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Commentary on Roofing

May 1, 2007

Last month's column, Taking Control of Roofing, focused on sources of quality information on roofing systems. As mentioned there, the U.S. Department of Defense (DOD), in its Unified Facilities Criteria Commentary on Roofing Systems , attempted to organize this material in a useful way for its designers and contractors. The entire document is available on the Whole Building Design Guide website.

Highlights of the introductory section of this document are reproduced for you here. Also, since the document was first published May 1, 1999, there have been some changes to websites, resources, and available roofing systems. Updates and changes are in progress. What follows is an excerpt from the Commentary on Roofing Systems ...

Because at no time in the history of roofing has such a wide choice of materials and roofing system options been available, the objective of these instructions is to provide a road map for the selection of appropriate roofing systems. To utilize a road map, the user must know the point of origin and the destination. In the case of roofing, there are two distinctively different starting points:
  1. In new design, the roof system selection can be part of the building design. For example, the building can be strengthened to support a heavy roof system or the slope can be increased to accommodate the minimum required for the desired roof system. When dealing with an existing structure, weight, existing slope, and compatibility with existing materials all become constraints.
  2. With an occupied building, construction noises, fumes, fire hazards, and building access during re-roofing all take on increased importance. Arrival at the destination (e.g. a satisfactory roof system) can take more than one route.
Each path has considerations unique to that particular roof system. The principal variables involved are discussed in this document and supplemental resources are provided. Satisfactory roofing performance comes from the careful selection, specification, installation, and maintenance of roofing systems. Design alerts are provided in each chapter where special attention may be needed. Chapter 2 gives a general guide to selecting a path, while Chapters 3 through 10 provide more detail on that particular route.

This column will address some of the general selection information provided in Chapter 2 of the DOD guide. Figure 2-1 of the guide depicts a general scheme for classifying roofing systems:

View larger image

Starting Points for Roof System Selection

  • New vs. re-roofing/re-cover. It is estimated that approximately 75 percent of current roofing activity is re-roofing.
  • Steep slope vs. low slope. In new construction, the designer is very likely to have a preconceived notion as to whether a highly visible sloped roof is wanted or whether a less visible low-slope roof design is acceptable.

For a PDF of Appendix 2-1, which gives some typical slopes and fire ratings for various systems, click here.

In roof design for new construction, roof slope, and fire and wind resistance are basic design parameters. The previous tables provide typical applications and compliance to building codes.

Fire ratings are generally established by following ASTM E-108. It this test method, fire tests are conducted on both combustible (wood, plywood, OSB) and non-combustible (steel, concrete, and gypsum) decks. The entire roof system is assembled, from the roof deck up to the roof surfacing and everything in between. All systems are tested for spread of flame (surface burning), with the highest rating designated Class A, intermediate resistance Class B, and some resistance, Class C. Building codes generally specify what the minimum rating should be. (These ratings should not be confused with quality or durability; in some cases, durability may have been sacrificed to achieve a higher fire rating.) If the roof deck is combustible, two additional tests are conducted: burning brand and intermittent flame exposure. These establish whether the fire could penetrate through the roof/insulation system and could ignite the combustible deck material.

While gravel-surfaced bituminous roof systems and ballasted single-ply systems will generally always meet Class A on both combustible and non-combustible decks, “thinner” systems, such as mineral-surfaced modified bituminous systems, mechanically and fully adhered single-ply systems, and even metal systems, may require the use of fire-resistant underlayment products and fire-resisting (FR) formulations for combustible decking applications.

Verify ratings using Underwriters Laboratories, FM Global, or other testing and label service organizations once specific products or systems are selected.

Water-Shedding vs. Waterproof
Steep roof systems (hydrokinetic) rely principally upon water shedding, while low-slope (hydrostatic) systems must be waterproof. This is illustrated in Figures 2-2 and 2-3:

In next month's column, I will begin discussing both steep- and low-slope roofing, addressing the strengths and limitations of each.

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