12/10/2008

Roof-System Performance III

By Richard L. Fricklas

 

Under requirements for components, the document provides two full pages of detail pertaining to steel roof decks. Of particular note is yield strength:

The minimum yield strength of the steel (Fy) should be 228 MPa (33,000 psi). (Currently, many roof systems require 80,000 psi [80 ksi] in order to achieve uplift forces.)

Allowable Span determination shall be the lesser of concentrated load deflection, uniform live load, or concentrated loading deflection. Concentrated load is based upon a 300-pound load on the midspan of a single-span deck, representing a hefty roofing installer carrying an armload of materials or perhaps a fully loaded ballast-spreading machine. Roofing mechanisms should be limited to 300 pounds per wheel located not closer than 30 inches apart and bearing no less than 4-inch tread width.

All deck openings that exceed 12 by 12 inches should be reinforced.
At changes in deck direction or plane, such as at ridges, valleys and hips, a sheet-steel closure plate not less than 0.024-inches thick by 8-inches wide, bent to conform to the deck planes, should be provided.

Vapor retarders are also discussed. Another excerpt follows:

Permeance should be less than 2.87 • 10-11 SI perms (0.5 Perms) when tested by Test Method E 96, Procedure A, Desiccant Method at 23°C. Retarders shall be covered by the insulation and roofing membrane at the end of each working day.

Preformed Roof Insulation
While many requirements are covered by individual ASTM specifications, shape stability deserves particular attention, especially with ASHRAE’s upgrading of total thermal resistance to R-20 (single layers of Isoboard, for example, would be thicker than 3 inches).

  • Shape stability. Insulation units should not curl or bow, when properly adhered or fastened, more than 3 millimeters (1/8 inch) in 1200 millimeters (4 feet).
  • Insulation boards should be butted together. All joints over 6 millimeters (1/4-inch) wide should be filled with insulation.
  • Insulation installed in multiple layers should have the joints offset, preferably on-half board (minimum of 6 inches) between layers.
  • Attachment of the bottom layer should be by mechanical fasteners.

This is still valid today. Low-rise foam adhesives are not currently approved by FM Global over steel roof decks.

Insulation Fasteners
Mechanical fasteners are well understood by the roofing industry. According to ASTM E936, “The hardness of the steel deck should be considered when selecting the insulation fasteners” and “corrosion resistance shall exhibit (not more than) minimal traces of rust spots when tested by Method B117 for 48 hours.” (This is a salt spray test that should be updated to Kesternich or Sulfur Dioxide Testing [ASTM G87]).

Bituminous Roofing
This section addresses conventional built-up roofing (BUR), but could be adopted for modified bitumen (MB) systems as well. Perhaps, in the future, we will see a document such as E936 dedicated just to MB or single-ply systems.

Other Components
There is little doubt that flashings are more troublesome than any other component of a roof system. Under general guidelines, the following information from E936 is of interest:

Pitch pans filled with asphalt or coal tar bitumen or plastic cement are the least satisfactory means of flashing roof penetrations and should be avoided. (Pourable sealer or premolded boots are vastly superior.)

All units installed on supports above the roof shall provide easy access for a worker to reach at least half the width under the unit if necessary, to repair the roof or the equipment. An access height of 600 millimeters (24 inches) is considered minimum to facilitate satisfactory repair work when necessary, or higher if size of unit warrants.

Roof System Evaluation
Field inspection, testing, field verification, and certification are all addressed in E936, even including a sample historical record form.

ANNEX to E936 (mandatory information) contains several pages of standard roofing details, excerpted from The NRCA Roofing and Waterproofing Manual.

APPENDIX to E936 (non-mandatory information) includes a section on roof consultants, including the following definition:

An individual or firm of established competence having professional qualifications as a roofing consultant and who is engaged in the field of roofing technology. They may also maintain a regular force of professionals and technicians.

Similar sections can be found on testing laboratories, owners and designers, fire performance, mediation services, and moisture evaluation. The entire E936 document can be obtained from ASTM Intl.

While much of the above information is available from NRCA, other trade associations, FM Global, and Underwriters Laboratories (UL), this document represents a consensus of the best prescriptive information on low-slope roofing. Since the industry may never see true performance specifications, documents such as E936 can play a major role in insuring that roof systems perform as intended. Since sustainability has become a buzzword of this decade, good design, materials, and installation are more important than ever.

Resources:


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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.


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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.
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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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