GYPSUM BOARD, GYPSUM PANEL PRODUCTS, AND PLASTER

Section 2501 Scope

Section 2502 Definitions

Section 2508 Gypsum Construction

Section 2510 Lathing and Furring for Cement Plaster (Stucco)

Section 2511 Interior Plaster

Section 2512 Exterior Plaster

Key Points

Section 2501 Scope

This chapter of the International Building Code® (IBC®) covers the installation requirements for wall- and ceiling-covering materials, including their method of fastening and, in the case of plaster, the permitted materials for lath, plaster, and aggregate.

Although plaster has many uses, including ornamental and decorative work, its use in the IBC is regulated purely as a wall- and ceiling-covering material.

The IBC regulates the installation of wall- and ceiling-covering materials as well as quality standards for the materials themselves. The primary wall- and ceiling-covering material in use today is gypsum wallboard; however, other gypsum panel products, as well as lath, plaster, and wood paneling, are sometimes utilized. As wood paneling is covered in Chapter 23, it follows that Chapter 25 only regulates gypsum wallboard, gypsum panel products, lath, and plaster. However, in this section the code permits the installation of other wall- and ceiling-covering materials, provided the materials have been approved. On this basis, the manufacturer’s recommendations and conditions of approval should be consulted.

Gypsum wallboard is a relatively new material for covering walls and ceilings. On the other hand, plaster is among the oldest of building materials still in use. The use of gypsum plaster dates back to about 4000 B.C., when the Egyptians applied it to the interior and exterior of the pyramids.

Section 2502 Definitions

Definitions specific to the provisions addressing gypsum board, gypsum panel products, and plaster are listed in this section and defined in Chapter 2. In short, the determination of whether or not a surface is considered interior or exterior is based on how it is viewed in relationship to the definition for weather-exposed surfaces. Any surface that can be considered weather-exposed under the definition in Section 202 is considered an exterior surface. Surfaces other than weather-exposed surfaces are viewed as interior surfaces.

Surfaces of walls, ceilings, floors, roofs, soffits, and similar elements, where exposed to the weather, are typically considered weather-exposed surfaces. There are three exceptions to the general criteria that would define such surfaces as interior where applying the code provisions. Those exterior conditions considered other than weather-exposed surfaces are illustrated in Figure 2502-1.

Figure 2502-1   Weather-exposed surfaces.

Section 2508 Gypsum Construction

This section addresses the installation of gypsum materials, primarily that of gypsum wallboard installed as wall and ceiling membranes. As gypsum board is a construction material utilized in almost every construction project, it is important that the application is in compliance with the IBC and the appropriate referenced standards.

2508.1   General.   The primary installation criteria for various gypsum materials are found in the referenced standards identified in Table 2508.1. The application of gypsum board varies based on gypsum board thickness, wall or ceiling installation, orientation of gypsum board to the framing members, and maximum spacing of the framing members. Based on the specific conditions encountered, the maximum fastener spacing and size of fasteners are identified.

2508.2   Limitations.   Because gypsum plaster and gypsum board are subject to deterioration from moisture, the code restricts their use to interior locations and weather-protected surfaces. The definition for weather-exposed surfaces is found in Section 202 and illustrated in Figure 2502-1. Even where installed in interior locations, it is important that gypsum wallboard not be installed in areas of continuous high humidity or wet locations. The IBC further requires that interior gypsum board, gypsum plaster, and gypsum lath shall not be installed until the installation has been weather protected.

2508.3   Single-ply application.   The application of gypsum wallboard is specified in this chapter for locations where fire-resistance-rated construction is not provided or for construction where diaphragm (shear wall) action is not required. Chapter 7 and fire-test reports will establish the means of fastening and supporting the ends and edges of gypsum wallboard for fire-resistance-rated assemblies. Table 2508.1 provides the installation standards for various types of gypsum construction.

The code requires that the fit of gypsum wallboard sheets be such that the edges and ends are in moderate contact. However, wider gaps are permitted in concealed spaces where fire-resistance-rated construction or diaphragm action is not required. This requirement is based primarily on appearance. Therefore, where the wallboard application is concealed, it is not objectionable to have wider gaps than those resulting from moderate contact. However, where the wallboard surface is exposed as it normally is, moderate contact is required so that there will be no objectionable cracking when the joint between the sheets is finished.

Unless the wallboard is considered a shear-resisting element or an element of a fire-resistance-rated assembly, fasteners may be omitted at certain locations. It should be emphasized that where a fire-test report or other installation standard indicates that fasteners are required on supports or edges, the fastening pattern may not be modified. Otherwise, those fasteners located at the top and bottom plates of vertical assemblies are permitted to be omitted. In addition, fasteners need not be provided at the edges and ends of horizontal assemblies perpendicular to supports and at the wall line. Note that fasteners are to be applied in a manner in which the face paper is not fractured by the fastener head. The intent of the requirement is to provide a tight fastening but not damage the gypsum board to the extent its nail-holding power may be affected. Proper construction procedure for the nailing of gypsum wallboard panels is to use a drywall hammer that has a crowned head and use wallboard nails that have concave heads. The intent is to create a dimple in the wallboard with no projection of the nail head above the wallboard.

2508.4   Joint treatment.   Although as a general rule the IBC requires joint and fastener treatment for fire-resistance-rated assemblies, the code exempts in Exception 1 those locations where the wallboard is to receive a decorative finish or any other similar application, which is considered to be equivalent to the joint treatment. Also, joint treatment is not required where joints occur over wood framing members, or where square-edge or tongue-and-groove edge gypsum board is used. In addition, Exception 5 indicates that joint treatment is not required for assemblies tested without joint treatment. In general, joint treatment does not materially increase the fire rating, and many partitions have passed the fire test without joint treatment. As indicated earlier in this section, joint treatment is primarily used for aesthetic reasons. One further exception addresses the condition where a multilayer system is constructed. Where two or more layers of gypsum board are utilized in the assembly, joint and fastener treatment is not required where the joints of adjacent layers are offset from each other.

Section 2510 Lathing and Furring for Cement Plaster (Stucco)

Cement plaster (stucco) used in both exterior and interior locations must be installed in accordance with this section. In order to ensure reliability and consistency, the materials of construction must comply with the appropriate standards listed in Table 2507.2. Of particular importance is the installation method of the water-resistive barrier that is required over wood-based sheathing where exterior plaster is applied. The installation must have a performance level “at least equivalent to two layers” of a water-resistive barrier that complies with ASTM E 2556. ASTM E 2556 includes house wrap materials, building papers, and felt, and is representative of the state of the industry. Within ASTM E 2556, Grade D paper is a Type I WRB and 60 minute Grade D paper is a Type II WRB.

The greatest benefits of using two layers of water-resistive barrier (WRB) can only be realized if the method and manner of the installation establish a continuous drainage plane, separated from the stucco. In a two-layer system, each layer provides a separate and distinct function. The primary function of the inboard layer is to resist water penetration into the building cavity. This interior layer should be integrated with window and door flashings, the weep screed at the bottom of the wall, and any through-wall flashings or expansion joints. This inner layer becomes the drainage plane for any incidental water that gets through the outer layer or at one of the joints or openings or where the outer layer is damaged. The primary function of the outboard layer (layer that comes in contact with the stucco) is to separate the stucco from the water-resistive barrier. This layer has historically been called a sacrificial layer, intervening layer, or bond break layer. See Figure 2510-1.

Figure 2510-1   Installation of water-resistive barrier.

Section 2511 Interior Plaster

Multicoat plastering has been the standard in the western world for over 100 years. It is generally the consensus of the industry that, particularly where plaster application is by hand, multicoat work is necessary for control of plaster thickness and density. Most of the materials used for plaster densify under hand application because of the pressure applied to the trowels, and it is believed that this change in density is more controllable and will be of a more uniform nature where the plaster is applied in thin, successive layers. For these reasons, the IBC requires three-coat plastering over metal or wire lath and two-coat work applied over other plaster bases approved for use by the code. Reducing the requirement for plaster bases other than metal or wire lath is based on the rigidity of the plaster base itself. More rigid plaster bases are not as susceptible to variations in thickness and flatness of the surface. In fact, it may be considered that the first coat applied in three-coat work on a flexible base, such as wire lath, is used to stiffen that base to provide the rigidity necessary to attain uniform thickness and surface flatness.

Fiber insulation board does not have the qualities necessary for a good performing plaster base. It absorbs excessive moisture from the plaster mix, creating problems of workmanship, and it does not have the stability and rigidity required for a proper functioning plaster base. Also, in the colder and damper climates, the fiberboard insulation retains the moisture absorbed from the plaster for a relatively longer period of time than other bases, causing premature failure of the plaster. For these reasons, the code prohibits its use as a plaster base.

Because portland cement plaster does not bond properly to gypsum plaster bases, the code prohibits its use over gypsum plaster bases. However, the code permits exterior plaster to be applied on horizontal surfaces, soffits, and so on, over gypsum lath and gypsum board when used as a backing for metal lath.

Plaster grounds are utilized to establish the thickness of plaster and usually are wood or metal strips attached to the plaster base. The intent is that plaster grounds are used as a guide for the straightedge in determining the thickness. In many cases, door and window frames are used as plaster grounds.

In plaster work, a base coat is any coat beneath the finish coat. This is true whether the plaster is of two-coat or three-coat application. In three-coat work, the first coat is usually referred to in the trade as the scratch coat. It is usually applied over flexible bases, such as metal or wire fabric lath, and is intended to stiffen the base and provide a mechanical bond to the base. Also, as its name implies, the first coat is scratched with a scarifying tool, which provides horizontal ridges or scratches that are intended to provide mechanical keys for the application of the second coat (or brown coat). The brown coat usually constitutes the major bulk of the plaster and, consequently, materially affects the membrane strength. As a result, proportioning and workability are critical, and the mix should have high plasticity for proper application. The term brown coat is utilized by the trade to differentiate the relative color of the second coat to the finish coat, which is usually much lighter in color and is sometimes white, depending on the constituents.

The base coats in plaster work provide the strength for the plaster membrane but generally do not provide a proper surface texture for a finished surface. Therefore, a thin, almost veneer, coat of plaster is applied to the base coats as a finish coat. The finish coat may be applied in such a manner as to provide an ornamental or decorative finish, or it may be applied as a smooth surface to act as a flat base over which paint and wallpaper may be applied.

Section 2512 Exterior Plaster

2512.1   General.   Portland cement plaster is the only material approved by the code for exterior plaster. Gypsum plaster deteriorates under conditions of weather and moisture, which are prevalent on the exterior surfaces of buildings. For this reason, Section 2512.3 states that gypsum plaster cannot be used on exterior surfaces. Exterior portland cement plaster is required by the code to be applied in not less than three coats when applied over metal lath, wire-fabric lath, or gypsum board backing for the same reasons as discussed for interior plaster. When the portland cement plaster is applied over other approved plaster bases, the code requires only two-coat work. The code permits plaster work that is completely concealed to be of only two coats, provided the total thickness is that required by ASTM C 926, insofar as the finish code of plaster is to provide a surface for exterior finishes (such as paint) and to provide an aesthetic appearance. Thus, where the plaster surface is to be completely concealed, it is not necessary to provide a finish coat.

The code requires that the exterior plaster be installed to completely cover, but not extend below, the lath and paper on wood or metal-studded exterior wall construction supported by a nongrade concrete floor slab. This requirement, combined with the requirement in Section 2512.1.2 for a weep screed, is intended to prevent the entrapment of free moisture and the subsequent channeling of the moisture to the interior of the building. This requirement is depicted in Figure 2512-1.

Figure 2512-1   Termination of exterior plaster at on-grade concrete floor for stud walls.

2512.1.2   Weep screeds.   Water can penetrate exterior plaster walls for a variety of reasons. Once it penetrates the plaster, the water will run down the exterior face of the water-resistive barrier until it reaches the sill plate or mudsill. At this point, the water will seek exit from the wall, and if the exterior plaster is not applied to allow the water to escape, it will exit through the inside of the wall and leak into the building. Thus, the IBC requires a weep screed that, when constructed as shown in Figure 2512-2, will permit the escape of the water to the exterior of the building. In addition, where weep screeds are not provided for plaster exterior walls constructed in cold-climate areas, it is possible that the trapped moisture will freeze and cause a premature failure of the exterior plaster. The water-resistive barrier required by the code must lap the weep screed’s vertical flange. Although this section does not specify the amount of overlap, at least 2 inches (51 mm) should be adequate in keeping with the typical weather-resistive-barrier lap requirements.

Figure 2512-2   Weep screed installation.

2512.2   Plasticity agents.   Admixtures such as plasticizers should not be added to portland cement or blended cement unless approved by the building official. Some admixtures can have deleterious effects that more than offset the desired improvement in plasticity. It is preferable that plasticizers be added during the manufacture of the cement in order to ensure product uniformity and proper proportions. When plastic cement is used, the code does not permit any further additions of plasticizers as it is assumed that the amount added during the manufacturing process is adequate and is the maximum permitted. Hydrated lime and lime putty are time-tested plasticizers used with portland cement plaster, and their use is permitted by the code in the amounts set forth in ASTM C 926.

2512.4   Cement plaster.   Portland cement plaster is affected by freezing in the same manner as portland cement mortar or portland cement concrete. When portland cement plaster is applied during freezing weather, it loses a high proportion of its strength and, therefore, does not meet the intent of the code. In addition to protecting the plaster coats from freezing for at least 24 hours after set has occurred, application of the plaster should only be done when the ambient temperature is higher than 40°F (4°C). Plaster may be applied in colder temperatures where provisions are made to keep the cement plaster work above 40°F (4°C) during application and for at least 48 hours thereafter.

It is also important that the plaster not be applied to frozen bases or those covered with frost, which will not only weaken the bond of the plaster to its base but will also freeze the layer of plaster adjacent to the frozen base. In those cases where portland cement plaster is mixed with frozen ingredients or applied to a frozen base, it loses a high percentage of its strength.