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FIELD TECHNIQUES - What To Ask - For Roofs That Last

   Author: MetalMag

THERE IS SO MUCH HYPE about the potential for roof-related catastrophes that we often lose sight of the simple fact that, for the vast majority of applications, metal roofing is not rocket science. There are, in fact, some basic fundamentals of design that virtually guarantee the success of all but the most highly specialized metal projects when properly applied. Identifying these principles and using them as guideposts in selecting your roofing partners will maximize your potential for success.

If your goal is to consistently specify watertight roofing solutions that last, the top 10 critical factors of concern are:

  • Code compliance
  • Weather exposure
  • Termination details
  • Thermal expansion and contraction
  • Moisture condensation
  • Sealant composition
  • Insulation composition
  • Venting requirements
  • Overflow capacity
  • Metallic coatings and paint finishes
Building codes consist of minimum standards, primarily devised to prevent the most serious types of losses. Nevertheless, they are the place to start, and complying with them is legal requirement.

Except for California, the International Building Code (IBC) has bee adopted most everywhere as the model code standard. It combines into a single, universal code formerly used codes, such as the Building Officials and Code Administrators International Inc., International Conference of Building Officials and Southern Building Coed Congress International Inc. Also of special concern for roofing application are local codes, which frequently impose more stringent standards than IBC and equally mandatory.

Determining precisely which requirements apply to your situation demands knowledge and expertise because every roof has its own unique design configurations. The code books comprise innumerable categories and conditions representing particular types of buildings, materials, environments and design configurations.

There is no such thing as a generic, one-type-fits-all metal roof system. Choices in metal roof products only can be made once design and materials have been evaluated according to local environmental factors. These include:

  • Humidity
  • Temperature range
  • Rapidity of temperature changes
  • Degree of ultraviolet (UV) exposure
  • Rainfall
  • Snow load
  • Marine (salt water) exposure
  • Elevation
  • Wind

Building owners, specifiers, manufactures and installers share the responsibility to make sure these factors are evaluated. Shared information through a team approach is the only way to ensure a successful metal project.

For environments subject to frequent and severe thermal changes, cool metal roofing is an ideal energy-saving solution that keeps rooftop temperatures down. Lowering temperature fluctuations reduces unsightly issues, such as oil canning, and ultimately extends the life of the roof.

Using quality materials and trained metal contractors will vastly increase your chances of ensuring watertight details. But poorly designed details will fail, even if the materials are great and installation is proper. Due diligence must be exercised in selecting and designing flashings, curbs, gutters, and other details.

Redundancy in termination details is the best way to make sure a metal roof system is watertight. There is no coined phrase that rings more true than "the devil is in the details". Select a panel system that incorporates details that include three design features that stop water infiltration.

Lastly, manufactured-provided detail drawing packages are essential for every metal roofing project. With a wide variety of standing-seam and flat-seam metal roof system available on the market today, there is no such thing as generic details. Shop drawings are the key to giving installers the information they need and to make sure a specifier's design is carried out.

For a long-term metal roof solution, a design should be chosen that completely hides the panel sealants, in the seams, away from UV exposure.

All buildings move. What takes place under them and what takes place in the air outside them will determine the degree and rapidity of thermal expansion and contraction. It's also important to keep in mind different metals expand and contract at different rates. Junctions between metals and nonmetals particularly are vulnerable because of the different expansion and contraction rates of the dissimilar materials.

Properly designed metal roof systems make allowances to accommodate thermal movement. The best standing-seam systems are designed with clip-and-panel configurations that allow for complete unlimited thermal movement.

This particularly is important with panels more than 50 feet (15m) in length will change significantly.

For environments subject to frequent and severe thermal changes, cool metal roofing is an ideal energy-saving solution that keeps rooftop temperatures down. Lowering temperature fluctuations reduces unsightly issues, such as oil canning, and ultimately extends the life of the roof.

Whether moisture is being generated from outside or within a building, vapor retarders are your best defense against roof-related "sick-building" syndrome. Vapor retarders are impermeable membranes positioned between the layers of insulation-generally with about one-third of the insulation under and two-thirds of the insulation on top.

Sometimes overlooked during new construction, vapor retarders are even more commonly neglected during renovations. Vapor retarders particularly are critical for buildings that house pools, atriums or moisture-generating process.

The most successful metal roof designs provide a vented space between the panels and roof substrate to allow for free air movement that can help reduce moisture build-up.

There are two main categories of metal roof system sealants; exposed and unexposed. Within each of these categories are a number of sealant compositions with varying performance characteristics.

As a general rule, sealants that remain exposed to UV eventually will crack and dry out. Exposed sealants installed at transition details and counter flashing tend to be a maintenance item every five to 10 years. To be safe, specify a high-performance urethane sealant in exposed applications for the best performance.

High-performance metal roof systems will use a butyl rubber compound to seal the seams or joints between panels. Factory-installed sealant application is the only way to ensure the best use of these sealants. Commodity metal systems frequently rely on though -fastening systems capped with neoprene or EPDM washers, a lower-cost solution that only should be used when system longevity is not a major consideration. For a long-term metal roof solution, a design should be chosen that completely hides the panel sealants, in the seams, away from UV exposure.

The R-value of insulation is of primary consideration when determining what materials should be specified. For built-up roof systems, it is equally critical to consider whether the insulation specified can be compatibly adhered to the specified roofing material.

For example, expanded polystyrene insulation is not compatible with solvent-based adhesives. However, newer urethane asphalt-based adhesives are compatible with most insulations.

The International Energy Conservation Code, the energy-conservation portion of IBC, includes ASHRAE standards related to heating, ventilation and air conditioning. ASHRAE 90.1 defines the required R-values and amount of insulation required to achieve certain R-values in specified geographical areas. Other insulation-related concerns are application category; local fire-code regulations; and Underwriters Laboratories, FM Global and other performance-related ratings for materials used.

Venting serves two purposes in a metal roof system. Venting equalizes the pressures over and under the roof for improved wind-uplift resistance and provides an added defense against condensation (see the moisture condensation section for information about condensation barriers.)

With metal, venting of the panels through roof vents and/or separating the panels from direct contact with an air space from the underlying deck is recommended. This particularly is critical with specialty metals, such as zinc, where any moisture can be immediately corrosive.

When negative pressures are generated by excessive wind, proper venting equalizes the pressure outside and inside the roof to resist wind uplift. Venting also keeps the air moving underneath the roof and above the substrate, discouraging condensation from forming.

Venting equalizes the pressures over and under the roof for improved wind-uplift resistance and provides and added defense against condensation.

A roof system's overflow capacity is another design consideration that too often is overlooked. Not typically a problem with metal roofing is something to consider with internal gutters. The installation of secondary drains may be required during retrofit, especially with older buildings that may have been built before today's more stringent codes.

As with so may roofing design considerations, ensuring overflow capacity is the responsibility of the designer during building construction and frequently is overlooked when renovation existing roofs. A properly installed roof not only should accommodate the rainfalls typical in your region but also those occasional, excessive rainfalls that might otherwise result in damaging pools of standing water.

Metallic coatings are the preferred means of surface protection for steel roof systems. These literally are thin coatings of a combination of metals to protect the steel that otherwise would rust easily.

A variety of metallic coatings are available, offering distinct performance characteristics. Depending on whether or not the metal eventually will be painted and whether the metal application will require significant bending, some metallic coatings will be more effective than others.

For metal applications that eventually will receive a paint finish, Galfan is the metallic coating of choice, offering optimum performance. For applications that are going to remain unpainted, Galvalume with a clear topcoat is the preferred choice. Where economy is a major consideration, galvanized coatings offer excellent paint-adhesion properties and present a viable alternative. G-90 always should be considered the minimum thickness of galvanized coating for a long-term application.

The most commonly recommended paint finishes are polyvinylidene fluorides (PVDFs) sometimes identified by the trade name Kynar or Hylar. These surface coatings typically offer a 20-year warranty. Although polyester-based paints also are available for metal roof systems, they typically do not have the color retention of PVDFs.

For specialty application such as food-processing facilities, marine applications and some chemical-processing facilities high-performance materials, such as aluminum, stainless steel, copper and zinc, may be appropriate. Such surfaces may last 100 years or longer.

Roofing may not be rocket science, but it a science. Like all sciences, if you adhere to its basic principles, you'll get the desired results 99.9 percent of the time.

Whether you're working with a material manufacturer, a design professional or a roof consultant, addressing the top 10 concerns discussed in this article will help guarantee the long-term performance of your roof.

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