1.1 General
1.2 Hail
1.3 Wind
1.4 Fire
1.5 Cold Climate

Code requirements change periodically as building codes are updated. Within the U.S., most jurisdictions that adopt building codes have moved toward some edition of the International Code Council’s (ICC) International Building Code (IBC) and the International Residential Code (IRC). Some jurisdictions also use the International Energy Conservation Code (IECC), the International Existing Building Code (IEBC), and the International Green Construction Code (IgCC). The legacy codes ¹ may have been used for the design and construction of many older existing buildings prior to their merging and formation of the ICC in 2000. State and local jurisdictions frequently make amendments to the model building codes, and as the authority having jurisdiction, these will represent the required code for a particular area. Most commonly, these modifications are made to the IRC, IECC, and the IgCC, and sometimes to the IBC. Local building officials also have discretion in determining product approvals and their use, and in authorizing variances.Note: Ask to see ‘approved’ variances in writing, as some parties will claim they are allowed but do not have tangible proof. Consequently, it is always wise to check local requirements and modifications to the model codes with the authority having jurisdiction if observed conditions or practices differ from manufacturer or industry recommendations.

Building codes also require that many products and materials comply with applicable test standards, which are referenced in building codes. Standards are developed by independent laboratories and industry groups. Like building codes, standards are also updated on a regular basis. Compliance with test standards is verified by independent testing laboratories, and confirmation of compliance is typically noted on package labeling or direct product labeling.

1.1 General

Minimum requirements for roof assemblies are found in Chapter 15 of the IBC, Chapter 9 of the IRC, and Chapter 7 of the IEBC. Fire-resistance classification requirements are based on the type of construction and the building use group. Most other roof system requirements in the building codes (i.e. IRC or IBC) focus on weather protection, structural performance, roofing materials, and installation.

1.1.1 Type of Construction

The building code designates the type of construction based on size and occupancy (i.e., assembly, residential multi-family, business) and establishes minimum fire-resistance classification of a roof assembly. Certain buildings are required to have a minimum level of fire resistance based on the occupancy and use of the building.

1.1.2 Determining the Current Code Adopted

It is important to know the building code that is adopted in the jurisdiction having authority. While not all jurisdictions require a permit for roofing work, repairs or replacements will need to comply with the applicable code in effect at the time of the repairs or replacement. Similarly, the building code that was in place at the time of the original construction or when the roof cover was last replaced should be known in order to verify compliance of the roof cover. Users should contact the authority having jurisdiction if there are questions. This would commonly be the “building department” or “building inspection department” of the village, town, city or county in which the building is located. Qualified building officials and building envelope specialists are both useful contacts to help determine local area requirements and if new-to-market products are acceptable.

1.1.3 Dead Load/Live Load

The addition of an extra layer of roof cover or the change of a light weight roof cover to a heavier one will result in an increase in the dead load on the roof structure. The roof structure should be assessed and verified to determine if it will be able to support this additional weight plus the construction and live loads (e.g., people on the roof, along with the material and equipment loads that will be encountered during installation of the roof covering system) associated with the roofing project.

1.1.4 Recover (Overlay) vs. Replace

A roof recover or overlay is the process of installing a new roof covering over an existing roof covering. A roof replacement is the process where all existing layers of roof covering are completely removed down to the roof deck before a new roof covering is installed. During roof repairs or replacement, the condition of the roof deck and framing must be documented. The new roof covering must not be installed on damaged structural roof components. Similarly, if a roof is recovered, the existing roof covering layer must be in good condition to support the new covering. Contact an engineer or design professional for more information.
Building codes may provide specific instructions and limitations for conditions when recovering is permitted and when replacement is required.
Note: Not all jurisdictions will allow the installation of a new roof covering over an existing roof covering. Building codes may require an evaluation of the decking and roof-to-wall connections when a substantial portion of the roofing materials are removed from the roof diaphragm of a building located in a high or special wind region.

1.2 Hail

There are no specific requirements for hail resistance of roofing products in model building codes. In moderate hail zones, recover of an existing roof is often not permitted (please refer to the jurisdiction having authority). Many manufacturers have some of their roofing products and assemblies tested using standards such as UL 2218 and FM 4473, but many products are unrated for impact resistance as it pertains to hail performance.

1.3 Wind

Roof coverings and assemblies must be capable of resisting the required wind loads for the location in which the building is located. Design wind speeds are much higher along coastal areas of the hurricane-prone regions, but are much lower over the majority of the U.S. Compliance with wind loading requirements in the building code will require a roof system to be tested and designed to meet the applicable wind speeds and the associated uplift pressures. Design wind loads are addressed in Chapter 16 of the IBC and Chapter 3 of the IRC. Wind design requirements for specific types of roof covering are addressed in Chapter 15 of the IBC and Chapter 9 of the IRC.

1.3.1 Design Wind Speed

Design wind speeds are provided by the adopted building code, and are assigned based on the location and risk category of the building. Risk category is a categorization of buildings/structures based on the occupancy and use of the building and risk associated with protection of the occupants. Design wind speeds for particular buildings are determined from maps included in the locally adopted and enforced building codes. The design wind speed is used directly in the selection of roofing products and their installation specifications.

For More Information:
Maps used to establish design wind speeds and the definitions of wind speeds used in the maps have changed over the years. Early maps used what is called the fastest-mile wind speed as the reference standard design wind speed. Beginning with the 1995 edition of ASCE 7, the design wind speed basis was changed to a 3-second gust wind speed, which is different than the 1-minute average speed used in the Saffir-Simpson Scale for hurricanes. An existing roof should be evaluated based on design wind speeds and uplift loads that were in place at the time of permitting, and it is important to recognize the difference in the definition of the reference speeds.

Generally, design wind speed maps from ASCE 7 are adopted into the model building codes during the next update cycle. Once the model building codes have been updated, the updates will begin to make their way into locally adopted codes and standards.

1.3.2 Wind Exposure Category

The wind exposure category of a building or structure is determined by the terrain and type, size and concentration of surrounding vegetation, buildings and structures immediately windward of the building/structure under consideration. Buildings located in areas with closely spaced surface obstacles, such as trees and buildings that slow down the wind near the surface of the earth² typically experience wind forces of lesser magnitude than those with fewer terrain irregularities³. The exposure category may vary for a certain building depending on the terrain in a given upwind direction. For example, a building located in a coastal, urban environment will have one exposure for wind coming from the ocean side and another for wind coming from the city side. In these instances, the exposure category resulting in higher wind forces will generally be selected for design.

Variations in wind loads with wind exposure category are incorporated in ASCE 7 as part of the velocity pressure exposure coefficients. The IRC uses Exposure B as the default wind exposure category, while the IBC uses Exposure C as the default wind exposure category. The default category applies if the site does not fit other exposure definitions. The ASCE 7 Commentary notes that the majority of buildings (perhaps as much as 60%–80%) have an exposure category corresponding to Exposure B.

1.3.3 Roof Design Wind Loads

Roof design wind loads are the wind pressures and resulting uplift forces for which the roof structure is designed. This includes structural components such as roof sheathing or rafters, or roof cover components and the connections of these elements. ASCE 7 wind loads are specified for the Main Wind Force Resisting System (MWFRS) and for Component and Cladding (C&C). C&C loads are used to design roof sheathing elements and their attachment to the roof framing as well as secondary members such as purlins, rafters, and truss top-chord members between panel points. These loads are calculated based on the criteria from ASCE 7. Equations for wind loads on roof systems are based on several variables, including:

• Basic wind speed
• Exposure and height, K_z
• The importance factor of the building, I_w (pre-2010 editions of ASCE 7)
• Topographic factors, K_zt
• Wind directionality, K_d
• Air density factor, K
• Design pressure coefficients, GC_p, which vary based on characteristics of the building (e.g., roof geometry, roof slope, and whether the area is a roof overhang)
• Internal pressure, GC_pi, which is based on whether a building is enclosed, partially enclosed, or open.

ASCE 7 provides the procedures needed to calculate design wind pressures for the components and systems outlined above. It also contains tables listing design pressures for buildings up to 160 feet in height based on some simplifying conservative assumptions. The IRC contains tables of design pressures for buildings with heights below 60 feet. Both sets of tables include roof design pressures suitable for design of the elements identified above. The ASCE 7 calculation procedures and the tables of design pressures are not reproduced here; please refer directly to ASCE 7 to obtain that information.

Research has demonstrated reductions in wind loads for roof covers, such as asphalt shingles and roofing tiles, where the capacity for pressure equalization has been demonstrated. Consequently, building codes allow different approaches for determining design wind loads for these systems. These design approaches are not included in this document but can be reviewed in the IBC and in other industry literature.

1.3.4 Roof Height

Wind speeds vary with height above ground and typically increase with increasing elevation. Effects of the variation of wind speed with height on roof design pressures are included in the velocity pressure exposure coefficient, K_z. The equation and parameters used to calculate K_z are based on measurements of winds at various elevations in extra-tropical frontal system winds. The resulting variations are also reasonable for hurricane winds, but are likely non-conservative for thunderstorm outflow winds and tornadoes. Roof design wind pressures are determined using the wind speeds at the mean roof height of the building (i.e., average of the eave and ridge heights). For buildings with low-slope roofs (less than 2:12 slope), the wind speed at eave height is used.

1.3.5 Design Pressure Coefficients for Roof Zones

Wind produces different magnitudes of pressure acting on the surfaces of a building due to the building’s geometry, and can create small areas of higher pressure on a surface while at the same time other areas on the same surface experience lower pressures. Because geometric variations can be quite large, Component and Cladding (C&C) design pressure coefficients decrease as the area over which the pressure acts increases. Building codes generally divide roofs into three zones: perimeter, corner, and field (i.e., areas away from discontinuities). Zoning can also vary based on roof slope and geometry (e.g., low-slope versus steep-slope or gable roof versus hip roof). For low-slope roofs, wind pressures typically have the greatest magnitude at the roof perimeter and corners. Steep-slope roofs have the greatest pressures in the perimeter and corners, as well as the roof peak and eaves. Other geometric discontinuities may have increased pressures as well, especially those at greater height on the structure. These pressures can be positive or negative depending on the direction of the wind and the geometry of the building. This is reflected in building code provisions by dividing the roof into zones and specifying different design pressure coefficients, GC_p, for the various zones.

For More Information:
C&C design pressure coefficients in building codes have increased substantially over the years as wind tunnel modeling technologies have improved and databases of pressure coefficients for different shapes and roof slopes have been expanded. C&C design pressure coefficients and the associated roof zones in ASCE 7-16 reflect results of a major reanalysis of databases produced from wind tunnel results.

1.3.6 Variances in Local Requirements

Today, most jurisdictions that have adopted building and/or residential codes use some edition of the IBC and/or IRC. However, older buildings have been built to a variety of building codes or in some cases to no building code. Consequently, it is important to know which code (if any) was adopted and enforced when the building was permitted.

Some local building codes establish a threshold that requires the roof structure to be evaluated for conformance with the latest building code provisions when more than a certain percentage of the roof is being replaced or if remodeling exceeds a certain threshold. Main Wind-Force Resisting System (MWFRS) loads have changed less over the years than C&C loads. Therefore, it is more likely that the roof-to-wall connections, roof sheathing attachment and roof covers are under-designed, than is the roof structure when compared with current design loads and construction practices. This also means that when re-roofing there are opportunities to improve the roof’s wind performance by following best practices for roof-to-wall and sheathing attachment.

For More Information:
As an example, most building codes being used before Hurricane Andrew allowed roof sheathing to be attached using 6d nails spaced 6 inches along panel edges and 12 inches along intermediate members. Testing of panel uplift resistance following Hurricane Andrew led to significant changes in prescriptive roof sheathing nailing requirements throughout much of the U.S. As a result, most nailing patterns in high-wind areas require 8d nails at a maximum of 6-inch spacing along all framing members. In the highest wind areas, the fastener spacing within 4 feet of gable ends is reduced to a 4-inch maximum in some standards and/or the fastener size is increased to 10d.

1.4 Fire

Roofing assemblies and roof coverings are rated as Class A, B, and C or may be non-classified, all of which is based on their effectiveness against fire exposures. Class A, B, and C assemblies and coverings are those that are effective against severe, moderate, and light fire text exposures, respectively. When required, testing is in accordance with ASTM E108 or UL 790. Minimum roof fire-resistance ratings apply based on the type of construction, and some building types do not require a fire rating.

1.4.1 IBC

The IBC Table 1505.1 lists the minimum requirements for roof covering fire-resistance classifications A, B, or C for all types of construction. The IBC allows non-classified roof coverings to be used on Residential Group R-3 (see IBC Section 310.5) and Group U (see IBC Section 312) buildings such as agricultural buildings, barns, carports, if the separation between the leading edges of the adjacent roofs is less than 6 feet unless otherwise required by local requirements, such as the International Wildland Urban Interface Code.

1.4.2 IRC

The IRC Section R902.1 requires Class A, B or C fire-resistant roofing only in jurisdictions designated by law, or where the edge of the roof is less than 3-feet from the property line. Jurisdictions in the US do not commonly require fire-resistance classifications for roof coverings on one- and two- family residential dwellings. Thus this should be verified with the local jurisdiction for the building of interest.

1.4.3 Insulation

The use of thermal insulation in roofs or roof assemblies must be evaluated in conjunction with the type of roof system. Installers should follow code requirements, energy standards, and manufacturer’s guidelines for the use of insulation in an approved roofing assembly, as they may differ across industries and product types.

1.4.4 Roof Decking and Fire-Resistance-Rated Walls

The IBC and IRC have specific requirements for roof construction where certain types of fire-resistance-rated walls that terminate at the underside of the roof decking. For buildings constructed in accordance with the IBC, the requirements are fairly complex and depend on the type of construction, use group, and rating of the wall, as well as fire separation distance between buildings and the construction of the roof assembly. See Section 705.5 (Fire-resistance ratings) and 705.6 (Structural stability) of the IBC for requirements of structural stability and parapet construction when roof decking needs to be replaced near fire-resistance-rated exterior walls.

Under the IRC, to maintain continuity of townhouse fire separation walls are generally required to extend above the roof deck except in certain conditions. See Section R302.2.2 of the IRC when roof decking needs to be replaced on townhouses.

See Section 603.1 of the 2015 IEBC, which requires that repairs maintain the level of existing fire protection.

1.4.5 Wildland-Urban Interface Requirements

Wildland-Urban Interface (WUI) requirements need to be verified. For example, the State of California has requirements for certain types of roof coverings, vents, and/or openings in areas of the state prone to wildfires that fall under the WUI restrictions. The International WUI Code is often adopted in these regions.

1.5 Cold Climate

Cold climates create unique conditions for roof design and prevention of ice and snow-related damages. In areas where there has been a history of ice forming along the eaves causing a backup of water, an ice barrier must be installed in accordance with Chapter 15 of the 2015 IBC or Chapter 9 of the 2015 IRC. Proper ventilation is also important for performance in cold climates, and the ability to shed or retain snow must also be considered. Contact the authority having jurisdiction for information regarding cold climate requirements for a particular location. Manufacturers may also provide general recommendations for product uses in cold climates. See sections 1.5.2 and 1.5.4 for more details.

1.5.1 Definition

For the purposes of roofing, cold climates are those in which ice dams form and snow retention may be needed. In a cold climate, snow may remain on roofs and ice dams may occur where temperatures stay below 32°F (0°C) for several days and nights.

Local building departments identify areas where there has been a history of ice forming along the eaves, and should be consulted for additional cold climate requirements.

1.5.2 Ice Damming Recommendations

For some roof assemblies in cold climates, ice may form along eaves, which may impede the drainage of meltwater from higher up on the roof surface. In such situations, an ice barrier is required to prevent dammed meltwater from penetrating through the roof assembly and into the interior spaces. When required for steep slope roofs, Section 1507 in the IBC and Section R905.1.2 of the IRC specify the use of an ice barrier that consists of either two layers of underlayment cemented together, or a self-adhering polymer modified bitumen sheet at eaves and valleys. These assemblies have temperature ratings that vary for different roof coverings. The ice barrier should extend from the lowest edges of all roof surfaces and extend at least 24 inches inward from the exterior wall line of the building. Many manufacturers limit the use of the Ice barrier to 24 inches maximum from the exterior wall line, or require special ventilation if the ice barrier extends inward for more than 24 inches. Consult the manufacturer’s specific instructions for details.

1.5.3 Ventilation Considerations

Special attention should be given as to how attics are ventilated, as these areas are subject to water vapor migration and the development of excessive temperatures which may affect the performance and durability of the roof coverings. The types, sizes, and locations of attic vents are important parts of moisture/temperature management of the building envelope. Intake and exhaust vents are the preferred method of attic ventilation. In older buildings, attic ventilation was mandatory; however in more recent editions of the code, unvented (i.e. conditioned) attic spaces are permitted. For vented and unvented (i.e. conditioned) attic spaces, the 2015 IRC Section R806 and IBC Section 1203 have specific requirements. Building code plus a building envelope specialist should be consulted in the design and specification of attic ventilation. Please refer to the ventilation manufacturer for product installation and limitations.

1.5.4 Snow Loads and Retention or Guards

Snow retention systems are intended to prevent snow from sliding off the roof, which may harm roofs, buildings, equipment or persons below. An engineered snow retention system should hold snow or ice on steep slope or slippery roofs. Such systems can be composed of special barriers, fences, cleats, guards, cables, beams or other approved devices. Refer to the jurisdiction having authority to determine if snow retention systems are required. They may also be included by owner’s choice.

Proper installation of snow retention systems is critical to achieve the intended performance. The manufacturer of the products should be consulted for the proper number, location and fastening requirements to resist the loads associated with the anticipated snow accumulations. The roof system and the building must be designed to resist loads from retained snow.

Building codes, such as the IBC, reference ASCE 7, Minimum Design Loads for Buildings and Other Structures, for the determination of design snow loads on buildings. The IRC does not require any engineering design revisions for most residential structures, unless the ground snow load exceeds 70 psf. The roof snow load is defined as the weight of snow on the roof surface. Calculated values in ASCE 7 are based on factors such as: ground snow load value; importance factor based on use or occupancy; wind exposure; roof slope and shape; roof obstructions; thermal factor related to expected roof temperature; and snow drifting.

For More Information:
See the FEMA Snow Load Safety Guide.

The ATC snow load website provides users with location-specific ground snow loads: snowload.atcouncil.org. (Note that an understanding of how to calculate and apply design snow loads is required to use this site.)

1.5.5 Variances in Local Requirements

Local jurisdictions may have additional requirements or design criteria for roofing assemblies in cold climates. Verify any local requirements with the authority having jurisdiction.

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