An SBS system from Soprema will provide a roof for the new 59-storey Encana headquarters in Calgary.

Recent years have seen SBS overtake BUR in popularity.

"At one time, BUR was the only system for a flat roof," Sziva says. "It's dropped off in market share from 30 years ago when it was around 60 to 70 per cent. It's now about 20 per cent. SBS now has about 60 per cent of the market and overtook BUR because of its higher performance."

Single-ply systems like TPO, EPDM and PVC now have roughly 10 per cent of the market. Unlike BUR or SBS, many of these systems include an option for white or reflective membranes. In big southern U.S. cities such as Atlanta, where the ratio of cooling days to heating days is around 8:1, reflective membranes make sense. They reduce urban heat island effects and cut air-conditioning costs.

But how useful is a reflective roof in Alberta's climate?

"The biggest recent roofing change in Alberta has been the switch to more use of reflective membrane," Sziva says. "They started out in the southern United States. But we have winters; they don't. The benefits of reflective membranes are not there above the 49th parallel. The cooling/heat days ratio is reversed in Canada, with a heating to cooling ratio of 8:1 in favour of heating in much of Canada. [Leadership in Energy and Environmental Design] didn't address regional issues. Durability is more important. That's where lifecycle costs make a difference."

Stephen Teal, a green roof and envelope specialist at Flynn Canada Ltd., agrees that green roofs are a more sensible option in our climate.

COOL ALTERNATIVE

"Green roofs can reduce cooling loads on a building significantly-up to 30 per cent. People tend to go to a reflective roof, but in Alberta it's probably an energy negative," he says. Green roofs, on the other hand, can provide some insulation in winter and a water-management function in summer, reducing the load on a municipal stormwater system.

Noting that Flynn is a founding member of Green Roofs for Healthy Cities, Teal says there is now a considerable wealth of knowledge and experience supporting the green roof sector.

Alberta's two main cities, Calgary and Edmonton, lag behind some other centres here and in the United States in their number of green roofs. In Alberta, Teal points to several recent projects that had included green roofs in their plans, only to cancel them at the construction phase.

Because of the climate, the range of green roof options in Alberta is perhaps more limited. But as more focus is placed on green building techniques, that is sure to change.


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