By Niklas W. Vigener and Nicholas A. Piteo
We recently consulted on a gem of historic architecture - a 1920s medical center in North Central Pennsylvania. It was an optimistic time, and the original donor mandated that "the structure last 300 years." His architects used the most time-proven materials money could buy. Today, the building - a city block's worth of granite, exquisitely decorated terra cotta, and bronze windows - does not look much the worse for wear. Its roof is covered with clay tiles, one of the oldest and most durable building materials.
Of course, such a timeframe far exceeds what reasonable designers will promise, but many institutions still expect their buildings to last for generations. Clay tile roofs, which can last 75 years or more, are frequently used on monumental buildings. On the downside, such roofs represent a significant initial investment that can be quickly destroyed by faulty design, workmanship, or materials. Here are some key requirements for long-lasting clay tile roofs.
Material. Not all clay tiles are created equal, and we investigated many roofs that were ruined by frost damage shortly after installation. As a general rule, the tiles must have a track record of successful performance in the climate where they will be installed (clay tiles that look great in Los Angeles may fall apart after 1 year in Michigan). In addition to reviewing actual performance, prudent designers will gauge material performance with laboratory tests.
Underlayment. Clay tiles are "water-shedding systems," which means that some water is expected to find its way under the tiles (windblown rain, for example) and must be intercepted on a waterproofing membrane. The most reliable assemblies will have a layer of self-adhered waterproofing membrane, which sticks to the roof deck and prevents leakage when water backs up behind ice dams and also seals around roof fasteners. One important caveat: Self-adhered membranes can invite condensation in some buildings and climates.
Gutters and flashing. The notion that all materials combined in one assembly should have the same life expectancy is an elementary principle of building design. Clay tile roofs include important perimeter flashing components that must be constructed from durable, heavy-gauge copper or stainless steel rather than aluminum or galvanized sheet metal.
Attachment. Clay tiles must be attached to the roof deck to prevent wind uplift and avoid "chatter" (the noise generated when tiles rattle against each other in gusty wind). Look for corrosion-resistant copper or stainless-steel nails; along roof perimeters, look for nose clips that restrain the front edge of the tiles. Avoid foam adhesives that are sometimes used to attach clay tiles and provide much less reliable attachment.
Does durability equate to a low life-cycle cost for your clay tile roof? Unfortunately not. The least expensive steep roof for your building is made with asphalt shingles. The late Carl Cash, a prominent roofing specialist and principal with our firm, researched the age and performance history of hundreds of roofs and compared these parameters to initial construction cost. His somewhat surprising conclusion: Asphalt shingles keep you dry for about one-quarter of the cost per year of service of clay tiles.
Niklas W. Vigener is a principal and Nicholas A. Piteo is a senior engineer in the Washington, D.C. office of Simpson Gumpertz & Heger Inc. (www.sgh.com).