by Katie Daniel | September 8, 2017 12:28 pm
By Rich Thomas, LEED AP, and Steven H. Miller, CDT
Many commercial buildings eliminate the roof as a visual element. Their flat or low-slope roofs function as necessary, but are considered so ugly they must be concealed from view behind parapets.
A pitched tile roof, by contrast, can contribute to the architectural statement while meeting the highest performance standards. More than simply shedding the sun, wind, rain, and snow, it also catches the eye. Concrete tile roofs are energy-efficient, durable, and long-lasting, while presenting a designable surface that can be the building’s crowning touch.
The design and composition of a roof determines not only its appearance and achievement of basic roof functions, but also how it plays its starring role in the energy performance of the building as a whole. The roof is the exterior surface that gains the most heat, directly exposed to solar radiation. It is also the ultimate barrier moderating the loss of rising interior heat. While reflectivity on top and insulation underneath are generally considered the main ingredients of a roof’s energy performance, tile roofs possess features in its shape and structure that increase performance in ways other types of roofs cannot.
Banff, a town located within a national park, has an unusual structure of civic authority, with multiple levels of governing bodies that present unique challenges to new building projects. New commercial projects, for example, are required to enhance the surrounding views of Alberta’s Rockies. The new 174-room Moose Hotel does just that by giving nature a beautiful frame in the form of a distinctive concrete tile roof. The three-storey complex is one of the largest developments built in the town since the 1998 enactment of the Commercial Growth Cap. Due to Banff’s strict growth limits and unusual way of allocating development permits (which includes a lottery), it was 15 years before the final property size and shape was acquired and approved.
The design that finally won approval, by Calgary-based architect Ted Darch, principal of E.J. Darch Architect LLP, resembles a village with a central courtyard. The numerous conjoined buildings are connected to each other by interior corridors on the lowest level and exterior walkways on the two upper floors. Virtually all the upper-storey rooms and walkways have scenic views.
“You can see most of the major mountains around town from one location or another,” explains Darch. “Being able to see views of the mountains sort of takes the guests outdoors without having to leave the hotel.”
The exteriors of the buildings are clad in thick natural stone masonry veneer and make extensive use of exposed cedar wood panels and trim, which the architect terms “traditional materials” for mountain locations.
The building complex is crowned with a series of bright red concrete tile roofs. The façade presents a series of dormers, some with raised sections, and additional canopies over the porches—a profusion of roof surfaces at varying angles. All these individual roofs “give it a fine texture,” as Darch describes.
“It’s also nice to have some protections over the entry doors to the suites,” he says, adding the biggest challenge of the roof design was calculating the effects of water drainage and snow-slide.
The slate-style concrete tiles in Mission Red set off and complement the majestic surrounding mountain peaks. Red roofs are also traditional in Banff, going back to small buildings in Banff Park built in the 1920s, with often painted metal or painted cedar shingles.
Darch originally selected a concrete tile made by a Calgary manufacturer, in a red colour from Frank Lloyd Wright’s Taliesin Colour Palette. When that product became unavailable, the roofing subcontractor suggested a manufacturer in California that makes slate-style concrete tiles. The manufacturer’s standard Mission Red colour almost matched the original choice, but was slightly brighter. The manufacturer usually used Mission Red only for Mediterranean-style arched tiles, but was able to do a custom run in slate style. They sent a small quantity to Banff for a mockup, “about enough to do a single-car garage,” Darch recalls. “We put it up on a small shed and looked at it for about a month and said, ‘That’s it!’”
However, the selection of the tile roof was based not only on appearance, but also on several performance factors.
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:
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).
Concrete tile is a material with high thermal mass that moderates heat flow; it stores heat like a sponge that can soak up a certain amount of water before any starts to drip out. This buildup delays the shedding of heat through the bottom of the tile and toward the building. During daylight hours, roof tile heats up slowly, and does not begin to transfer heat toward the building until well into the day. After the sun goes down, the heat continues to transfer out of the roof tile, and may contribute to lowering the need for heating during cooler nighttime temperatures.
Additionally, the tile’s structure adds an insulating effect, using one of the world’s best insulators—air. Concrete tile is a hard, structurally robust object installed onto raised battens (nailing strips attached to the roof deck parallel to the roof ridgeline) so it stands off from the underlayment and roof deck below. Contact (and therefore thermal bridging) is limited to a few points on the underside of the tile. This forms an air channel under the tile assembly with two important effects:
Arched tiles, such as classic clay barrel tile and S-Mission tile, have sizeable air channels running perpendicular to the ridgeline, under their arches. Flat tiles, such as the slate style used in the Moose Hotel, can also be provided with air channels. Tile is normally fastened to a single layer of battens. A variant on this method involves counter-battens fastened to the deck running perpendicular to the ridge, and then parallel battens atop those. This creates an air channel under the parallel battens. A study by the Oak Ridge National Laboratory (ORNL) in the United States found slate-shaped concrete tile fastened in this manner caused substantial cooling. (For more, see William A. Miller and Hashem Akbari’s 2005 ORNL report, “Steep-slope Assembly Testing of Clay and Concrete Tile With and Without Cool Pigmented Colours.” Visit info.ornl.gov/sites/publications/files/Pub6113.pdf.)
With flat tile, the air channel can be provided at lower installation expense by using specialized, raised battens (available from some tile manufacturers), eliminating the need for counter-battens. Air channels are cut through the raised batten’s underside, creating the cooling effect.
The ORNL study also found tile roofs mitigated the winter heating penalty of a reflective roof. A tile roof improves interior heat retention because of the air channel’s insulating effect, while the thermal mass effect may add heat during the night.
Darch also notes an important effect of this air gap in snowy environments such as Banff. Despite roof insulation, some heat inevitably escapes through the roof sheathing. If it transfers to the roofing material, it can melt the bottom layer of snow buildup, causing a snow-slide ‘avalanche’ that could easily injure a person standing beneath it. For this reason, the Moose Hotel roof was installed with counter-battens to maximize ventilation beneath the tile and thermally isolate it from the roof sheathing.
Additionally, the roof’s underside may be provided with a radiant barrier. Thermal insulation may be installed below the roof assembly and above the conditioned space. Both improve thermal performance. The Moose Hotel used sprayfoam insulation at R-20 in the walls and R-40 in the roof.
|UNDERSTANDING THE SOLAR REFLECTANCE INDEX|
|The Solar Reflectance Index (SRI) has become the chief quantitative measure used to define cool roof materials. The ability of a roofing material to reflect incoming radiation (i.e. reflectivity) and shed heat back to the exterior (i.e. emissivity) is measured using standard procedures, and expressed as a single, combined value.
The U.S. Green Building Council (USGBC) defines this SRI as:
a measure of the constructed surface’s ability to stay cool in the sun by reflecting solar radiation and emitting thermal radiation. It is defined such that a standard black surface (initial solar reflectance 0.05, initial thermal emittance 0.90) has an initial SRI of 0, and a standard white surface (initial solar reflectance 0.80, initial thermal emittance 0.90) has an initial SRI of 100. To calculate the SRI for a given material, obtain its solar reflectance and thermal emittance via the Cool Roof Rating Council Standard (CRRC-1). SRI is calculated according to ASTM E1980 [Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces]. Calculation of the aged SRI is based on the aged tested values of solar reflectance and thermal emittance.
Ratings systems take into account the possibility the reflectance of a material may change as it ages, due to colour darkening or lightening. Therefore, both an initial SRI and a three-year aged SRI are standard measurements. To find cool-roof-rated products, visit CRRC’s online database at coolroofs.org/products/results.
Concrete tile is manufactured in many parts of Canada and is often sold in the region where it is manufactured to reduce costs associated with shipping. Concrete tile may qualify as regional materials under certification systems such as LEED v4. However, not all colours and styles are equally available in all regions.
Cement manufacturing is a high-heat process, and is associated with carbon dioxide (CO2) emissions. However, modern manufacturing technologies have significantly lowered the energy consumption and increased the efficiency of these processes.
Concrete tile is generally expected to last at least 50 years. It is likely the tile will outlast the underlayment and sheathing. In fact, there are specialized roofing serves that lift tile, replace deteriorated materials beneath, and reinstall the tile. High durability and long service life reduce material consumption by lengthening the replacement cycle of the roof. Concrete tile does not suffer noticeable degradation in performance over time unless the tiles are physically damaged, which takes considerable abuse.
High-sloped roofs may be less prone to accumulate dirt or soot, maintaining appearance and reflectivity better. High-sloped roofs of all types may have greater longevity of the underlayment materials due to their being able to dry out more effectively if moisture gets under the roofing material.
Integrally pigmented concrete may exhibit slight colour fading, especially during the first few years, as some less-adhered pigment particles in the surface layer get washed out by rain or snow. This slight lightening of colour can result in a slightly higher three-year aged SRI than the initial SRI, possibly resulting in improved cool-roof performance.
Tile roof systems offer a broad range of esthetic options. A roof can create a connection to traditional architecture, or express an entirely contemporary, breakaway idea. Pitched roofs on low-rise commercial buildings often convey a ‘home-like’ quality with a safe and welcoming sensibility—an effective strategy for any hotel, restaurant, public library, or other building seeking to encourage public access. Steeply pitched mansard roofs on high-rise buildings reference late 19th-century grandeur.
Concrete roof tiles are available in several styles. Classic barrel tile, a style originating with clay, dates back thousands of years. Concrete can also be moulded to resemble other traditional materials, such as slate tiles, wood shingles, and wood shakes.
Roof tiles can also support contemporary designs. Some generally resemble traditional shapes such as slates, but do not try to emulate traditional colours or textures. Instead, they may feature smooth textures, and colours not associated with wood, clay, or stone.
Many linear and geometric effects can be created by staggering the tile placement. Combined with the possibilities of using multiple colours—distributed randomly or arranged in patterns—a tile roof becomes a canvas for architectural expression.
Considerations for design
Roof pitch is both an esthetic and performance choice. Steeper pitches shed water and snow more readily, and may stay cleaner because rain tends to wash them easily. In regions with intense rainfall or high snow loads, lower slopes may be unsuitable.
The ORNL study found some lighter-coloured tile experienced a slight reduction in reflectance due to soiling by air pollution and airborne dust over the course of two years of exposure. This effect was more pronounced at test sites in a highly populated urban area of California and in the desert than it was in a more rural (and moist) region of Tennessee. A steeper-pitched roof was also observed to show less loss of reflectance than a lower-slope (2:12) roof. The hypothesis is wind action kept the tile cleaner. For pitched roofs in urban areas, or regions where dry and dusty conditions prevail, slopes of 4:12 and greater may be advisable to limit reflectance loss.
Concrete tile roof systems provide high-performance roofs that also serve as designable surfaces. The construction of the roof system gives it unusual thermal efficiency, and the physical properties of the material bestow great durability and a long life cycle. Designers can leverage this performance and bring the roof out from behind the parapet to participate in the architectural statement. The pitched roof is a piece of architectural vocabulary with long traditions, and therefore strong cultural and emotional associations. A concrete tile roof allows the architect to reference those traditions and draw on those emotions without sacrificing performance or sustainability.
|A concrete tile roof may contribute to prerequisites and credits under various Leadership in Energy and Environmental Design (LEED v4) Building Design and Construction rating programs:
Rich Thomas, LEED AP, is product manager for Boral Roofing, a manufacturer of clay and concrete roof tile. He has been involved in the roofing industry for 28 years, focusing on energy-efficient roof systems as a key aspect of roofing design. Thomas can be reached via e-mail at email@example.com.
Steven H. Miller, CDT, is a freelance writer and photographer, and a marketing communications consultant specializing in the construction industry. He can be reached at firstname.lastname@example.org.
|After we published this article in the September 2017 issue of Construction Canada, a reader had concerns regarding the testing method. To read the letter and see the author’s response, click here.|
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