CHAPTER
15
ROOF ASSEMBLIES AND ROOFTOP STRUCTURES
Section 1504 Performance Requirements
Section 1505 Fire Classification
Section 1510 Rooftop Structures
In addition to the requirements for roof assemblies and roof coverings, this chapter regulates roof insulation and rooftop structures. Rooftop structures include such elements as penthouses, tanks, cooling towers, spires, towers, domes, and cupolas. Roofing materials and components are regulated for quality as well as installation.
The provisions in Chapter 15 for roof construction and roof covering are intended to provide a weather-protective barrier at the roof and, in most circumstances, to provide a fire-retardant barrier to prevent flaming combustible materials such as flying brands from nearby fires from penetrating the roof construction. The chapter is essentially prescriptive in nature and is based on decades of experience with the various traditional roof-covering materials. These prescriptive rules are very important to ensuring the satisfactory performance of the roof covering, even though the reason for a particular requirement may be lost. The provisions are based on an attempt to prevent observed past unsatisfactory performances of the various roofing materials and components.
Those measures that have been shown by experience to prevent past unsatisfactory performance generally are included in the manufacturer’s instructions for application of the various roofing materials. In many cases, the manufacturer’s instructions are incorporated in this code by reference. The code intends, then, that they be followed as if they were part of the code.
The overriding safety need of roofs is resistance to external fire factors. In this regard, the enforcement of this chapter is driven by Table 1505.1, as well as the appropriate standards for fire-retardant roof assemblies and roof coverings, including ASTM E 108, UL 790, and ASTM D 2898. Typically, a roof covering by itself cannot be a listed fire-retardant roof. Therefore, the regulations clearly separate assemblies and coverings to enforce construction of listed roof assemblies to the level at which listed wall and floor/ceiling assemblies are regulated.
Section 1502 Definitions
As with other industries supplying specialty building products, the roof-covering industry has a language of its own. In order to properly understand and apply the provisions of the International Building Code® (IBC®), the unique terms employed in the IBC must be understood. The roofing industry publishes several publications containing excellent glossaries of the terms of their industry. This section identifies those terms specifically defined in Chapter 2 that are related to roofs, roof assemblies, roof coverings, and roof structures.
Section 1504 Performance Requirements
Roof decks and roof coverings must be able to withstand the effects of nature in a satisfactory manner. This section of the code regulates the performance of a roof against three concerns: wind, weathering, and impact. For wind resistance, roofs must comply with this section and Chapter 16. Low-slope roofs must demonstrate that they are resistant to both weathering and impact damage by complying with the appropriate standards.
The use of aggregate as a roof-covering material and aggregate, gravel, or stone as ballast is prohibited in specified locations in an effort to reduce property loss that is due to high winds. Field assessments of damage to buildings caused by high-wind events have shown that gravel or stone blown from the roofs of buildings has exacerbated damage to other buildings because of breakage of glass. The code prohibits the use of aggregate, gravel, or stone on roofs of buildings in hurricane-prone regions. These regions are defined as areas along the United States Atlantic Ocean and Gulf of Mexico coasts where the basic wind speed exceeds 90 miles per hour (40 m/s), and also include the islands of Hawaii, Puerto Rico, Guam, Virgin Islands, and American Samoa. Aggregate, gravel, and stone roof-covering materials are also prohibited on those buildings where the mean height exceeds that allowed by Table 1504.8 on the basis of the nominal design wind speed and exposure category. Under these conditions, there is a great enough potential for gravel stone, debris, or other unsecured objects to become airborne and possibly break glass in buildings downwind.
Section 1505 Fire Classification
As a minimum, the IBC generally requires Class B or C roof coverings for most buildings. These are roof coverings that provide protection of the roof against moderate and light fire exposures, respectively. The various sizes of brands used for testing are shown in Figure 1505-1. These exposures are external and are generally created by fires in adjoining structures, wild fires (brush fires and forest fires, for example), and fire from the subject building that extends up the exterior and onto the top surface of the roof. Wild fires and some structural fires create flying and flaming brands that can ignite nonclassified roof coverings. With regard to clay tile roofing, which is defined in Section 1505.2 as a Class A roof assembly, it is of interest to note that the Spanish missionaries shipped clay roofing tile to North America to protect their mission buildings from fire caused by flaming arrows shot onto the roofs.
Figure 1505-1 Brands for Class A, B, and C tests.
The roof assembly classifications required by the code, which are related primarily to type of construction, are delineated in Table 1505.1.
Section 1505 defines the following roof assemblies and roof coverings:
1. Class A roof assemblies. Roof assemblies recognized as Class A are effective against severe fire exposures. A variety of prescriptive materials and assemblies are considered as Class A roof assemblies, as well as any roof assembly or roof covering tested and listed as Class A. Class A roof assemblies are permitted for use on all buildings, regardless of the building’s construction type.
2. Class B roof assemblies. Class B roof assemblies are effective against moderate fire exposures and are considered appropriate for all types of construction. There are no prescriptive roof assemblies or roof coverings considered as Class B, as they must be listed as such. Consistent with the universal acceptance of Class A assemblies, Class B roof assemblies are also permitted for use on all buildings.
3. Class C roof assemblies. Buildings not required to be of fire-resistant construction—Types IIB, IIIB, and VB—are permitted to use Class C roof assemblies. Such assemblies are effective against light fire exposures.
4. Fire-retardant-treated wood shingles and shakes. Where fire-retardant-treated wood shakes and shingles are components of Class A, B, or C roof assemblies, they must comply with the criteria of Section 1505.6. AWPA C1, All Timber Products-Preservative Treatment by Pressure Processes, is referenced as the standard for regulating the pressure treatment for fire-retardant purposes. In addition to the required markings that identify the shakes or shingles and their manufacturer, a label is mandated to identify the appropriate Class A, B, or C classification.
5. Nonclassified roofing. Roof coverings that are considered nonclassified roof coverings are approved for use by the IBC on Group R-3 and U buildings where the roof is located at least 6 feet (1,829 mm) from all lot lines. These roof coverings have been shown by experience to provide the necessary resistance to weather as intended by the code when the qualities of the materials comply with the appropriate requirements.
6. Special-purpose roofs. These roofs are either of wood shingles or wood shakes and are applied with a minimum ⅝ inch (15.9 mm) Type X water-resistant gypsum backing board or gypsum sheathing panel. The intent of the provisions for special-purpose roofs is to provide a roof covering that, although it may be ignited by flying brands, will not burn through to the interior of the building. Also, the special underlayment tends to prevent fires from the interior of the building from burning through to and igniting the roof covering, which helps prevent flying brands. Special-purpose roofs are permitted in limited applications on buildings of Type IIB, IIIB, and VB construction by Footnote c to Table 1505.1.
Because of the inconsistent use of the terms roof assembly and roof covering throughout Chapter 15, there is confusion as to the proper use of combustible materials at the roof. Based on Item 4 of Section 603.1, roof coverings that have an A, B, or C classification are permitted in buildings of Type I or II construction. This does not include the structural deck materials, which must be of noncombustible construction or fire-retardant-treated wood in compliance with Item 1.3 of Section 603.1. It would, however, permit the use of wood structural panels or foam plastic insulation boards as a part of the classified roof covering where used in combination with a noncombustible roof deck. Where a Class A, B, or C roofing assembly includes a combustible structural deck, other than fire-retardant-treated wood where permitted, it is limited to use on a building of Type III, IV, or V construction.
Section 1506 Materials
Certainly, roofing materials must comply with quality standards embodied in the IBC for Chapter 15. Furthermore, identification of the roofing materials is mandatory in order to verify that they comply with quality standards. In addition to bearing the manufacturer’s label or identifying mark on the materials, roof-covering materials are required by the code to carry a label of an approved agency having a service for inspection of materials and finished products during manufacture.
Section 1510 Rooftop Structures
Penthouses and other roof structures are regulated by the IBC as if they were appurtenances to the building rather than occupiable portions.
In fact, if a penthouse is used for any purpose other than shelter of mechanical equipment or shelter of vertical shaft openings, the code requires that it be considered an additional story of the building.
As intended by the IBC, roof structures are equipment shelters, equipment screens, platforms that support mechanical equipment, water-tank enclosures, and other similar structures generally used to screen, support, or shelter equipment on the roof of the building. This section also regulates towers and spires, which are addressed separately in Section 1510.5.
The IBC regulates penthouses, roof structures, tanks, towers, and spires to prevent hazardous conditions that are due to internal and external fire concerns or structural inadequacy, and to ensure their proper use as equipment shelters.
1510.2 Penthouses. The code does not regulate the height of penthouses and roof structures on Type I buildings. However, for buildings of other construction types, the code limits the height of penthouses and roof structures to 18 feet (5,486 mm) above the height of the roof, except where the penthouse is used to enclose a tank or elevator. In such cases, a maximum height of 28 feet (8,534 mm) is permitted. As this section also limits the aggregate area of all penthouses and roof structures to one-third the area of the roof, the additional height permitted for penthouses and roof structures does not pose any significant fire- and life-safety hazard that is due to other restrictions that this section places on construction. See Figure 1510-1.
Figure 1510-1 Penthouse limitations.
Penthouses and other rooftop structures are also regulated by the exception to Section 504.3, where the requirements are based on the effect of a rooftop structure on the allowable height permitted for the entire building. Where applicable, the provisions from both Chapters 5 and 15 are in effect, and where there is a conflict, the most restrictive condition will apply.
As the code has reduced requirements for construction of penthouses and roof structures, it is logical that their use should be limited as specified in this section. Thus, if other uses are made of penthouses or other roof structures, it also seems appropriate that they should be constructed as would be required for an additional story of the building. It is the intent of the code that a penthouse or roof structure complying with this section not be considered to create an additional story above that permitted by Section 504.
If a rooftop structure qualifies as a penthouse, it is intended that the floor of the penthouse is only required to meet the roof provisions of Table 601. As an extension of this recognition, any fire-resistance-rated shaft that extends to or through the penthouse floor does not need to be protected at the floor line. Under both conditions, the floor of the penthouse is solely regulated as the building’s roof. Regarding any required means of egress from the penthouse, the provisions for an occupied floor are not applicable, nor are the requirements of Section 1006.3 for an occupied roof. It would seem appropriate that the access provisions of the International Mechanical Code® (IMC®) for rooftop equipment also provide for adequate egress.
1510.2.5 Type of construction. The intent of the code is that penthouses be constructed with the same materials and the same fire resistance as required for the main portion of the building. However, because of the nature of their use, the code does permit exceptions for exterior walls, roofs, and interior walls. Where the exterior walls of penthouses are at least 5 feet (1,524 mm) [or in some cases 20 feet (6,096 mm)] from the lot line, reductions in any required fire resistance are typically permitted. See Figure 1510-2.
Figure 1510-2 Penthouse construction.
1510.5 Towers, spires, domes, and cupolas. The IBC intends that towers, spires, domes, and cupolas be considered separately from penthouses and other roof structures. The towers contemplated in this section are towers such as radio and television antenna towers, church spires, and other roof elements of similar nature that do not support or enclose any mechanical equipment and that are not occupied. As with penthouses and other roof structures, the code intends to obtain construction and fire resistance consistent with that of the building to which they are attached. Under a variety of conditions, however, towers and similar elements are required to be constructed of noncombustible materials, regardless of the building construction.
• Roofs and roof coverings are addressed for both weather protection and fire retardancy.
• The performance of a roof for weather protection is regulated against three concerns—wind, weathering, and impact.
• The IBC generally requires roof coverings that provide protection for the roof against moderate or light fire exposures.
• Roof assemblies and roof coverings are classified as either Class A, B, or C roofing assemblies; nonclassified roofing; fire-retardant-treated wood shingles and shakes; or special-purpose roofs.
• Penthouses and other roof structures are regulated by the code as if they were appurtenances to the building rather than occupiable portions.