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Asv 21st century tile roofs 2008-10-vandewater

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While work is ongoing to further define the benefits of what has been termed Above-Sheathing Ventilation (ASV), the significance of this information is likely to create a very strong impetus for ...

While work is ongoing to further define the benefits of what has been termed Above-Sheathing Ventilation (ASV), the significance of this information is likely to create a very strong impetus for change in how tile roofs are installed.

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Asv 21st century tile roofs 2008-10-vandewater Asv 21st century tile roofs 2008-10-vandewater Document Transcript

  • A lthough tile roofs are one of selecting a less expensive alternative that quickly driving the entire industry to adopt the oldest and most enduring he knows will not last as long but at least a “new” method that provides a solution types of roof covering known will cost less initially. that heretofore had not been considered as to man, the methods of appli­ The good news is that there are other an element of tile roofing. The green-build cation continue to evolve due factors that are now starting to have an movement has forced designers and to both innovations from impact on this scenario, and the net result builders to reevaluate all of the methods manufacturers and requirements for will be a much more satisfied owner/ and materials that are used in construction improved performance. At a very rudimen­ investor. Even though there are many con­ in order to reduce the waste and excess of tary level, clay or concrete roof tiles provide tractors who are aware of better ways to traditional construction. There are a great a hard surface that sheds water away from install tile roofs so that they will measure many programs and standards that have the structure that it protects. The inert up to consumers’ expectations, a much been developed to guide the industry to properties of the raw materials make the larger number continue to install tile in the more responsible and sustainable selec­ tiles durable against a range of destructive cheapest method possible, with no concern tions that will reduce the impact of con­ forces such as wind, rain, snow, and hail, or awareness of climate issues or other con­ struction on our environment and on future while resisting the less obvious effects of ditions that impact the long-term perfor­ generations. ultraviolet rays, heat, fire, and vermin. mance of roofs. As the various elements of construction The long-term performance of a tile roof, While most roofers will argue the merits have been evaluated, tile roof manufactur­ however, depends a great deal on how it is of their own particular method of installa­ ers have participated in a number of stud­ installed. While most tiles carry a 50- or 60­ tion, there are recent changes that are ies that were performed to determine the year product warranty, there are tile roofs that have lasted ten times that long with only minor maintenance or repair. On the other hand, there is no greater disappoint­ ment than that expressed by a building owner who has invested in a tile roof only to have it require major overhaul within the first 20 years of service life. In most cases, there is nothing wrong with the tiles, but due to either flashing or underlayment fail­ ure, the roof is leaking and the recommend­ ed solution is typically a total reworking of the roof. Even though the tiles are fine and could be reused, it is often more cost effective to discard them and replace the entire assem­ bly. This is certainly not viewed as a good value to the building owner, since the expectation was that a greater investment at the time of original construction should have yielded a much longer return on that Figure 1 – Heat penetrating the roof of each attic assembly being tested on the Envelope investment. In many cases, this disappoint­ Systems Research Apparatus (ESRA) by the Oak Ridge National Laboratory. (Graphic ment may simply drive the owner into courtesy of ORNL.) OCTOBER 2008 INTERFACE • 33
  • Figure 2 – Above-sheathing ventilation (ASV). impact of roof coverings on potential energy either lighter colors or special pigments that the impact of differences within the savings through the roof. Most of the early mimicked light colors, and more and more installed systems. Studies in the U.S. studies focused primarily on higher surface municipalities are making these roofs included work done by the Florida Solar reflectivity and emissivity that would reduce mandatory in many sections of the country. Energy Center1 and Oak Ridge National the amount of heat that could enter into the Some of the building, thinking this could reduce the studies, however, amount of energy required to cool the inte­ went beyond the rior of the building. Significant improve­ simple measure of ments were made by promoting the use of surface issues and “cool roofing” materials that incorporated started to evaluate Figures 3A and 3B – Counter- batten (A, at left) or shimmed battens (B, above) improve ASV and drainage. 34 • INTERFACE OCTOBER 2008
  • Laboratory,2 both of which showed conclusively that the airspace created by the man­ ner of installation of tile roofs played a significant role in the reduction of heat gain into the structure (Figure 1). Numerous studies conducted by Lafarge Roofing Technical Center further validated the studies conducted at FSEC and ORNL and formed the basis for submittals that have been reflected in the new California Title 24 Energy Code. While work is ongoing to further define the benefits of what has been termed Above- Sheathing Ventilation (ASV), the significance of this infor­ mation is likely to create a very strong impetus for change in how tile roofs are installed (Figure 2). Whereas the ASV of a high-profile tile is quite good regardless of how Figure 4 – Standard batten system. the tile is installed, lower pro­ files and flat tiles have a fairly limited ASV unless they are elevated above the roof deck. Fortunately, there are time- tested methods already in place for accom­ plishing this task, even though traditionally they have been used only for low-slope applications or in regions habitually sub­ jected to severe winter weather. The solution is elevated or counter-bat­ ten installations (Figures 3A and 3B). While most concrete tiles and some clay tiles are designed to be hung on batten strips, the most common practice in America is to fas­ ten these battens directly to the roof deck (Figure 4). Unfortunately, this practice is perhaps the greatest contributor to prema­ ture failure of tile roofs, particularly in cli­ mates subject to severe weather. Tile roofs are water-shedding assemblies that, like other roofing materials, require a back-up underlayment system to prevent intrusive water from entering into the roof during extreme conditions. In standard applications where tiles or battens are attached directly to the roof deck, if water somehow gets under the tile, either from severe weather conditions or from less-than-perfect installation, that water has a tendency to collect and pool wherever it does not have a clear channel of escape. It is this repeated and long-term occurrence of wetting that ultimately caus- OCTOBER 2008 INTERFACE • 35
  • Another issue that is quick­ ly entering the mainstream of green awareness is the sustain- ability of materials and meth­ ods that will extend their ser­ vice life. As this relates to tile, it only stands to reason that any­ thing that will help prolong ser­ vice life will be considered more desirable than a material that has to be periodically replaced and sent to a landfill. Since most tiles have a life expectan­ cy greater than 50 years, if ele­ vating the battens above the roof deck prevents leaks, then those roofs will not have to be replaced and will become the preferred choice for purveyors of green construction. With this new interest in roofing as an integral compo­ nent in energy-efficient design, this author believes that sys­ tems that provide longevity and energy solutions will start to Figure 5 – Trapped moisture gradually destroys most underlayments, resulting in roof leaks. move into the construction mainstream, and tile roofs that es tile roofs to fail. While the underlayment So, as the benefits of ventilated tile roofs won’t leak while they are providing environ­ may keep the water out for a period of time, become better known, it is most likely to mental advantages will become the norm in eventually, the repeated exposures will stimulate a move toward the elevated batten North America. They will very simply cause it to fail, resulting in a roof leak systems that accentuate this feature in become the most responsible choice. (Figure 5). order to save as much energy as possible. In With the growth of the green movement general terms, the more air space between References and attention being paid to finding as many the roof deck and the bottom of the tile, the 1 Beal, D. and Chandra, S., “The ways as possible to use responsible building more effective the assembly will be in reduc­ Measured Summer Performance of techniques and materials, the opportunity ing heat transfer. Add a light or cool color to Tile Roof Systems and Attic to use roofs to save energy is likely to gath­ the surface of the tile, and the energy sav­ Ventilation Strategies in Hot er momentum as more people become ings increase some more. Climates,” Thermal Performance of aware of this opportunity. Many cities and This means that tile manufacturers will the Exterior Envelopes of Buildings states around North America are already be adding more colors that are capable of VI, US DOE/ORNL/BETEC, Decem­ rolling out plans to either create incentives satisfying the demands of this new market. ber 4-8, 1995, Clearwater, FL. or require cool roofing on both new con­ Most manufacturers have already begun to 2 Miller, W., Desjarlais, A., Atchley, J., struction and particular reroofing projects have their certified products listed with the Keyhani, M., MacDonald, W., Olson, in their jurisdictions. As this happens, roof­ Cool Roof Rating Council or Energy Star® to R., and Vandewater, J., “Cool ing professionals will have to become famil­ help designers and consumers make selec­ Roofing…Cutting through the Glare,” iar with the particulars of compliance. tions that will satisfy their needs and RCI Foundation, Atlanta, GA, May The good news about tile roofs is that requirements. 12-13, 2005. they do not have to go through major mod­ ifications to become effective energy-saving Jerry Vandewater assemblies. The tiles themselves offer a Jerry Vandewater has been involved in the roofing industry much higher thermal mass than other roof­ for over 30 years, working primarily with clay and concrete ing materials, and even the simplest roof tiles. As the technical services manager for method of application creates an air space MonierLifetile, and as the Technical Committee chairman for that serves as an effective buffer against the Tile Roofing Institute, he has overseen the creation of heat transfer. While this is most effective in numerous installation manuals and tile-related building predominantly hot climates, the ventilated codes. Vandewater is a frequent speaker for roofing organiza­ tile roof has also been shown to be effective tions such as WSRCA, NRCA, RCI, and home inspection asso­ in the prevention of ice damming that can ciations such as CREIA, ASHI, and NARI. occur in cold climates. 36 • INTERFACE OCTOBER 2008