Concrete Tiles: Durable, Sustainable Roofing Materials Integrate Design and Performance  

Concrete roof tile, used for centuries, is a dependable, durable, and sustainable material that enhances energy efficient design, while exhibiting a distinct architectural character. In recent years, the material has grown in popularity because of its superior strength compared to other traditional roofing materials, like wood or asphalt.

Because of concrete tile's long history, a large number of roof tile shapes have been developed. There are flat tiles, Roman tiles with a concave curve at one end and a convex curve at the other (to allow interlocking), S-shaped pan tiles, and semi-cylindrical Mission or barrel tiles. It is red-hued tile that most often comes to mind with concrete tile, but the material is actually available in a variety of shapes, designs, and colors. Whether for a Craftsman bungalow, Spanish colonial, Cape Cod, or Queen Anne home, this material is suitable for many types of residential styles. Concrete roof tile is also appropriate for commercial structures, schools, churches, and other building types, because of its durability and aesthetic design qualities.

Concrete roof tiles better insulate a building against summer heat than comparable roofing products, such as asphalt or wood shingles, and have a lifespan that's often two or even three times longer. During project budgeting, life cycle costs are often evaluated against initial costs.

A multi-year span of historically low interest rates during the late 1990's through 2005 has generated a boom in residential design and construction. Homebuilders have found by using quality materials, such as concrete tile, they can add to value to their projects, and distinguish them from other countless subdivisions. "Tile roofs have such a rich heritage, appreciated by today's homeowner who is interested in craftsmanship and permanence," says Donald A. Gardner, AIA, founder, Donald A. Gardner Architects, Greenville, South Carolina. "Concrete tile meets these objectives by providing a durable roofing solution that emulates natural materials."

Photo courtesy of MonierLifetile This home was roofed with concrete tile designed to resemble traditional split shake shingles.

Architects and design professionals should be aware of the required conditions sometimes resulting from the additional weight of concrete roof tiles, and the potential structural loads to be accommodated on each project. Although in many cases a roof can accommodate concrete tile without any corresponding structural adjustment, it is important that proper weight measurements be made before proceeding with installation.

Concrete Tile Design Issues

Concrete roof tile is suitable for use in a variety of climates, and reflects tradition, function, and technology. "The roof changes the whole appearance of the outside of a house," says Mac McKinney, president, McKinney Builders, Newnan, Georgia. There are, however, certain design criteria that merit consideration.

Weight: While concrete tile is unequivocally heavier than other roofing materials like asphalt and wood, rarely in single-family home construction does it require special structural accommodation. In new home construction, trusses are pre-designed to support the load of standard weight concrete tiles. When re-roofing an existing home in the West, a lightweight concrete tile is an option at less than six pounds per square foot, which in most municipalities is below the weight necessary for an extra structural engineer's report.

Photo courtesy of MonierLifetile The concrete tile topping this home resembles classic wood shingles but will last far longer.

Colder climates: The material has in the past enjoyed greater popularity in the southern United States and South Pacific than it has in northern portions of the Western Hemisphere. But concrete tile roofs actually have been mainstays for hundreds of years in the coldest climates in Europe.

Variety of shapes and colors: Concrete tile is often associated with rounded shapes and red tones, as frequently seen in churches and homes in Spain, Mexico, and Southern California. But concrete roof tiles are available in a variety of shapes and colors appropriate for most architectural styles. Concrete tile can be made to look like many other natural materials, such as slate, clay and wood in various shapes and profiles.

Fiber-cement products: Fiber-cement roofing products are made from cement and wood fiber cellulose. Cellulose is an organic material that is susceptible to moisture and resultant deterioration if the cellulose product is not formulated and manufactured properly. Many fiber-cement roofing products have failed or deteriorated more quickly than expected.

Seismic properties: A study commissioned by the National Tile Roofing Manufacturers Association suggests that concrete and clay roof tiles withstand seismic forces far greater than the 0.8g (gravity acceleration) that the Uniform Building Code requires for other building components.

Costs: With the rising cost of petroleum-based products, such as asphalt shingles, concrete tile has become more comparatively priced. When life cycle costs and sustainability are considered, concrete tile is an economical choice. According to the nonprofit Committee for Firesafe Roofing, measured by life cycle costs in 2005, concrete tile averaged approximately six dollars per 100 square feet, compared to 22 dollars for wood shake roofing material, eight dollars for metal, or a comparable six dollars for asphalt or fiberglass heavy laminate shingles.

Design Considerations

Concrete tiles incorporate many design features for optimal performance. Special tile head lugs are used to engage the battens to which roof tiles are affixed, assuring a seamless fit among components. Weather checks are performed at the nose to reduce water intrusion. Interlocking side laps channel water off the roof and protect the underlayment.

An elevated batten system allows unrestricted water runoff that may occur due to condensation, broken tiles, or an unusually severe weather event, such as high winds or heavy rains. The batten system also promotes increased airflow under the tile and reduced penetration into the underlayment. Flashing maintains water flow on top of the tile, while containing and maintaining unrestricted water flow under the tile. Eave closures support the eave course in proper plane to the field tile. Weep holes are drilled to supplement proper drainage, and vents promote increased airflow.

In coastal areas with more severe weather, standard flashings are upgraded in strength with self-adhered or multi-ply underlayment, along with a two-component adhesive that expands to establish contact with both the underlayment and the bottom of the roof tile.

Structural Issues

Although concrete tile weighs two to three times as much as asphalt and fiberglass shingles, and about twice as much as wood shakes, most roofs are designed to allow for two layers of asphalt shingles, which is not necessary with concrete tile. As a result, depending on the project conditions, additional structural engineering to compensate for the weight of the roofing material may be minimal.

Photo courtesy of MonierLifetile Concrete tile, such as the Chestnut Brown Villa style topping this home has become a mainstay of larger high-end luxury homes.

Basic design principles allow aesthetic and structural advantages of concrete tile and maximize efficiency. A wood frame load path for receiving concrete roof tile is built with a foundation, sill plate, stud wall, headers, a top plate, ceiling joists, and rafters. For a rafter span chart, the maximum weight a rafter span can withstand, possible required adjustments include purlins (horizontal members fastened perpendicularly to the underside of a rafter and braced to the bearing wall) and ties, all as part of system joining relevant components. Rafter span charts deal only with gravity loads and the support capabilities of the rafters to control deflection. When measured spans exceed the chart criteria, additional bracing is required.

After the rafters have been reinforced to prevent deflection of the load, attention must be shifted to the design aspect of load transfer. The easiest way to understand this concept is to picture a simple triangle and realize that the top two diagonal chords are the rafters holding the weight of the roof and the bottom point is the tie that keeps them from spreading apart. Ties can be standard ceiling joists or T-ties when ceiling joists are not possible. Ultimately, concrete roof tile can be installed on virtually any roof configuration.

Although concrete tiles can be made to work on steep roof slopes, more moderate roof slopes can achieve the same design effect while saving both construction materials and installation costs. For example, a 12:12 roof slope (or 45 degrees) compared to a 9:12 (75 degrees) roof slope achieves the same architectural intent, while saving approximately 15 percent in materials and 20 percent in installation costs. Proportionate savings would accompany lower roof slopes.

Excessive use of multiple hips, valleys and offset eaves are effective when using roofing materials with limited depth and dimension. But they are not as necessary when using concrete roof tile. Cut-up roofs, those which are of unique shapes differing from standard rectangular forms, are more expensive because they require more cuts on field tiles and trim at transition points where a portion of tile ends, such as the edge or the top of a pitched room. Trim tile along the edges of these transition points is also more expensive to install than standard field tile.

Larger roof sections increase roofer efficiency. Designing roof sections to accommodate even tile coursing reduces cutting and lowers installation costs without compromising desired architectural elements.

Durability of Standard-Weight Tile

Concrete tile is a noncombustible roofing material; it's proven to be fire resistant. Additionally, the Universal Building Code requires that concrete tiles be able to withstand 50 cycles of freeze thaw and still maintain break strength (the amount of weight it can withstand upon initial testing). Concrete tile passes the freeze and thaws tests for clay, brick and structural material, as conducted by the American Society for Testing Materials (ASTM). This is an important quality not just in cold climates, where the tiles must withstand temperatures well below freezing for weeks on end, but also in regions or climates with wide swings between daily low and high temperatures.

Concrete tiles are also wind resistant. The material is wind tunnel tested by the ASTM to withstand winds of up to 125 miles per hour, an important feature for regions subject to tornadoes and hurricanes. Fastening options for concrete tile, such as nails, wind clips, screws, and adhesive foam, are tested to resist winds up to 140 miles per hour.

Concrete tile is also hail resistant. In 2005, the State of Texas Department of Insurance approved a new testing method for concrete roof tile developed in accordance with the Roof Tile Institute. In Texas and the greater Midwestern United States subject to annual hail season, the new Factory Manual 4473 will allow for reduced insurance rates for buildings with concrete tile.

Photo courtesy of MonierLifetile It may look like slate, but this building's tiles are made of concrete too.

In earthquakes, a study commissioned by the Tile Roof Institute suggests concrete and clay roof tiles withstand seismic forces far greater than the figure of 0.8 gravity acceleration the Uniform Building Code requires for other building components.

Beyond meeting the demands of certain extreme weather and climate conditions, concrete roof tiles must also meet several code requirements. According to the Uniform Building Code, the tiles must have enough transverse break strength, for example, to be able to withstand a load of at least 300 pounds placed on the center of the tile. In many areas of the country, concrete tile's durability will not only help protect the building physically, but can also contribute to lower insurance rates.

Moisture passing through the tile body must also be monitored on a regular basis. Code requires that the density of the concrete must be such that water cannot pass through the tile during a 24-hour test. Water absorption for standard weight tile should average between eight and 12 percent.

Concrete possesses ideal thermal and radiant properties, making it an attractive sustainable building choice. According to a 2000 study by the Florida Solar Energy Center, a concrete tile roof reduced the transfer of solar heat, or ceiling flux, by 48 percent compared to a black shingle roof. Much like a basement, it experiences more narrow temperature fluctuations.

Sustainability and durability are additional advantages of concrete tile. Since concrete tiles can last as long as 100 years, the material indirectly reduces construction waste because it has to be replaced less often than other roofing materials. Construction waste accounts for a sizable portion of total landfill space. Concrete tile also is not made with petroleum-based products, as asphalt shingles are, and therefore its cost may not be as vulnerable to oil price fluctuations.

Manufacturing Process

There are four basic ingredients for making tiles: sand or aggregates, cement, color or pigment for aesthetics, and water. These are mixed together to form a solid concrete material.

Not just any sand can be used for making concrete or concrete roof tiles. First, the sand must form to the correct grading specification. Grade refers to the size of different grains of sand. When sand is too coarse, the cement cannot fill the void space between sand grains. The effect on the final product is an open or coarse surface texture leading to increased permeability and higher potential for efflorescence. Sands that are too fine tend to produce tiles that are less strong and less durable than expected. These mixes require a high water ratio, increasing the chances of the grains' interlock not being straight (lock splay) or of surface bubbling, the presence of small bubbles or rings on the surface.

Photo courtesy of MonierLifetile Concrete tile can invigorate commercial buildings as well as homes.

Sand must be chemically, mineralogically, and physically suitable. It must be free of contaminants such as chloride, which is present in marine sand, can interfere with the cement hydration, and may reduce long-term strength and durability performance. Physical properties of the sand, including shape, may also affect its suitability. Finally, the sand must be of consistent quality. When selecting aggregates, adequate reserves of sand must be available.

Once the raw materials have been selected, they are broken, crushed, sampled, and fed to a rotary kiln. The kiln typically operates at 2600 degrees Fahrenheit for the production of Portland cement. Heating to this temperature results in decomposition of the clay minerals and de-carbonation of the calcite, enabling the production of calcium silicates. This process makes the concrete stronger. Finally, the powdered cement from the kiln is cooled before milling to the required fineness. Gypsum is added to control its setting rate, producing cements for different purposes. These are referred to as Type I, II, III, IV, and V.

Type I Portland cement is known as common cement. It is generally assumed unless another type is specified. It is commonly used for general construction especially when making precast and precast, pre-stressed concrete that is not to be in contact with soils or ground water. Type II is known to have moderate sulfate resistance with or without moderate heat of hydration. This type of cement costs about the same as Type I. Cement is increasingly sold as a blend of Type I/II on the world market. Type III is known for its high early strength. Type IV Portland cement is generally known for its low heat of hydration. Type V is known for its extreme sulfate resistance.

Coloring and Curing

Coloring concrete can be done in two basic ways. Natural coloring consists of iron oxides, while synthetic metal oxides can be made from iron to make red, yellow or black pigments, from cobalt chrome aluminates to generate blue tones, or chrome oxides for green.

The concrete mix is fed into a tile-making machine, where it is extruded under high pressure into molds (to make various shapes and sizes of tile) that continuously pass through the machine, and immediately proceed down a conveyor belt to receive nail holes and surface treatments. The tiles are transported and collected into curing racks that are then moved into curing chambers for the initial cure prior to packaging.

The curing process accelerates the rate of cement hydration so that the tiles made are strong enough to be de-palleted (separated from the mold). The curing process also impacts other product quality factors affected by cement hydration, such as color variation, efflorescence, and moisture resistance.

There are two types of cures: ambient and heated. After six days of curing, the results of these two processes are the same. The heated cure reaches a stable point much faster, sometimes in one day. For this reason, a controlled heating process is most prudent while tiles are in the curing chambers. This eliminates weather as a factor in curing in order to better assure consistency.

Curing also represents the difference (aside from basic raw materials) between concrete, clay, and natural slate roof tile. Chambers producing cured concrete roof tile reach controlled temperatures of 150 degrees Fahrenheit. Clay tile kilns produce peak heat levels between 2000 and 2200 degrees Fahrenheit. The lesser curing time indicates concrete's greater inherent strength. Natural slate is not manufactured and therefore subject to natural structural inconsistencies which may result in reduced durability. Concrete tile, on the other hand, is manufactured to help ensure a consistent product.

In some areas, particularly warmer regions not subject to severe cold temperatures, a slurry coating, consisting of cement, pigment, sand, and water, may be applied to the tile in the factory. The slurry coating must be mixed to specifications, and is then applied evenly over the tile to a thickness of 400 microns while the tile is traveling on its palette mold at a speed of 1.6 tiles per second.

Efflorescence

For a few months after their manufacturing, concrete tile may exhibit efflorescence, a natural process of water penetrating the capillary structure on the surface and extracting soluble salts from the tile body. Efflorescence is a temporary condition and does not impact the functional qualities of the tile. Deposits from the efflorescence process on the tile surface will wash away in rain or by cleaning once the supply of salts accessible to water is exhausted. The duration depends on the amount and cycle of rain the tile is exposed to.

Efflorescence can be reduced or eliminated by applying an acrylic sealer to form a continuous film over the tile surface. The sealer blocks the migration of calcium hydroxide to the concrete's surface, while allowing carbon dioxide to pass through to form a plug of calcium carbonate in the capillaries.

In addition to efflorescence, the appearance of the tiles may at first be affected by slight mismatches in color from different palettes during curing. When the roofing contractor assembles a roof load, tiles should be gathered from two or more palettes in order to blend shades and reduce grouping of shades.

Installation

The Uniform Building Code stipulates that tiles must be accompanied by installation instructions. In moderate climates, specifications are designed to restrict water intrusion and extend the life of the assembly components. High wind areas specify installation geared to stabilize roof tile during a wind event.

Moisture control is vital in any roofing assembly regardless of the added investment in components, such as underlayment or flashing materials. With any concrete tile, water is directed to runoff on top of the tile and control drainage in pre-formed flashing at critical roof plane junctions under the tile. The system limits extended moisture exposure to the most vulnerable assembly components by providing unrestricted water passage off the roof, while providing airflow under the tile. This allows the air space below the tiles to dissipate heated air, before it gets into the structure and warms the building, thus extending life of the tile.

Maintenance

It is recommended that concrete tile roofs undergo a yearly visual inspection. This can help limit accumulation of leaf debris in the valleys of the tiles or moss growth that could create a damming effect with rain. Moss and algae themselves do not harm concrete tiles, but to prevent them from affecting the aesthetic appearance of the tile, periodic cleaning with a power washer by a professional can remove them. Biocides or zinc strips may also be utilized to slow the growth of these organisms on the tile. After periods of high winds, earthquake, or extensive hail, a visual inspection of the roof should also be made to ensure that there are no cracked, broken, or loose tiles (or their fastening apparatus) in need of replacement.

Brian Libby is a freelance architecture writer living in Portland, Oregon who frequently writes about sustainability.