12/10/2008

Roof-System Performance III

By Richard L. Fricklas

 

The last two issues of this newsletter looked at historical attempts to develop performance specifications for certain roofing projects. The CIB/RILEM FIT system was an attempt to quantify attributes that might be needed for a specific roof project. I listed some levels of performance for a few roof systems that, in general, would meet specific project requirements.

Another example in this performance series is ASTM E936 (last updated in 2004), titled Roof System Assemblies Employing Steel Deck, Preformed Roof Insulation, and Bituminous Built-Up Roofing

Bill Maroni of Factory Mutual (now FM Global) conceived this amazing document back in the 1970s. From an insurance perspective, it became apparent to FM Global that most roof failures could not be attributed to just a single element; it was the system that needed to be correctly put together. Since bituminous roofing was dominant in the 1970s, the committee that Maroni assembled focused on bituminous roofing, with steel decking and rigid roof insulation as the major components. Much of the work of this Committee on Roof Assemblies (CORA), however, is equally relevant to single-ply systems as well.

The scope of ASTM E936 is worth reviewing:

This practice covers the performance requirements for the design, components, construction, and service expectations of new roof system assemblies. For this purpose, the roof system always includes steel deck, preformed roof insulation, and bituminous built-up roofing, and their attachment. It may also include fire-resistive components, integral acoustical treatment, vapor retarder, adhesive or mechanical fastener attachment, and aggregates.

Since this newsletter is part of a series on roof performance, the next paragraph of E936 sets the stage for our discussion:

The objective (of this document) is to provide realistic criteria for the overall performance of the roof assembly and its components because by necessity and custom, a roof assembly contains a variety of components and is subject to varied environmental conditions.

Performance Concepts
Design. The roof system should be designed in accordance with this practice to resist the effects of the usual or normal weather and loading conditions, which can cause excessive deflection, destroy adhesive bond, fracture the insulation, and result in premature failure of the roof system. Such weather and loading conditions may include, but are not confined to, water, wind, hail, snow, ice, uniform and concentrated loading, and thermal expansion and contraction of building units. The roof system should be sloped to provide drainage under design loading conditions, and the design should sustain the anticipated live load if drainage is obstructed.

Construction. During construction, the partially completed and the completed roof assembly should:

  • Be protected against construction traffic and equipment to be used in the construction of the roof assembly and subsequent traffic and use by other trades.
  • Provide weather protection consistent with the construction schedule requirements as determined by existing weather conditions.

Service. The roof system assembly, when in service, should:

  • Be protected against anticipated building maintenance procedures.
  • Provide weather protection.
  • Provide thermal insulation.
  • Provide a vapor retarder, if required.
  • Provide fire-safety and uplift resistance as required by the building owner, applicable building codes, or insurance underwriters.
  • Carry anticipated design dead loads and live loads.
  • Receive proper and periodic maintenance over its service life.
  • The components used in the roof system assembly should be compatible with each other.

Design, Materials, and Construction Requirements. All components of the roof system should conform to specific design criteria essential to provide an assembly capable of fulfilling the performance concepts.
Note 1: The spacing and straightness of the steel deck supports are important to proper deck installation and should be confirmed by the designer or their representative.
Note 2: For locations other than roof edge and nonwall supported details, the need for wood nailers should be determined by the designer or specifier.
Note 3: The first layer of the preformed insulation can be more positively secured by mechanical fasteners with the additional layers of preformed insulation fully adhered to the first layer.

The ASTM document goes on to discuss testing of components and protection of materials, and recommends that a pre-roofing conference be conducted prior to the erection or assembly of the roof system.


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Visit our website today to learn about the design flexibility of a Morton building and the endless possibilities of partnering with our designBUILD team.


Wood construction is both cost and energy efficient. Check out Morton Buildings and our designBUILD team online today to discover all the benefits of post-frame construction.


When choosing a metal-clad building for your next construction project, consider Morton Buildings, Inc., and their designBUILD team, we’ll make your dream a reality.

We Can Help You Reduce Energy by 30%

Our mission is to help our customers manage their buildings' energy costs, improve reliability, and enhance performance while having a positive impact on the environment.
CLICK HERE to find out how.

Add highly responsive multi-zone comfort to any building project, in any climate. Our CITY MULTI H2i R2- and Y-Series VRF systems give you flexibility to fit the needs of any building. Enjoy 100% heating capacity at 0°F outdoor ambient, and 85% heating capacity at -13°F outdoor ambient.  For more information, log on to www.mitsubishipro.com

 
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