Section VII – Special Construction

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This document may be revised or withdrawn from circulation at any time.


Most special building systems designed to improve sound isolation, fire resistance, moisture resistance, or thermal condition depend on gypsum board assemblies built to specifications proven to provide higher performance than standard systems typically provide. Items specified that help achieve higher performance include framing techniques, positioning of boards, spacing and selection of fasteners, caulking, and taping and finishing of joints. To ensure the desired performance from a gypsum board system, always closely follow the manufacturer’s installation specifications.

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The first essential for attaining airborne sound isolation of any assembly is to close off air leaks and flanking paths – routes by which sound can escape from one area to another. Because sound can travel over, under, or around walls, through windows and doors adjacent to them, through air ducts and through floors and crawlspaces below, these flanking paths must be correctly treated to reduce the transmission of sound. Hairline cracks and small holes permit the transmission of sound that can reduce the acoustical performance of an assembly, adversely affecting the Sound Transmission Class (STC), particularly in higher rated assemblies. Failure to properly follow special construction and design details can ruin the effectiveness of the best assembly.

To improve sound isolation:

·Separate framing of the two sides of the wall.

·Mount gypsum board on resilient channels.

·Install sound absorbing materials in the wall cavity.

·Use adhesively applied gypsum board of varying thicknesses in multi-layer construction.

·Caulk the perimeters of gypsum board partitions, openings in walls and ceilings, partition-mullion, intersections, outlet box openings, etc.

·Locate recessed wall fixtures in different stud cavities. (Avoid installing medicine cabinets, electrical, telephone, television and inter-com outlets, plumbing, heating and air conditioning ducts back to back.)

·Carefully cut to proper size and caulk any opening for fixtures, piping and electrical outlets.

·Caulk the entire perimeter of sound insulating partitions around gypsum board edges to make them airtight – see Figure 52.

·Caulk with non-hardening, non-shrinking, non-bleeding, non-staining, resilient sealant.

·Ensure that sound control sealing is covered in the specifications, understood by the workmen of all related trades, supervised by the foreman, and inspected carefully as construction progresses.

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Separated Partitions

A staggered wood stud gypsum board partition placed on separate plates will effectively decouple the system and provide an STC of 40-42. The addition of a sound absorbing material between the studs of one partition side can increase the STC as much as 8. Attach kitchen cabinets, lavatories, ceramic tile, medicine cabinets and other fixtures to a staggered stud wall or a metal stud chase wall rather than a resiliently mounted wall. The added weight and fastenings of these items will acoustically “short circuit” a resiliently mounted wall.

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Resilient Mounting

Resilient attachments, acting as “shock absorbers,” reduce the passage of sound through the wall or ceiling and increase the STC rating. To achieve higher STC ratings, it is necessary to use more complex construction methods,

such as separated partitions, multiple layers of gypsum board, and sound absorbing materials.


Attach resilient furring channels with the nailing flange down (open edge facing up) and at right angles to the wood studs (Fig. 46c). Drive 1-1/4 -inch type W or S screws or 6d coated nails through the pre-punched holes in channel flange. Use 7/8-inch or 1-in Type S screws with extremely hard lumber. Locate channels 24 inch from the floor, within 6 inch of the ceiling line and no more than 24 inch o.c. Extend channels into all corners and fasten to corner framing. Attach a resilient channel or ½ -inch by 3 inch wide continuous gypsum board filler strips to the bottom plate at floor line. Splice channels directly over studs by overlapping ends and fastening both flanges to the support.

Apply gypsum board horizontally with long dimension parallel to channels using 1 -inch Type S screws spaced 12 inch o.c. along channels. Center abutting edges of board over channel flange and securely fasten. Using a long fastener that contacts framing members will defeat the purpose of the resilient channels, which is to isolate the gypsum board from the framing.


Attach resilient furring channels at right angles to wood joists in ceilings using 1-1/4 inch Type W or Type S screws or 6d coated nails 1-7/8 -inch long (screws preferred). Locate channels within 6 inch of the wall-ceiling intersection and no more than 24 inch o.c. for joists spaced 24 inch o.c. maximum. Extend the channel into all corners and fasten to corner framing. Splice channels under joists by nesting channels and screwing through both flanges into the supporting framing member. Attach the gypsum board with Type S screws placed 12 inch o.c. in the field and along abutting ends. Apply the long dimension of the board at right angles to the channels with end joints neatly fitted and staggered in alternate rows. Use pieces of resilient channel for back blocking butt joints not falling on furring members.

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Fig. 45 – Designs for Sound Isolating Assemblies

Sound Isolating Materials

·Mineral or glass fiber blankets and batts for in metal and wood stud assemblies.

·Mineral (including glass) fiberboard.

·Gypsum core sound insulating board used behind gypsum board applied adhesively or mechanically fastened.

·Rigid plastic foam board used in exterior wall furring systems.

·Lead or other special shielding material.

Install mineral wool or glass fiber insulating batts and blankets in assembly cavities to absorb air-borne sound within the cavity. Place batts in cavities and carefully fit behind electrical outlets, around blocking and fixtures and around cutouts necessary for plumbing lines.

Some insulating batts and blankets have paper facings or other vapor barriers and flanges. Others have no facings and flanges. Install batts by friction fitting the batts inside the stud cavities. To ensure good board-to-stud contact, avoid stapling or fastening the flanges to stud faces. In metal framed and in laminated gypsum board partitions, attach the blankets to the back of the gypsum board. Install batts and blankets without facings by friction-fitting within the stud space.

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Fig. 46a–46d – Details of STC Rated Assemblies


General Procedures

Gypsum board may be applied over rigid plastic foam insulation on the interior side of exterior masonry and concrete walls to provide a finished wall and protect the insulation from early exposure to fire originating within the building. These systems provide high insulation values needed for energy conservation. Gypsum board applied over rigid plastic foam insulation in the manner described in this section may not necessarily provide finish ratings required by local building codes. Many building codes require a minimum fire protection for rigid foam on interior surfaces equal to that provided by ½-inch gypsum board. Review manufacturer’s literature when working with these materials as flammability characteristics of rigid foam insulation products

vary widely.

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Mechanical Application

When protecting rigid foam insulation with gypsum board, cover the entire insulated wall surface, including the surface above ceilings and in closed, unoccupied spaces. Screw attach single or double-ply, 1/2 -inch or 5/8 -inch thick gypsum wallboard to steel wall-furring members attached to the masonry per the insulation manufacturer’s directions, or attach gypsum board with nails or screws driven directly into wood framing.

Choose furring members designed to minimize thermal transfer through the member and to provide a 1-1/2 -inch minimum width face or flange for screw application of the gypsum board.

Install furring members vertically, spaced 24 inches o.c. Provide blocking or other backing as required for attachment and support of fixtures and furnishing. Attach furring members at floor-wall and wall-ceiling angles (or at the termination of gypsum board above suspended ceilings), and around door, window and other openings.

Apply single-ply gypsum boards vertically with the long edges of the board located over furring members. Place gypsum boards so that end joints are avoided. Take care that fastener spacing meets requirements for single-ply application over framing or furring.

Attach base ply of double-ply applications vertically over framing. Attach face ply either vertically or horizontally. Offset edge joints of vertically-applied face ply and end joints of horizontally applied face ply at least one furring member space from base ply edge joints. Use fastener spacing for two-ply application over framing or furring (see Table V in Section IV).

Use mechanical fasteners of such length that they do not penetrate completely to the masonry or concrete. In single layer application, reinforce all joints between gypsum boards with tape and finish with joint compound. In two-ply application, the base layer joints may be concealed or left exposed.

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Tile Backer

Water-resistant gypsum backing board, commonly called “green board” because of the color of its face paper, is specially designed and formulated to serve as a base for ceramic and similar tile. The core, face paper, and back paper are formulated and manufactured to resist the effects of moisture and humidity. Water-resistant gypsum backing board contains additives in its core that provide water resistance and its face and back paper are treated to limit water absorption; however, water resistant gypsum backing board is NOT WATER-PROOF. Consequently, water resistant gypsum backing board is not suited for use in areas with direct exposure to running water or high humidity, but it is very well suited as a tile backer in areas directly adjacent to wet areas or areas of high humidity.

Model building code requirements typically prohibit the use of gypsum board as a tile or panel backing material for tub and shower enclosures. In non-wet areas, water resistant gypsum backing board or regular gypsum board (ASTM C 1396) may be used as a base for application of ceramic or plastic wall tile or plastic finished wall panels. Do not use gypsum board in extremely critical exposure areas such as hot tubs, swimming pools, saunas, steam rooms, or gang shower rooms. Do not use water resistant gypsum backing board on ceilings without installing additional framing; do not exceed spacing of framing members beyond 12 inches o.c. with 1/2 –inch-thick board and 16 inches o.c. with 5/8-inch-thick board.

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Before beginning the application of water-resistant gypsum backing board to walls or partitions, inspect the framing for proper spacing and alignment. Space framing no more than 24 inches o.c. and reinforce all interior angles with framing to provide rigid corners. Install shims or furring as needed to align fixtures flush with the face of the gypsum board.

Where framing is spaced more than 16 inches o.c., or where ceramic tile is more than 5/16-inch thick, install blocking between the studs. Regardless of framing spacing, install suitable blocking, headers, or supports for plumbing fixtures. Take care that plumbing fixtures are properly installed, securely attached, and adequately supported.

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Application of Tile

Follow American National Standard Specifications for Installation of Ceramic Tile With Water Resistant Organic Adhesives, ANSI A 108.4., for ceramic wall tile application to gypsum board. Use an adhesive that meets the specifications of the American National Standard for Organic Adhesives for Installation of Ceramic tile, ANSI A 136.1, Type I.

Gypsum board joints, including those at all angle intersections that are under areas to receive tile or wall panels, must be treated with joint treatment and tape.

Caulk the cut edges and openings around pipes and fixtures flush with the surface using a flexible sealant caulking compound or adhesive complying with the American National Standard for Organic Adhesives for Installation of Ceramic Tile, Type I (ANSI A 136.1). Always follow the directions of the manufacturer of the tile, wall panel, or other surfacing material.

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Gypsum sheathing is a versatile, economical, water-resistive sheathing consisting of a water-resistive gypsum core and water-resistive paper or other facing. It serves as a substrate under exterior wall claddings. Gypsum sheathing is designed to be mechanically attached to the outside surface of exterior wall framing spaced up to 24 inches o.c. using either nails, screws, or staples. Gypsum sheathing is available in a wide range of lengths to meet the demanding needs of the construction industry. Easily sized, cut, and installed, gypsum sheathing is the sheathing of choice wherever fire resistance ratings or noncombustible materials are required.

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Gypsum sheathing is suitable for use with virtually all exterior wall claddings, including wood siding and shingles, panel siding, vinyl or metal siding, masonry or brick veneer, Portland cement stucco, and Exterior Insulation and Finish Systems

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Fire Protection

The natural noncombustible core of gypsum sheathing protects the framing from the hazards of fires occurring outside the building, even when covered by combustible siding. When exposed to high temperatures, chemically combined water in the gypsum is gradually released, providing protection until all of its combined water is completely driven off. In addition to its fire resistance properties, gypsum sheathing has a low flame-spread rating of less than 25.

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Code Compliance

Gypsum sheathing complies with building code requirements for use as structural sheathing in frame construction. Consult the local building code for specific details of construction and permissible shear values.

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Application of Gypsum Sheathing

General Provisions

Do not apply gypsum sheathing to ceilings, soffits or sills unless otherwise recommended by the manufacturer.

Ensure that all wood framing members to which gypsum sheathing will be fastened are straight and true. Do not exceed stud spacing greater than 24 inches o.c. Ensure that the fastening surface is no less than 1-1/2 -inches wide and does not vary more than 1/8 -inch from the plane of the faces of adjacent framing.

Ensure that all steel framing members to which gypsum sheathing will be screw-attached are straight and true and are spaced no more than 24 inches o.c. Ensure that steel studs meet the design thickness required and are finished with a protective coating to prevent corrosion. Ensure that the fastening surface to which gypsum sheathing will be attached is no less than 1-1/4 –inches wide.

Ensure that gypsum sheathing used in building construction is more than 8 inches from the finished grade in fully weather and water-protected siding systems, and more than 12 inches from the ground for properly drained and ventilated crawl spaces. Where ground moisture or humidity are extreme and/or continuous, cover the ground’s surface with a vapor retarder.

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Nails – Use minimum 12 gauge, galvanized, roofing nails.

Staples – Use staples made of galvanized steel, minimum 16 gauge, with 7/16 -inch crowns, and with divergent points.

Screws – Use screws that meet ASTM C 1002, Standard Specification for Steel Self-Piercing Tapping Screws for the Application of Gypsum Panel Products or Metal Plaster Bases to Wood Studs or Steel Studs. Use Type W screws for fastening to wood framing. Use Type S screws for fastening to light gage steel framing or wood framing. When attaching gypsum sheathing to heavier gauge steel framing, use screws that meet ASTM C 954, Standard Specification for Steel Drill Screws for the Application of Gypsum Panel Products or Metal Plaster Bases to Steel Studs from 0.033 -inch to 0.112 -inch in Thickness.

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Framing Members

Ensure that wood framing members conform to American Softwood Lumber Standard, Voluntary Product Standard PS 20. Ensure that steel framing members conform to ASTM C 645, Standard Specification for Nonstructural Steel Framing Members (light gage), or ASTM C 955, Standard Specification for Load-Bearing (Transverse and Axial) Steel Studs, Runners (Tracks), and Bracing or Bridging for Screw Application of Gypsum Panel Products and Metal Plaster Bases (not less than 0.033 -inch thick). For more on this topic, see Section XI.

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Delivery, Identification, Handling, and Storage

Confirm that all materials are delivered in original packages, containers or bundles bearing brand name, applicable standard designation, and name of manufacturer or supplier for whom product is manufactured.

Take care that gypsum sheathing and accessories are properly supported on risers on a level platform and are fully protected from weather, direct sunlight exposure, and condensation.

Neatly stack gypsum sheathing flat and in such a manner that prevents sagging or damage to edges, ends and surfaces. Refer to GA-801, Handling Gypsum Panel Products, at, for proper storage and handling requirements.

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Methods of Cutting and Installation

Cut gypsum sheathing by scoring and snapping or by sawing, working from the face-paper side. When scoring, use a sharp knife to cut through the face paper into the gypsum core. Then snap the gypsum sheathing back away from the cut face. Break the back paper by snapping the gypsum sheathing in the reverse direction, or preferably by cutting the back paper. Neatly trim all cut edges and ends of gypsum sheathing to obtain neatly fitting joints once the sheathing is installed. Cut out holes for pipes, fixtures or other small openings with a saw or special tool designed for the use. Neatly scribe and cut gypsum sheathing where it meets projecting surfaces.

When a mechanically attached water-resistive barrier is required, attach the water-resistive barrier over the face of the gypsum sheathing and to the framing, with the upper layer lapped over the lower layer as set forth in the


Take care to properly flash gypsum sheathing at openings, and locate sheathing so that no joint will align with an edge of an opening. Stagger all joints.

Protect holes and cutouts for pipes or fixtures, or other small openings with flashing or caulking.

Do not exceed spacing of 24 inches o.c. for framing members.

Install control joints in walls wherever specified.

Where shear values are not required, gypsum sheathing 48 inches wide may be applied parallel or perpendicular to framing with vertical joints over framing members and with gypsum sheathing fitted snugly around all window and door openings.

Cover square-edge gypsum sheathing applied with its bound edge perpendicular to the framing with a water-resistive barrier or seal horizontal joints.

Install Tongue & Groove (V-edge) gypsum sheathing perpendicularly to framing members, with grooved edges down, interlocking the tongue of the panel below without forcing. Abut ends of panels over centers of framing members and stagger all end joints between adjacent courses. Fit gypsum sheathing snugly around all window and door openings.

When shear values are not required, space fasteners no more than 8 inches o.c. along vertical ends or edges and intermediate supports. When wall bracing or wall shear values are being assigned to the installed gypsum sheathing, space fasteners as specified by the gypsum sheathing manufacturer.

Space fasteners no less than 3/8 -inch from the ends and edges of the gypsum sheathing.

Drive nails so that the heads are at or slightly below the surface of the gypsum sheathing. Take care to avoid damaging the face and core.

Drive screws so that the screw heads are at or slightly below the gypsum sheathing surface paper without breaking the surface paper or stripping the framing member around the screw shank.

Drive staples with crowns parallel to framing members and in such a manner that the crowns bear down tightly against the gypsum sheathing without cutting into

the face paper.

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Exterior Soffit and Ceiling Construction

Exterior gypsum soffit board and gypsum wallboard are suitable for commercial use as ceilings for covered walkways and malls, large canopies and parking areas, and for residential use in open porches, breezeways, carports and exterior soffits. These areas must be horizontal or sloping downward away from the building.

Space framing no more than 16 inches o.c. for ½ – inch thick gypsum board, nor more than 24 inches o.c. for 5/8 – inch thick board. Use an application method identical to that for interior ceilings. Treat joints and fastener heads with setting-type joint compound. Provide suitable fascias and moldings around the perimeter to protect the board from direct exposure to water. Unless protected by metal or other water stops, do not place the edges of the board less than 1/4-inch away from abutting vertical surfaces (Fig. 47).

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Fig 47a

Fig 47b

Fig 47c

Fig 47d

Fig. 47 (a, b, c, & d)Protect gypsum board from wetting in outdoor locations with drip edges at facia boards. Separate gypsum board edges from dampness by metal trim or by a 1/4 inch space. Spaces enclosed by gypsum board must be adequately vented.

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Fig. 47 – Details Showing Proper Installation of Soffit Board

Seal the exposed surfaces of the gypsum board and metal trim with a premium exterior primer and two coats of exterior paint. Vent attic spaces to the outside where the area above opens to an attic space over habitable rooms. Install vents at the end of each joist space where the gypsum board is applied directly to joists or rafters. Ensure that vents are screened, are a minimum 2 inches wide, and span the full width between the joists. Locate vents 6 inches from the outer edge of eaves.

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A minimum 5/8-inch thick layer of Type X gypsum board added to a roof structure will help stop fire from burning down into a building. It will also help prevent the lateral spread of flame under the roof surface.

Be sure to provide adequate ventilation in attic areas to control moisture and reduce temperature. Careful attention to design is critical to ensure proper

air flow to all areas.

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Methods of Installing Gypsum Roof Underlayment

The Gypsum Association has developed the following three recommended methods of installation. Although the model codes permit any of these three alternates to parapet construction, individual code provisions may vary. Consult local code authority for provisions specific to your jurisdiction.

The three methods of applying gypsum board as a roofing underlayment, in order of preference, are described below.

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Ledger Strip Method

Attach nominal 2-inch x 2-inch ledger strips to form continuous supports along the sides of the roof framing so that the upper surface of 5/8-inch gypsum board will be flush with the top of the trusses or rafters. Use at least 1¼-inch-long fasteners spaced a maximum of 48 inches o.c. to attach strips to roof framing. Cut the gypsum board to fit snugly between the roof framing members and lay it in place. Take care to ensure that gypsum board end joints spanning between framing members fit snuggly.

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Fig. 48 Ledger Strip Roof Underlayment Installation Method

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Partial Roof Underlayment

Install 5/8-inch type X gypsum board under the combustible roof sheathing on both sides of the party wall for not less than the minimum distance required by the code. Apply the gypsum board on top of roof framing, with the paper-bound edge perpendicular to the framing, prior to the installation of combustible roof sheathing. Locate all gypsum board ends on the roof framing members. Nail the gypsum board to each framing member with at least two minimum 4d common nails, or 1-3/8-inch gypsum wallboard nails. Butt end joints loosely; fit edge joints snugly together without forcing. End and edge joints need not be taped.

Protect gypsum board against exposure to inclement weather during installation and until the roof membrane installation is complete. Shim all remaining trusses and rafters as necessary to provide a straight and uniform base for the attachment of the combustible roof sheathing. Attach roof sheathing through the gypsum board to the roof framing with sheathing joints offset from the gypsum board joints. Increase fastener length by 5/8-inch to accommodate the thickness of the gypsum board and shims. Apply the roof membrane as specified by the manufacturer.

This second method offers easy attachment of the gypsum board to the roof framing, compared to the ledger

strip method. It does however require shimming the roof framing that is not protected with the gypsum board. As an alternate to shimming, two different sizes of rafters or trusses may be used. In typical multi-family housing where individual units are relatively narrow, it is generally more economical to cover the entire roof.

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Fig. 49 Partial Roof Underlayment Installation Method

Full Roof Underlayment

Install 5 / 8 -inch type X gypsum board over the entire roof area. Apply the gypsum board perpendicular to and on top of roof trusses or rafters prior to the installation of combustible roof sheathing. Locate gypsum board end joints over and the roof framing. Nail the gypsum board to each framing member with two minimum 4d common nails, or 1-3 / 8-inch nails complying with ASTM C 514, Standard Specification for Nails for the Application of Gypsum Board. Butt end joints loosely; fit edge joints snugly together without forcing. End and edge joints need not be taped. Protect gypsum board against exposure to inclement weather during installation and until the roof membrane installation is complete. Attach combustible roof sheathing through the gypsum board to the roof framing with sheathing joints offset from the gypsum board joints. Increase fastener length by 5 / 8-inch to accommodate the thickness of the gypsum board. Apply the roof membrane as specified by the manufacturer.

This full roof underlayment method offers a number of advantages over the partial roof underlayment and ledger strip methods. First, it provides the greatest degree of fire protection. Second, it is the least complicated of the three methods to install, and finally, it is generally the most cost-effective of the three methods when labor costs are factored in.

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Fig. 50 Full Roof Underlayment Method


Shaft walls enclose elevator shafts, stairwells, air return systems, and mechanical utilities (which include plumbing, heating, electrical, cable, and telecommunication lines, among others) in multi-story buildings. Shaft walls protect occupants exiting burning buildings via the stairs; likewise, they protect fire fighters entering burning buildings using the elevators. Until the 1960s, shaft walls were typically constructed using masonry or concrete, but the advantages inherent in gypsum board construction – lighter weight, quick installation, no cure time, fewer weather restrictions, smaller footprint – have made gypsum board shaft wall systems the preferred choice for modern multi-story construction.

Gypsum board shaft wall systems vary some in the variety of their designs; however, most systems share several common elements. Most use specialized steel framing, including E-studs, C-H studs, C-T studs, I-studs, and J-runners. Most also use 1-inch thick gypsum liner panel as a component, along with one or more layers of fire-resistance-rated gypsum board, veneer base, or abuse resistant board.

Shaft walls must meet building code requirements and adhere strictly to specified fire-tested designs. Some shaftwall systems also allow for assembly from only one side because working from inside the shaft at elevated heights is extremely dangerous and financially prohibitive. Stairwell systems typically permit installation and finishing on both sides.

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The steps to installing a typical shaftwall system are as follows:

·Install J-runner to floor and ceiling using power-actuated fasteners spaced no more than 24 inches o.c.

·Seal joints with appropriate sealant.

·Measure width of opening and layout in 24-inch increments to ensure that the final panel will exceed 8 inches in width; adjust layout as necessary.

·Affix an end-piece framing member (e.g. E-stud or J-runner) at one end of opening to top and bottom J-runners.

·Cut 1-inch liner panel to a length ¾- to 1 inch shorter than the ceiling height and friction-fit into or screw attach using type-S screws to end-piece framing member.

·Cut C-H or C-T stud to length 3/4- to 1 inch shorter than ceiling height and friction fit into floor and ceiling channels and over the bare edge of liner panel.

·Repeat until opening is spanned.

·Attach facing panels to face of studs using S-type screws per the design requirements.

Note that shaft wall studs may not be spliced; they must be ordered in lengths greater than or equal to the floor-to-ceiling height.

Be sure to use an approved and tested shaft wall design, and carefully follow the manufacturer’s recommendations for the assembly of that system. See examples of shaft wall designs in the images below.

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Fig. 51 View of Shaft Wall Installation


Chase walls consist of two parallel rows of studs. Each row of studs supports one or more layers of gypsum board on one side – the side facing the adjacent occupied area – only. This creates an unfaced hollow area between two wall surfaces where plumbing, electrical, HVAC, and other services run between two spaces. A common use of chase walls occurs where bathrooms sit back-to-back, so that plumbing, heating, and electrical service reaches two areas from a common location. The open space between chase walls also reduces the amount of noise that travels from one occupied space to the next.

A typical steel-framed chase wall installation is as follows:


Attach tracks to concrete slabs with concrete stub nails or power-driven anchors 24 inches o.c.; attach track to suspended ceilings with toggle bolts or hollow wall anchors 16 inches o.c.


Position steel studs on opposite sides of chase directly across from each other with flanges facing in the same direction.

· Cut cross-bracing from gypsum board 12 inches high by chase wall width. Attach braces to webs of parallel studs no more than 48 inches from the floor and spaced 48 inches o.c. vertically using three 1-inch type S screws per stud.

· Attach facing panels to outside face of studs using Type-S screws per the design requirements.

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Fig. 52 Typical Chase Wall Design


Gypsum board area separation fire walls provide efficient and affordable fire protection and sound attenuation in multifamily construction up to four stories in height and light commercial construction. These systems only

require standard framing tools and techniques for their assembly. They can be installed under almost any weather conditions, enabling builders to reduce the number of trades people working on a jobsite and avoid the time and weather constraints typically associated with alternate masonry systems that require 28-day curing periods.

Because gypsum board wall systems weigh significantly less than alternate systems, they do not require large, costly footers and can be erected directly over poured concrete slab foundations. Moreover, they provide all these benefits while responding directly to contemporary building code requirements calling for individual townhouse units and similar multifamily dwellings to be separated by fire-rated construction.

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Code Compliance

The model building codes require all fire walls to meet the following design criteria:

·they must be continuous from the foundation to the underside of protected roof sheathing or continue through the roof to form a parapet, and;

·they must allow for the collapse of the construction on the side of the wall exposed to fire while remaining intact to protect the structure on its opposite side.

Any wall system used to separate buildings must pass a test conducted by an independent testing laboratory according to the provisions set forth in ASTM E119, Standard Test Method for Fire Tests of Building Construction and Materials, or its equivalent to ensure that the system meets the specified fire-resistance requirements—usually two hours for walls between dwelling units. As a consequence, builders must assemble individual walls exactly as designed and tested. In every instance, the components and methods must meet the specifications described in the fire test report to ensure that the system provides the required fire resistance.

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System Types

Gypsum board area separation firewalls are generally available in cavity-type or solid systems. Cavity-type systems typically incorporate components such as gypsum liner panels and methods similar to those for steel stud partitions or shaft wall construction to protect stairwells and elevator shafts. Solid gypsum board walls use three basic components: 1-inch-thick type X gypsum board liner panels that are 2-feet wide and either 8-, 10-, 12- or 14-feet long; metal framing members consisting of 2-inch-wide H-studs and U-shaped track; and “breakaway” L-shaped aluminum clips that soften at relatively low temperatures.

A solid gypsum wall built to resist fire for two hours uses two layers of 1-inch-thick gypsum liner panel supported by steel framing. The panels stand on end and slide into the bottom of a steel U-shaped track, which is attached to the foundation or to the top track of a section of wall on the story below. Steel H studs hold each pair of 1-inch panels together on their vertical edges. An inverted U-shaped track serves as a cap over the top ends of the panels and the H studs. Fasteners secure the H studs to the top and bottom tracks. Once a span of wall is completed, another section is assembled directly over it.

L-shaped aluminum clips connect the wall to the adjacent building’s structural framework. The clips connect each H stud on both sides with the adjacent floors or roof/ceiling intersections to keep the fire wall in place between the two structures.

When the aluminum clips reach a temperature exceeding 1,100 degrees F, they break away between the steel and the wood frameworks and allow the burning structure to fall away while leaving the fire wall in place. Because the gypsum board panels prevent the heat on the fire side from reaching the opposite side, the aluminum clips supporting the wall on the non-burning side remain intact and prevent the fire from spreading. Note that the L clips must appear in the tested design to ensure that they perform properly during a fire, and must be fastened and installed in conjunction with system directions.

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Proper Installation

To be fully code compliant, gypsum board fire wall systems used in townhouse or multi-family construction typically must be erected with a minimum set-back from adjacent combustible (wood) construction. This is usually accomplished by maintaining a gap of approximately ¾ inch to 1 inch between the outside face of the gypsum liner panels and the adjacent construction. The codes also generally require the void created by the set-back at the floor level to be in filled with a noncombustible fireblocking material. This can easily be accomplished by fitting mineral fiber insulation, scrap pieces of gypsum board liner panel or another code-approved material into the void. The fireblocking material must also be mechanically attached to the structure’s wood framing.

If the mandated set-back cannot be achieved, the steel surfaces of the H studs must be covered with a 6-inch-wide layer of gypsum board batten strips, full sheets of gypsum board or mineral fiber insulation. Construction details showing gypsum wall intersections with foundations, intermediate floor junctions, exterior wall intersections, roof junctions and parapets are available from component manufacturers.

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Sound Attenuation

In addition to defining fire-resistance requirements, the model building codes typically require walls separating townhouse units to maintain a minimum Sound Transmission Class (STC) rating of 45 to 50 to ensure that proper sound attenuation occurs between individual units.

Gypsum board wall systems must be constructed in accordance with the original sound test if they are to comply with sound transmission requirements. Any deviation from the requirements of the test report, including the addition of additional components, may result in unacceptable system performance.

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Fig. 53Typical Gypsum Board Area Separation Fire Wall Construction


Gypsum veneer plaster systems are thin coat, high strength plaster systems that can be quickly applied over a variety of substrates. These systems simulate in many respects the versatility, beauty, and durability of conventional plaster systems. Gypsum veneer plaster finishes provide hard, dense surfaces that are resistant to scuffing, denting, cracking, and abrasion. The variety of possible veneer plaster finishes ranges from a mirror smooth surface to any type of floated, swirled, or light texture. Veneer plaster is easily integrally colored with pigments or finished with any number of decorating products to achieve a broad array of looks.

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The setting time of veneer plaster materials is generally between 45 and 90 minutes. With sufficient manpower and favorable environmental conditions, a veneer plaster job can be completed, from bare studs to decoration, in only 48 to 72 hours.

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Gypsum Base for Veneer Plaster

Gypsum base, or “Blue Board,” is a gypsum board substrate specifically designed to support gypsum veneer plaster. Gypsum base is usually 48 inches wide and available in 8 foot, 10 foot, and 12 foot lengths. Other lengths are available by special order. Regular base thicknesses are 3/8-inch, ½-inch, and 5/8-inch. Type X special fire-resistant bases are available in ½-inch and 5/8-inch thicknesses.

Gypsum bases vary among manufacturers. Each manufacturer formulates its gypsum base specifically for the accompanying gypsum veneer finish. Follow the manufacturer’s instructions carefully to ensure the proper matching of the gypsum base and gypsum veneer plaster.

Installation of gypsum base is very similar to the installation of regular gypsum board; the tools and techniques used are essentially the same. The main difference in the installation between the two systems is that the surface of the veneer base is not dimpled at the fasteners when veneer base is nail-applied. Joint finishing techniques and materials are also different when gypsum base is installed.

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One or Two Component Systems

Gypsum veneer plasters are available in either one or two component systems. Each type is available pre-mixed in bags, ready to combine with water at the job site. One component systems provide quick and easy finishing, while two component systems provide improved aesthetics, especially where indirect lighting conditions exist or high gloss paint is specified. Both trowel-applied and machine-applied systems are available.

One component systems can be completed in one or two days and decorated after 24 hours under favorable conditions. Two component systems can be applied over a variety of substrates and enable the applicator to level out slightly uneven surfaces.

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One Component Systems

A one component gypsum veneer plaster system consists of one product ready to be applied as a thin membrane after the addition of potable water. This system is usually applied using the “scratch-and-double-back method,” producing a thickness of 1/16 inches to 3/32 inches. The surface can be trowelled to a highly polished finish or worked to achieve a float, swirl, pucker, dimple, Spanish, or other finish.

One component systems work best over gypsum base, but they also produce good results over other bases such as monolithic concrete or masonry block. Skilled applicators use coarser floats or add more sand for deeper texture


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Two Component Systems

Two component systems use two different, separately packaged products – the basecoat and the finish coat. Gypsum basecoat plaster, which is many times stronger than conventional plaster, is applied directly over the base. The finish coat can be applied as soon as two hours after the base coat has set or when proper “suction” is achieved. Two component veneer plaster systems work particularly well over masonry block walls because the trowelled-on base coat permits the application of a thickness sufficient to level out the surface. The finish coat after the base coat has set and achieved proper suction. A two component system may be finished to a smooth finish, a sand texture, or one of many other textures.


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Direct Application to Masonry Surfaces

Both one and two component veneer plaster systems provide high quality, durable finishes over a variety of masonry surfaces, particularly concrete block and monolithic concrete. One component systems work best over smooth, straight and true masonry surfaces, while two component veneer plaster systems allow for a modest amount of surface variation. Most masonry substrates will require some surface preparation and the application of a bonding coat prior to receiving a veneer plaster finish.

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Trowel Application

One Component

Time the application so that an entire wall or ceiling area can be completed from the same mix. Work to a ceiling angle or corner. If glass fiber tape is used on the joints, bed the tape first. This will keep the glass fiber tape from wrinkling, promoting uniform takeup and assuring positive keying of the veneer plaster into the mesh. After the joints have set, plaster the entire room, starting with the ceilings and then completing the walls. Tightly scratch in and immediately double back with material from the same mixer batch to a uniform thickness as recommended by the manufacturer. When the plaster mass has become firm (taken up) trowel the surface to remove trowel marks, surface imperfections and integrate the surface to a uniform smoothness. Just before the surface sets, trowel it to a smooth finish using water sparingly. Do not over trowel or trowel through the set. The set of most veneer plaster can be recognized when it appears to darken. Always carefully follow the manufacturer’s instructions.

Two Component

Apply the base coat evenly, yet rough and open enough to provide a key and proper suction for the finish component per ASTM C 843, Standard Specification for Application of Gypsum Veneer Plaster, or manufacturer’s specifications. Apply the finish coat with trowel or other finishing method. The finish may be a smooth or textured veneer plaster, lime putty gauging, or any other finish that can be applied to a conventional base coat plaster.

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Machine Spray-Application

There are veneer plasters made specifically for machine spray-application. This application method requires extreme care and cleanliness. The key concern associated with spraying veneer plasters is the time of set. Set time is especially critical in machine application because the machine accelerates the setting process of the plaster. Follow the veneer plaster manufacturer’s recommendation to select the proper veneer plaster for the machine. Accelerator addition must be adjusted at the nozzle to obtain no more than one hour (preferably 30 minutes) set time on the wall. Cleanliness of mixing and spraying equipment is of utmost importance. Veneer plaster must not be allowed to set up in machines. Plan the work so that the amount of veneer plaster in a batch can be mixed and sprayed in about half the published setting time. This will allow time to properly clean the machine between each batch.

Apply veneer plaster spray in two even passes by spraying at right angles between coats; for example, up and down on the first pass and side to side on the second pass. Allow base to set and dry sufficiently to provide proper suction for the finish. Hand-apply smooth finish as previously described. If a spray texture is desired, hand apply finish plaster by scratching in and doubling back, then spray texturing with the finish plaster to obtain the desired finish once the initial scratch and double-up application has become firm but has not set. Vary the texture by changing plaster consistency, adjusting the air pressure, changing the orifice size, or a combination. Experiment by spraying a piece of scrap gypsum base until the desired texture is achieved; then spray the whole room in one operation, if possible.

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Direct Application to Masonry Surfaces

Concrete Blocks – Ensure wall is straight, clean, and free of oil and dirt. Straighten any irregularities in the blocks by filling with base plaster as needed. Level but avoid smooth troweling. Leave the final surface rough and open to provide proper bonding of the finish coat. After set, apply either the smooth or textured finish coat.

Monolithic Concrete – Dense, smooth concrete requires a liquid bonding compound to receive gypsum veneer plaster. Grind flush all ridges and bumps, and then remove all loose dust. Apply the recommended bonding agent carefully

following the manufacturer’s instructions. Mix a small batch of base coat gypsum veneer plaster and spot fill any deep holes and crevices first. Then, apply the base coat of gypsum veneer plaster; level, but do not smooth. After the base coat has set, apply either smooth, floated, or textured finish coat.

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Fig. 54 Images of Veneer Plaster Application

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