Best Practices: Proper Roof System Design Standards
February 9, 2006
Roofs are an integral component of the building envelope, and although their initial cost constitutes as little as 5 percent to 7 percent of the total building cost, maintenance costs - throughout the life of the structure - typically exceed 60 percent. When taking these factors into consideration, the most economical choice would be to design a high-performance roof system at the outset. However, statistics prove that this is seldom the case. Over 50 percent of all construction litigation in the United States involves roofing. This is nearly four times more than the next highly litigated component, wall systems. Furthermore, 75 percent of all new roofs have reported leaks within the first five years, and 20 percent of all roof failures are due to improper design.
Improper design of roof systems can be attributed to a lack of substantial knowledge of the mechanics of system components. Currently, no major architectural program in the United States addresses roof design for more than a couple of hours of study. This leaves practicing architects to expand their role as a “generalist” tying in building components. Architects are often forced to rely on manufacturers to assist them in roof system design. Problems in this relationship arise because manufacturers often have limited analytical expertise in other manufacturers’ materials and systems, and their vested interests in selling their own products may preclude them from being impartial. In some cases, roofing contractors act as specifiers, and they are often inclined to recommend systems they are most familiar with and those which offer the biggest profit margin. Contractors often lack the knowledge, expertise, and resources to objectively evaluate the problems and suitability of materials that will perform best under field conditions.
Manufacturers on Design and Warranties
A disturbing practice often utilized by roof designers is the insertion of manufacturers’ technical requirements as specifications. These documents are in fact only guidelines established by manufacturers to set tolerances for the application of their materials. Manufacturers state in these documents that they are not intended for use as design documents, and the language is designed to shield the manufacturer from all liability in design issues. The guidelines and details provided by manufacturers often change, particularly with regard to single-ply systems.
Reliance on manufacturers’ warranties as design criteria provides another set of problems. As I’ve spoken to architectural groups and designers throughout the country, I have found that many designers have misconceptions about the value of these warranties. Warranties in the roofing industry were initiated as a marketing tool for manufacturers who were placing new and “unproven” products on the market. Warranties limit the manufacturers’ liabilities, and most warranties provide no more retribution than the coverage of material costs for roof leaks that are reported within 24 hours of occurrence. The limitations often exceed the coverage. The roof designer should read all warranties closely and decide what type of warranty is best suited for the project. Remember - no warranties include coverage for design issues.
Improper Design and the Average Roof Life
Improper design and other contributing factors have led to the steady decline of roof system service life throughout the years in the United States. There is good historical evidence of this decline. At the end of World War II (and for some time thereafter) built-up roofs lasted an average of 20 to 30 years or more. The decline in average service life in roofs began in the 1950s and 60s with the introduction of new technologies and a change in the basic relationship between contractors and manufacturers. The downward trend in average roof service life continued to accelerate during the 1970s and 80s, as new manufacturers entered the marketplace, inundating the U.S. market with a variety of unproven and marginal materials and systems. The situation worsened with the promulgation of artificially imposed energy codes and programs. The United States Air Force, publishers of AFM-91-36 Built-up Roof Management Program, determined that by 1980 the average built-up roof was lasting approximately 12.5 years.
The decline of new construction in the United States in the past five years has dramatically increased the number of remedial construction projects. Most companies and organizations have recently opted to expand or renovate existing facilities in lieu of building new facilities. This trend is projected to continue for some time.
Remedial roof design limits specifiers’ choices when selecting a system. Remedial work can only be completed upon the thorough investigation, research and analysis of the problem that led to the original roof failure. Once the problem has been determined, the specifier must provide the design solution by focusing on stringent criteria. Building design and use, roof access, the number and type of penetrations, installation requirements, durability, geographic weather conditions, expected foot traffic, economic concerns and budgetary constraints are all important criteria in the final system selection.
The design standards that are referenced in bid documents are important for proper roof application. The acronyms that are strategically placed throughout the bid documents are more than simply “CYA” phrases used by designers. The standards are inserted to ensure that the installed roof system meets the requirements of the roofing industry, as well as local and national building codes and insurance requirements. When properly presented in the bid documents, the standards can be a valuable asset to the installing contractor.
The responsibility for deciphering the standards and determining how and where they apply in the design documents is the responsibility of the specifier. Simply providing a statement such as “comply with the (standard)” is inadequate if it does not provide the exact design standard requirements for the system. For instance, in the case of Factory Mutual wind uplift requirements, the specifier should go beyond the statement “apply in accordance with FM 1-90” and state the required attachment method for the specified deck and insulation components. This would include items such as providing the exact number of fasteners required per square foot to comply with the standard.
If this information is not clearly identified by the specifier, the contractor is forced to provide bids based on assumptions.
Deciphering the standards by the exact name and/or number is also a critical issue. Most of the standards have several different classifications. For example the statement “complete the metal coping in accordance with NRCA details” is vague and confusing due to the fact that there are several variations of this type of detail. The specifier should state the exact standard or be prepared to allow the contractors to complete the detail based on their own interpretation. This type of practice could lead to inaccurate bids (as each contractor’s interpretation might be different), jobsite disputes, change orders, confrontations between the specifier and the contractor and - in the end - an unsatisfied building owner. The specifier must determine the standards that apply to the roof system, list them in clear and concise language, and - where necessary - define the methods required to meet the standards during installation.
Although the specifier bears the responsibility for determining the design standards for the roof system, the contractor should have a basic understanding of the standards and why and where they are required. The most prominent standards used in roof system design are ASTM International, Factory Mutual (FM), Underwriters Laboratories (UL), and various details from manufacturers’ literature, the National Roofing Contractors Association (NRCA) or the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA).
Founded in 1898, ASTM International is a not-for-profit organization that provides a global forum for the development and publication of voluntary consensus standards for materials, products, systems, and services. Over 30,000 individuals from 100 nations are the members of ASTM International, including producers, users, consumers, and representatives of government and academia. In over 130 varied industry areas, ASTM standards serve as the basis for manufacturing, procurement, and regulatory activities. Formerly known as the American Society for Testing and Materials, ASTM International provides standards that are accepted and used in research and development, product testing, quality systems, and commercial transactions around the globe.
In the roofing and waterproofing industry, ASTM committees are comprised of industry volunteers, predominately material manufacturers and producers, who outline the test methods and standards for material specifications. The ASTM standards referred to in bidding documents are specifically related to component materials. Most roofing and waterproofing materials must comply with minimum ASTM requirements.
All of the ASTM standards have reference numbers for indication. All of the standards in the roofing and waterproofing division begin with the letter D, followed by the standard number and the year of origin of the standard, as in D4- 86 (1998) Standard Test Method for Bitumen Content, for example. Changes or modifications to the standards are denoted with the year of revision in parenthesis after the original year.
The ASTM standards are classified in three categories: standard specifications, standard test methods and standard practices. The standard specifications provide the minimum specifications that a material must comply with to meet the standard. Some of the newer materials, such as TPOs and SBS, do not have ASTM standards at this time. Since the organization relies on volunteers and since most of the participants are representatives of material manufacturers (who have a vested interest in the requirements), development or revision of the standards follows a process similar to that of a legislative bill passing through Congress. In bidding documents, it is the standard specifications that should be referenced. If the specifier erroneously submits a standard test method in the materials section of the bid documents, the contractor should clarify that material testing is not required. Test methods and practices may be provided in the execution section of the bidding documents if the specifier requires quality assurance testing. Typically, test methods are used in forensics evaluations of applied materials. It is the responsibility of the specifier to verify that designed materials meet or exceed the ASTM standard’s specifications.
Factory Mutual Requirements
The Factory Mutual Engineering Corporation has published “Loss Prevention Data for the Roofing Contractor,” which provides guidelines for installation of roof systems to prevent substantial damage from severe weather. As an insurance company, Factory Mutual holds a vested interest in decreasing these types of failures.
In addition to the design standards, Factory Mutual also conducts testing to approve materials and systems. The materials and systems are tested (for a fee) to meet wind uplift and fire standards. The membranes are typically tested in system configurations over different deck types and with different insulations. Factory Mutual charges a separate fee for all tests, and each system configuration must pass on its own for individual certification. The specifier must determine that a proposed system has passed FM in the exact configuration specified.
The approvals are based on systems - not separate components. Factory Mutual design standards and testing procedures have been adapted as acceptable in the roofing industry. Most of the materials and systems require FM approval prior to market entry. However, not all of the roof systems on the market today have FM approval. For instance, loose-laid ballasted single-ply systems are not FM approved due to the probability of roof blow-offs in high wind conditions. Nevertheless, these systems are frequently installed.
Prior to design, the specifier should indicate whether the building owner is insured by Factory Mutual. Companies that are insured by FM must comply with the design standards set forth in the loss prevention guidelines using FM-approved roof systems. FM periodically inspects insured facilities to ensure compliance with the standards and oftentimes reviews design documents for compliance on new and remedial construction projects.
Although FM provides loss prevention standards for fire, hail and roof loads, FM is most recognized for wind uplift testing. The total roof system must be designed to comply with the standards at insulation and membrane attachment and at all perimeter flashings.
There are two common misconceptions associated with the FM wind uplift classifications. The first involves the number 1; the classification denotation is the numeral 1, not the letter “I,” as is frequently used. (The correct pronunciation is FM Class 1-60, not FM I-60.) The second misconception involves what the uplift classes actually mean. The common misconception is that Class 1-60 refers to withstanding wind speeds of 60 miles per hour. This is incorrect. The determination is actually based on pounds per square foot of pressure that the system can withstand. This is determined during testing procedures. In this process, the complete roof system - including the deck - is secured to the frame of the wind uplift test apparatus. During the test procedure, compressed air is slowly introduced below the deck in increments of 15 psf. If the sample maintains 60 psf for one minute without damage, it is classified as Class 1-60. The same procedure applies for Class 1-90, in which systems must maintain 90 psf for one minute, and Class 1-120, in which systems must maintain 120 psf for one minute.
The procedure used to determine wind uplift classifications on specific buildings during the design phase is based on a calculation that takes into account the basic wind speed in the geographical area, ground surrounding the building and the roof uplift pressure at the field of the roof. The building’s height and perimeter construction is also considered. It is the responsibility of the specifier to complete these calculations. It is the specifier’s responsibility to determine the proper wind uplift classification for the building.
Determining Design Requirements
Compliance to manufacturers requirements is important, particularly due to the fact that the manufacturer will be required to provide a warranty for the project. However, it is the specifier’s responsibility to determine the design issues that best meet the requirements of the building. Hopefully, the specifier has actually investigated the building and is aware of the as-built conditions that may not be covered in manufacturers’ literature or generic details. Generic details may not adequately illustrate actual field conditions. Furthermore, simply copying a detail from a book does not constitute true design.
In conclusion, the design standards are important for proper roof application. It is the responsibility of the specifier to determine the exact standards that are required for each building and to properly define the requirements of the standards so that the contractors can comply during installation. Manufacturers’ literature, NRCA details and all other industry organizations can serve as guidelines for design, and the specifier should be aware of the requirements established by these organizations. However, the design should be completed to best meet the requirements of the building - even if they are more stringent than industry requirements.