All Things Gypsum
The Basics of Substrates
This article will examine the three systems most commonly used as gypsum board substrates: wood framing, metal framing and solid-surface systems. Desired characteristics of each will be considered and discussed.
General wood framing requirementsAn application of gypsum board cannot compensate for improper or misaligned framing. Wood framing and furring should be aligned accurately so that the gypsum board fits flat against the framing surface at all points and all wood framing members are to be in the same plane. The fastening surface of any framing or furring member should not vary more than 1/8 of an inch from the plane or face of any adjacent framing, bridging, or furring member.
The spacing of framing members should comply with the requirements for the gypsum board thickness as defined by ASTM C 840, “Standard Specification for Application and Finishing of Gypsum Board,” or Gypsum Association GA-216, “Application and Finishing of Gypsum Board.” The requirements in the two documents are identical.
When selecting the wood for framing and furring, care should be taken to use only properly cured lumber. Excessive moisture in wood can cause it to warp as it dries. Wood-framing members should have a moisture content of less than 19 percent as defined by PS20, “American Softwood Lumber Standard.” If warped or crooked studs and joists have been used, they should be repaired or replaced using straight lumber.
Fasteners are to be installed so that the entire shank of the nail or screw is seated within the core of the wood and is not protruding from the side of the framing member. The attachment surface of the wood-framing members must be sufficiently wide to provide both a suitable backing surface for the edge of the panels and a dense wood core area for the fastener to grip properly. The surface to which the gypsum board is attached must not be less than 2 inches nominal width. Wood studs, when installed as part of a framing system, should not be less than nominal 2x3s in non-load-bearing partitions and nominal 2x4s in load-bearing partitions. Wood furring strips attached to concrete or masonry walls should not be less than nominal 1x3s. Cross furring members attached to a ceiling should not be less than nominal 2x2s.
Furring installationFurring strips and cross furring are typically used to correct surface unevenness in solid surfaces or existing framing. Spacing requirements for furring are the same as those for wood studs or other wood-framing members.
Particular attention should be paid to wood furring members that have been installed to solid substrates, such as masonry or concrete. Furring strips installed using cut nails or similar fasteners should be carefully examined to ensure that the furring members have not been split or fractured during the application process. To allow for sufficient attachment of the gypsum board, fasteners used to attach gypsum board to furring strips should not penetrate through the back face of the furring strips and come into contact with the solid surface beneath.
Wood joists and trussesWood ceiling joists or trusses should be evenly spaced with faces aligned in a level plane. If gypsum board is to be directly attached to framing members that are not in level plane or are irregularly spaced, the installation of leveling cross furring is recommended. Cross furring is to be installed in compliance with established framing spacing recommendations and may be either wood or steel.
Excessively bowed or crooked joists should not be used. Joists with a slight crown may be used if they are installed with the crown facing up. Slightly crooked or bowed joists can sometimes be aligned by nailing bracing members (strongbacks) across the joists approximately at mid span.
When trusses or joists change direction in the middle of a room, proper blocking should be installed so that maximum framing spacing does not exceed the requirements of ASTM C 840 or GA-216.
Metal framingMetal framing members are manufactured to industry standards that specify dimensional uniformity. As a consequence, concerns about attachment surface dimensions and overall member size are usually of lesser consequence with metal framing members than with wood-framing members.
As with wood framing, metal-framing members must be installed straight and in plane. Bent, damaged or rusted metal framing members should be removed and replaced. Framing screws should be judiciously installed so as not to place outward pressure on gypsum board edges and corners.
Metal framing members are installed based on specific spacing requirements. Details of the maximum spacing allowed for an application of gypsum board on metal framing may be found in ASTM C 840 or GA-216. In addition, ASTM C 754, “Standard Specification for Installation of Steel Framing Members to Receive Screw-Attached Gypsum Panel Products,” contains framing spacing and limiting height deflection tables that should be consulted prior to installation.
Solid steel framing members, such as open-web steel bar joists and steel beams and girders are not designed to receive a direct application of gypsum board, and are often spaced more than 24 inches on center. When constructing a ceiling beneath solid steel framing, it will be necessary to provide suitable furring or suspension systems with appropriate spacing for attaching the gypsum board.
Masonry and concrete surfacesGypsum board can be laminated directly to above grade interior solid wall surfaces, such as masonry and concrete, if the surface is dry, smooth, clean, and flat. Concrete and masonry surfaces must be made as level as possible. Any surface that cannot be readily prepared for a direct adhesive application of gypsum board should have furring or framing installed. If an adhesive application method is used, rough or protruding edges and excess joint mortar on the face of the solid surface must be removed. Depressions and indentations in the face of the solid surface must be filled with mortar to make the wall surface level.
Solid surfaces should be clean of form oil, curing compound, loose particles, dust, or grease to ensure an adequate bond between the surface and the adhesive used to install the gypsum board. Concrete should be allowed to cure for at least 28 days before gypsum board is laminated directly to it.
Supplemental mechanical fasteners spaced 16 inches on center may be used to hold gypsum board in place while the adhesive is developing a sufficient bond. The manufacturer of the adhesive should be consulted for specific requirements.
Other considerationsWood and metal framing members are often insulated to increase system sound attenuation or to improve the thermal properties of the structure. When used, paper- or scrim-faced batt insulation should not be installed using staples that are fastened onto the face of the wood-framing members as this may cause the edges of the gypsum board to seat improperly. Friction-fit batt or blanket insulation should be installed fully within the stud cavity so that edges or sides do not interfere with application of the gypsum board.
With truss roof construction, exterior walls and interior ceilings occasionally are finished before interior partitions are erected. When this method is used, all construction elements, primarily the roofing and roof-installed equipment, which will increase the roof load must be installed before the interior partitions are erected. If substantial roof loads are introduced after the interior partitions are erected, the ceiling may be forced down against the partitions and may become distorted as the roof trusses deflect.
It is difficult and expensive to repair a framing or a masonry flaw once the error has been concealed. Therefore, examining a substrate prior to the application of gypsum board can be as important as evaluating the finished drywall work itself. A brief time spent examining the substrate can save time and help prevent unnecessary, expensive and tedious callback work.