12/10/2008

Roof-System Performance III

The third in a series of three articles that discuss roof system performance

By Richard L. Fricklas

 

Richard (Dick) L. Fricklas

Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems, and continues to participate in seminars for the University of Wisconsin and RCI Inc.-The Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.

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.

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.


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.

Bluebeam® Revu® simplifies digital facilities document management from design review to leveraging as-builts, maintenance manuals and O&Ms submittals.

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.


 
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