Durability
Longevity was a factor that strongly influenced the selection of concrete tile for the Moose Hotel. The owner is a local Banff company that is invested for the long term.
“They’re looking at durability—at not ever having to replace things like the roof, if they can,” says Darch. “Concrete tiles are very good in that regard.”
Unlike more-traditional roof materials of the region (e.g. wood shingles or shakes) concrete tile does not support the growth of mould, mildew, or moss, and is not subject to rot or decay.
“The main issue with cedar shakes is moss buildup,” points out Darch. “They get a lot of airborne dirt and a few seeds—after a while there’s green on it, things are growing, sometimes a fair amount.” Concrete tile is a good match for a challenging climate like Banff, too, where winter temperatures can get down to −36 C (−34 F) and the average yearly snowfall is 1910 mm (75 in.). Concrete tile has excellent freeze-thaw performance. When specifying concrete roof tile, one can look for products complying with ASTM C1492, Standard Specification for Concrete Roof Tile, which includes freeze-thaw testing.
Another important aspect for Darch was concrete tile’s noncombustibility. Banff is in the forest, and while there have not been severe fires there recently, other incidents in the Rockies have demonstrated the potential for destruction. Elsewhere in Alberta, the May 2016 Fort McMurray wildfire was the costliest disaster in Canadian history, destroying more than 2400 homes and buildings. In fires of that nature, the roof is a key element in the spread of the blaze, says Darch.
“It rains down burning embers. If the roof gets embers on it, it’s a good thing to have a noncombustible material,” he says of the concrete tile.
When specifying concrete roof tile, one should look for projects compliant with CAN/ULC S107, Standard Methods of Fire Tests of Roofcoverings.
Energy and thermal performance
The thermal performance of roofs has become a focus of energy codes. Highly reflective roofs are mandated in many warmer climates to reduce solar heat gain and lower energy usage for air-conditioning. The observation that urban areas become heat islands—remaining measurably hotter day and night than adjacent rural areas—has prompted a broad push for reflective roof colours. A reflective roof bounces solar radiation back into outer space without it being converted into heat, which keeps the roof—and the building below it—cooler in the summer, but also deprives it of some solar heat in the winter.
In some Canadian climates (including Banff), solar heat gain is not a cause of significant net energy consumption. The winter ‘heating penalty’ of a reflective roof is theorized to exceed the summer energy savings. (In practice, the colour and reflectivity of a roof material may be irrelevant during cold months if the roof is covered in snow.)
The structure of a concrete tile roof system, however, has thermal performance properties that function regardless of its reflectivity. While some concrete tile colours qualify as cool roof materials depending on the rating system, all of them provide increased energy efficiency due to the nature of the material and mechanics of the roof system.
A tile roof is a multi-layered system. From the top down, the layers are:
- tile;
- air channel;
- battens (usually wood);
- underlayment;
- roof deck;
- radiant barrier (sometimes included, often as a foil layer laminated to the bottom of oriented strand board [OSB] decking); and
- insulation.
Each of these layers plays a role in the thermal performance of the system. The layer directly exposed to the sun, the tile, has two main responses to incoming solar energy.
‘Reflectivity’ is a surface’s ability to make incoming radiation bounce off. As is taught in elementary schools, lighter colours reflect more light and stay cooler in the sun because they also reflect infrared (IR). However, some materials with fairly dark colours—generally in the orange and red range—also reflect a significant proportion of infrared. Concrete colour is achieved by mixing integral pigments into the concrete during casting. Concrete tile cool roof colours tend to be lighter shades, although concrete tile made with high-IR-reflective pigments are also possible.
‘Emissivity’ refers to a surface’s ability to shed radiant energy (as distinct from conducting away heat). Concrete tile has high emissivity, so a portion of the radiation not instantly reflected is sent back upward, more slowly, in the form of radiation. Some radiant energy is absorbed by the tile as heat, and may be transferred by conduction to the air above and below the tile.
Sustainability rating systems such as the Leadership in Energy and Environmental Design (LEED v4) program utilize the Solar Reflectance Index (SRI), which takes into account both reflectance and emittance. (See “Understanding the Solar Reflectance Index,” page 40.) If LEED certification is being pursued, concrete tile colours with SRIs in the 40s and 50s may be used to satisfy LEED steep-slope cool roof requirements. However, as noted previously, the applicability of the cool roof concept varies depending on local conditions.
As to thermal response of the tile itself, these concrete products are not known to deteriorate or fail due to thermal expansion and contraction (unlike some other roofing materials).
