Fire Resistance Testing
Two important aspects of fire resistance testing are frequently misunderstood; this article will clarify the misperceptions surrounding the meaning of fire resistance ratings and the role of the hose stream test in assessing fire resistance. Understanding the basic concepts behind fire resistance and hose stream testing will provide the background needed to identify and correct misunderstandings and misinformation that may be perpetuated in the competitive arena regarding fire resistance ratings and gypsum board systems.
What Does A Fire Resistance Rating Mean?
A fire resistance rating is one of many tools used by designers to assess relative fire risk. In addition to fire resistance, other properties of the construction materials to be considered include burning characteristics, fuel load of the space, and the proposed use of the structure or occupancy. All of these features must be considered before an assessment of the actual fire risk can be made. Additional factors such as building location, distance to fire services, and the presence or absence of other fire protection systems are also part of this complex assessment process.
A fire resistance rating alone cannot predict the performance of a system or building in an actual fire. In fact, no fire test method that is conducted under laboratory conditions can predict what will happen in a real structure fire. Fire tests are simply convenient ways of classifying materials and establishing a ranking of performance among different materials so designers can compare and select materials and systems for specific projects. Fire test results, including fire resistance ratings, enable code officials to compare materials and systems against code requirements to determine compliance. It may help to think of a fire resistance rating in the same way you think of the mileage sticker on a new car. The mileage on the sticker is determined under very specific test conditions and your actual mileage will likely be either more or less than the mileage on the sticker.
How is Fire Resistance Measured?
The fire resistance test method used throughout the United States is ASTM E 119, Standard Test Methods for Fire Tests of Building Construction and Materials. This test procedure was first published by ASTM in 1918 as ASTM C 19-18. That first ASTM version prescribed two tests: 1) a Fire Endurance Test and 2) a Fire and Fire Stream Test (commonly referred to as the Hose Stream Test). Similar test methods are published by Underwriters Laboratories (UL) and by the National Fire Protection Association (NFPA).
The Fire Endurance Test subjects a specimen to a prescribed fire until certain conditions are met that indicate the end of the endurance test. This period of time is known as the “resistance period” of the Fire Endurance Test. All fire resistance rated systems, regardless of the materials from which they are built, are tested using this Fire Endurance Test.
The Hose Stream Test is divided into 1) a “primary” or “standard” method and 2) an “optional program” method. The optional program is referred to as an “exception” in the NFPA version. The standard method states that: “a duplicate specimen [is] subjected to a fire exposure test for a period equal to one half of … the resistance period of the Fire Endurance Test, but not for more than one hour.” The duplicate specimen is then immediately subjected to the impact, cooling, and erosion effect of a stream of water from a fire hose at a pressure and for a duration of time specified in the test method. If no significant amount of water passes through the test specimen, the fire endurance time of the first specimen becomes the fire rating for the system.
The duration of the hose stream exposure is a function of the fire endurance period of the original specimen and is keyed to the fire-resistance rating of the system being tested; i.e., the longer the rating, the longer and more severe the hose stream exposure. The “optional” program, which can only be used if both the testing laboratory and the test sponsor agree, is to administer the hose stream to the same specimen used for the Fire Endurance Test without the need for, and added cost of, constructing and burning a duplicate specimen as required by the standard method.
Role of Hose Stream Testing
Although the standard method and the optional program have remained essentially unchanged over the years as ASTM C 19-18 evolved into the current version of ASTM E 119, the value of the Hose Stream Test has been a subject of debate for years. It was eliminated from testing requirements for floor-ceiling and roof-ceiling systems in the 1950s because years of tests had resulted in no failures of systems because of the Hose Stream Test. The Hose Stream Test was removed from European and International test methods years ago and the British test method has not contained a Hose Stream Test for over 40 years. The Hose Stream Test is now found only in the ASTM, UL, and NFPA test standards, but even here, it is applied only to walls.
In spite of the fact that the international fire-safety community discontinued the Hose Stream Test years ago, the masonry industry would prefer to see fire testing in the United States take a step backward by eliminating the standard Hose Stream Test method and requiring what is now the optional program. The masonry industry often misrepresents the origin and purpose of the Hose Stream Test in an attempt to strengthen its technically deficient arguments in hopes of gaining a competitive advantage.
The ASTM E 119 procedure, including both the Fire Endurance Test and the Hose Stream Test, is intended to evaluate the performance of systems during the period of fire exposure. It is not, nor has it ever been, the intent of the tests to evaluate a system’s suitability for continued use after the fire exposure. Section 1.2 of the scope of ASTM E 119 states clearly, “It is the intent that classifications shall register performance during the period of exposure and shall not be construed as having determined suitability for use after fire exposure.”
Questions surrounding why the hose stream exposure is conducted on a duplicate specimen that has undergone a fire exposure for a period of less than the full fire rating were raised as long ago as 1930 when a representative of the gypsum industry asked fire test experts at the National Bureau of Standards (NBS) to explain the rationale for the procedure. In a letter dated January 2, 1930 , Mr. Fitzhugh Taylor, a fire test expert from NBS and Secretary of the ASTM Committee responsible for developing and maintaining the fire resistance test method, responded with the following explanation:
The second test of the prescribed pair, the Fire and Fire Stream Test [Hose Stream Test], in which the fire exposure is shorter than that of the Fire Endurance Test, is a means of investigating stability of the test subjects, including essential parts thereof, during the period in which they function effectively as fire barriers; this being the intent of the second test of the pair, it is prescribed that the fire hose stream shall be applied well in advance of the time when the fire resistance or the test subject has been taxed to its ultimate.
Mr. Taylor’s comments confirm that it was never the intent of the Hose Stream Test to evaluate a system’s performance after a fire of the full duration of the rating. In fact, the validity and value of the Hose Stream Test, in general, has been challenged by Mr. S. H. Ingberg, recognized as one of the father’s of modern fire testing, whose understanding of the rationale for the Hose Stream Test is:
In general, the justification of the hose stream test is predicated on the following sequence of events:
(1) The application of hose-streams to a construction exposed to fire causing damage making the construction vulnerable to fire exposure.
(2) The fire-fighting force leaves the scene or is driven away by the fire.
(3) The fire recurs in sufficient severity to overtax the damaged construction….
The occurrence of consecutive events (1), (2), and (3) is apparently of the order of a fairly remote probability, considering particularly that (3) is unlikely to occur after the amount of water application required under (1) by the test procedure. This formed the basis for the omission of the hose-stream test for columns in the present standard, and … in a forth-coming revision of the British Standard … on fire test methods, the hose stream is not required for any type of construction.
Although the validity and value of the Hose Stream Test is questionable and it has been removed from most international fire test methods, the Hose Stream Test remains a part of the fire resistance test methods used in the United States today. Its purpose is, and has always been, to evaluate physical stresses encountered during the period of fire exposure, not to evaluate performance after fire exposure. There is no added significance to the fire rating from using the “optional program” method, nor is the significance of the fire rating diminished by using the “standard” method.
Copies of ASTM E 119 are available from ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959
Water-Based Spray Textures on Gypsum Board Ceilings
The phenomenon of sagging 3/8 inch gypsum board on ceilings framed 16 inches on center and 1/2 inch gypsum board on ceilings framed 24 inches on center after application of water-based ceiling textures is well documented by field reports and laboratory experiments. A viable, low cost solution to the condition is to increase the minimum thickness of the gypsum board to 1/2 inch gypsum board for framing spaced 16 inches on center and 5/8 inch for framing spaced 24 inches on center. This recommendation has been reflected in gypsum industry literature for many years. The Gypsum Association is not aware of any scientific evidence that would warrant a revision to this recommendation.
Texturing compounds have been used for decades as an attractive decoration for ceilings in residential and commercial buildings. Not only does the wide variety of patterns offer dramatic aesthetic appeal, but textured ceilings also serve the functional purpose of enhancing sound absorption and masking minor defects in the finishing of the gypsum board ceiling.
Texturing materials are applied either manually or mechanically. Manual techniques include the use of trowels, brushes, rollers, and sponges. Mechanical application utilizes a power sprayer with adjustable tips to vary the spray pattern, rate of application, and thickness of the coating. The amount of material applied is also dependent on the specific type of material being used, the skill and technique of the applicator, and the density and viscosity of the texture material. Although manufacturers provide instructions regarding the proper mixing of their specific compounds, it is common for the applicator to adjust these proportions based on field conditions and personal experience. The most common adjustment is the addition of water to thin the slurry to increase the application rate and extend the area covered by a given amount of texture. As speed of application and coverage area are increased, savings in labor and materials occur.
By the late 1960’s and early 1970’s textured ceilings were used extensively. With labor costs soaring and construction scheduling of prime importance, textured ceilings became the preferred ceiling finish because of the time needed for plasterers and drywallers to properly finish the gypsum board to achieve smooth flat ceilings. With the advent of lightweight construction methods, less rigid buildings, and subtle changes to framing materials it became tougher to achieve a smooth ceiling; textured ceilings appeared to be the best alternative.
Around the same time, member companies of the Gypsum Association received many complaints from around the country related to textured ceilings. Although mold and mildew growth was responsible for part of the complaints, visible (as opposed to easily measurable) sag was the predominant problem. The term frequently used to describe the condition was “pillowing.” Gypsum board manufacturers believed that the sagging was directly linked to the application of water-based texturing compounds and environmental conditions. Laboratory testing(1,2) confirmed that the application of water-based ceiling textures, either hand or spray applied, adversely affected the sag resistance of gypsum board.
Mechanics and Chemistry of Gypsum Board
Gypsum board is the generic name for a family of sheet products consisting of a noncombustible core primarily of gypsum with paper surfacing. This is the preferred term for gypsum panel products used in the construction industry. Gypsum wallboard is a type of gypsum board that is designed to be used for walls, ceilings, or partitions and affords a surface suitable to receive decoration(4).
Gypsum is a mineral consisting primarily of fully hydrated calcium sulfate. Like most minerals and rocks, gypsum is strongest in compression. When attached to ceiling framing, gypsum wallboard acts like a simply supported beam between supports. All other factors being equal, the strength of gypsum board is directly proportional to thicknes(6,7).
The upper portion of the core of the gypsum board is in compression from the forces attributed to the dead load of the panel, loads applied to the backside of the board (such as by thermal or acoustical insulation, toggle bolt flanges suspending loads below the ceiling, etc.), and gravity. The lower portion of the core is in tension. A thin sheet of pure gypsum applied to a ceiling with an appreciable distance between framing members would not remain flat for long due to rupture as a result of these forces. Gypsum board takes advantage of the strength inherent in the core and enhances this strength by the use of admixtures and high tensile strength paper facers. The face papers effectively act as composite reinforcement to the core and are an important part of the panel’s ultimate strength and performance.
Effect of Water on Gypsum Board
When liberally wetted (such as by the application of water-based texturing materials) or exposed to prolonged conditions of elevated temperature and humidity, gypsum board usually undergoes several complex chemical and physical changes. When wetted, the face paper will absorb the water. This absorption tends to swell and elongate the paper fibers and reduces the tensile and shear strength of the face paper.
Gypsum board, although much more resistant to the effects of water exposure than most people think, does have an affinity for water. The gypsum core may be affected by the water when it is transmitted to the core by the saturation of the paper facers or around cut-outs. Gypsum board exposed to water for prolonged periods may experience reduction in tensile strength and shear resistance due to surface tension, lubrication, and chemical changes to the core. In severe cases, the gypsum may dissociate in the liquid causing further reduction in strength and the ability to recover once the volume of water is removed or reduced. Water also disrupts the critical interface where the paper is bonded to the core thereby jeopardizing the composite strength characteristics of the paper and the core.
Sagging of Gypsum Board Ceilings
In 1973, the Gypsum Association developed a special recommendation advising the use of 5/8 inch gypsum board when ceiling framing was spaced 24 inches on center and spray applied water-based textures were to be applied (8). This special recommendation was developed as a direct result of the previously described complaints and in-depth laboratory testing and analysis of humidified deflection and the effect of water-based textures on ceiling sag. Testing reported by the gypsum board manufacturers indicated that sag problems were dramatically reduced if not eliminated when (a) framing spacing was reduced, (b) proper orientation of the board was maintained (i.e., right angle vs. parallel to framing), and (c) board thickness was increased to 5/8 inch.
In 1975, the Gypsum Association published its version of a specification for the proper installation of gypsum board, GA-216-75. Table 1 of this industry specification indicated that only 5/8 inch gypsum board (applied perpendicular to framing) should be used on ceilings to receive a spray applied water-based texture finish.(10) Today, over 20 years after its first edition, GA-216 has not weakened regarding this recommendation. In fact, the present verbiage of GA-216 clarifies that the recommendation applies to both hand and spray applied textures and that 1/2 inch gypsum wallboard applied perpendicular to framing has been found to be an acceptable base (as opposed to a single layer of 3/8 inch gypsum wallboard) for ceiling textures with framing not exceeding 16 inches on center(11).
To date, no technical data has been submitted to the Gypsum Association which would support a change to this position.
In the late 1980’s, at least one gypsum board manufacturer began marketing a new product designed specifically to be sag resistant. Several other manufacturers quickly followed with similar specialized products. This sag-resistant gypsum ceiling board is typically 1/2 inch thick, but complies with the humidified deflection criteria found in ASTM C 36 for 5/8 inch thick gypsum wallboard. Laboratory tests and model building code acceptance reports indicate that 1/2 inch sag-resistant gypsum ceiling board has been found acceptable for use alone or as a base for ceiling textures with framing spacing a maximum 24 inches on center(12-19).
Prior to the 1995 edition, the One-and-Two Family Dwelling Code (OTFDC), published by the Council of American Building Officials (CABO), referenced GA-216 for the application of gypsum board, thereby including the previously mentioned provisions for the use of gypsum board on ceilings with water-based textures. By 1994, prescriptive installation provisions, including those addressing water-based ceiling textures, replaced reference to GA-216 in the OTFDC. In the mid-1990’s, a contractors’ trade association sponsored proposals to change the OTFDC to permit the use of 1/2 inch gypsum wallboard on ceilings textured with water-based texturing materials with framing spaced 24 inches on center. (20,21) The Gypsum Association vigorously opposed these changes. In all cases, the CABO Code Development Committee overwhelmingly rejected the proposals.
In 1997, the new International Residential Code (promulgated by the International Code Council) began development (22). A proposal was again submitted by a contractors’ trade association to permit the use of 1/2 inch gypsum wallboard with ceiling framing spaced 24 inches on center using water-based textures. The Code Development Committee, relying in large part on the technical information presented here, rejected the proposal and amended the draft to include proposed verbiage that recognized the use of 1/2 inch sag-resistant gypsum ceiling board in those locations(23).
For nearly a quarter of a century the Gypsum Association and its member companies have advised against the use of 1/2 inch thick gypsum board on ceilings where water-based spray textures are applied with framing 24 inches on center. This recommendation was derived after multitudes of complaints were received regarding unsightly sag after installation. Laboratory testing was able to replicate many of the complicated and intricate conditions which either cause or exacerbate this problem. The tests derived a substantive and measurable difference between the performance of 5/8 inch gypsum board and ½ inch gypsum board for a variety of conditions.
The Gypsum Association maintains that the original data used to support the recommendations was accurate, more recent humidified deflection tests have verified this position, and that the recommendations remain valid today. The gypsum board manufacturers cannot support an installation practice that violates and is contrary to longstanding, uniform, and technically sound industry recommendations. Further, it is neither in the best interest of the general public nor sound construction practice for building codes to contradict technically valid industry specifications and recommendations. The Gypsum Association urges rejection of any proposals made to the model building codes, state and local amendments to building codes, and product application standards that would permit this sub-standard installation practice.
July 13, 1995
(Revised January 26, 1998 and July 30, 2002)
Questions about this paper may be addressed to the Technical Services Division of the Gypsum Association at 301-277-8686; web site at www.gypsum.org.
1. Mikus, Ralph F. “Sag Tests of 1/2″ and 5/8″ Thick Gypsum Board Attached to Ceiling Trusses Spaced 24″ O.C.” Pittsburgh Testing Laboratory, Report SF-232, P-27-73, September 27, 1973.
2. Mikus, Ralph F. “Sag Tests of 1/2″ and 5/8″ Thick Gypsum Board Attached to Ceiling Trusses spaced 24″ O.C.” Pittsburgh Testing Laboratory, Report SF-232, P-28-73, December 4, 1973.
3. American Society for Testing and Materials. Standard Terminology Relating to Gypsum and Related Building Materials and Systems. ASTM Standard C 11 (97), page 2. West Conshohocken, PA: ASTM, 1997.
4. American Society for Testing and Materials. Standard Specification for Gypsum Wallboard. ASTM Standard C 36 (95b), page 1, section 1.1. West Conshohocken, PA: ASTM, 1996.
5. American Society for Testing and Materials. Standard Terminology Relating to Gypsum and Related Building Materials and Systems. ASTM Standard C 11 (97), page 2. West Conshohocken, PA: ASTM, 1997.
6. American Society for Testing and Materials. Standard Specification for Gypsum Wallboard. ASTM Standard C 36 (95b), page 1, section 4.1. West Conshohocken, PA: ASTM, 1996.
7. Entz, Richard P. Sag Resistance of ½-inch Gypsum Board: Finished with a Heavy Water-Based Texture (Ceiling Application). Building Standards – Part 1. International Conference of Building Officials, May – June 1975.
8. Gypsum Association. Application of Gypsum Wallboard on Ceilings to Receive Water-Based Spray Texture Finishes. GA-215-73. Chicago, IL: Gypsum Association, 1973.
9. Gypsum Association. Recommended Specifications for the Application and Finishing of Gypsum Board. GA-216-75. Evanston, IL: Gypsum Association, 1975.
10. Ibid, Page 4.
11. Gypsum Association. Recommended Specifications: Application and Finishing of Gypsum Board. GA-216-96. Washington, DC: Gypsum Association, 1996.
12. National Evaluation Service, Inc. Report No. NER-458. Falls Church, VA: Council of American Building Officials, 1993.
13. National Evaluation Service, Inc. Report No. NER-496. Falls Church, VA: Council Of American Building Officials, 1994.
14. BOCA Evaluation Services, Inc. Research Report No. 93-21. Country Club Hills, IL: Building Officials and Code Administrators International, Inc., 1993.
15. SBCCI – Public Safety Testing and Evaluation Services, Inc. Report No. 9387. Birmingham, AL: Southern Building Code Congress International, Inc., 1993.
16. SBCCI – Public Safety Testing and Evaluation Services, Inc. Report No. 93123. Birmingham, AL: Southern Building Code Congress International, Inc., 1993.
17. SBCCI – Public Safety Testing and Evaluation Services, Inc. Report No. 94110. Birmingham, AL: Southern Building Code Congress International, Inc., 1996.
18. Canadian Construction Materials Centre. Evaluation Report CCMC 12278-R. Ottawa, ON: National Research Council of Canada, 1995.
19. Canadian Construction Materials Centre. Evaluation Report CCMC 12349-R. Ottawa, ON: National Research Council of Canada, 1995.
20. Council of American Building Officials. 1995 Proposed Changes to the One and Two Family Dwelling Code. R-44-95. Falls Church, VA: Council Of American Building Officials, 1995.
21. International Code Council. 1996 Proposed Changes to the One and Two Family Dwelling Code. R-113-96. Whittier, CA: International Code Council (International Conference of Building Officials, Secretariat), 1996.
22. International Code Council. Minutes of the First Meeting of the International Residential Code Committee, September 23, 1997, Phoenix, Arizona. Whittier, CA: International Code Council (International Conference of Building Officials, Secretariat), 1997.
23. International Code Council. Minutes of the Third Meeting of the International Residential Code Committee, November 20-22, 1997, Rosemont, Illinois. Whittier, CA: International Code Council (International Conference of Building Officials, Secretariat), 1997.
Residential Job-Site Disposal of New Construction Waste Gypsum Board
The Gypsum Association recommends the following procedures for disposal of job-site new construction waste gypsum board on residential building lots based on information derived from scientific studies.
1. Waste gypsum board to be disposed of on site should be pulverized so that all pieces on the soil surface, including paper, will disintegrate in a reasonable period of time under local precipitation levels and other climatic conditions. This suggestion generally means that all pieces of waste gypsum board, including paper, placed on a residential building lot will be equal to or smaller than one-half inch square or in diameter.
2. Pulverized waste gypsum board may be placed on the soil surface or mixed with the top layer of the soil.
3. Waste gypsum board should be spread evenly over the entire lot where conditions of terrain and landscaping considerations permit.
4. Application may be at rates up to the equivalent of 22 tons per acre.
5. Pulverized waste gypsum board should be disposed of only on lots or in areas that have adequate drainage and aeration, i.e., no standing water or anaerobic conditions should exist until the waste gypsum board has completely disintegrated.
6. State, local, and federal regulations and statutes should be considered so as to ensure compliance with all environmental and other governing ordinances and rules that allow these types of utilization for waste gypsum board or if special permission is necessary to dispose of construction waste gypsum in this manner.
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During the past several years, drywall contractors, painters, and others involved with finishing gypsum board surfaces have occasionally encountered a phenomenon that has become known as “paint yellowing.” “Yellowing” is generally used to describe the characteristic; however, in many cases the manifestation of the problem is not yellowing, but rather a general, nondescript discoloration that often appears to be pink, tan, or some other color. Discoloration generally shows up only on the surface of the gypsum board and not in areas that have been coated with joint treatment compound. Discoloration of this nature is extremely rare but has been reported in widely differing areas of both the United States and Canada.
Historical Recommendations for Finishing/Decorating Gypsum Board
Since the 1950’s, standards relating to the finishing of gypsum board have strongly indicated that a sealer and primer be used to cover gypsum board surfaces prior to painting. At least as far back as 1953, the American Standards Association (ASA) recommended that the “surfaces be sealed and primed as required by subsequent finish” whenever gypsum board was to be painted (Standard Specifications for Gypsum Wallboard Interior Finishes, ASA A97.1-1953). Members of the ASA Project A97 Committee that developed this standard represented the Painting and Decorating Contractors of America Association, drywall contractor groups, the gypsum board industry, and a wide range of other interested organizations and individuals. This ASA recommendation remained relatively unchanged until 1965 when specific drying conditions (temperature and humidity limitations) were added along with a special caution to “apply sufficient quantity to assure that the surface is completely covered” and “do not over-thin.” The 1965 ASA recommendations also suggested the use of pigmented primers under certain conditions and advised that drying periods of as much as 36 to 48 hours may be necessary between coats of sealer in humid or cold weather and 12 to 18 hours under normal conditions. The gypsum manufacturers began to address the matter with publication of the 1978 edition of the Gypsum Association’s Recommended Specifications for the Application and Finishing of Gypsum Board, GA-216. The Gypsum Association’s recommendation called for priming or sealing the surface “with specific primer or sealer recommended by paint or texture manufacturer for application over gypsum board.” Skim coating with joint treatment compound was also recommended for surfaces that were to be decorated with glossy paints. The 1985 edition of GA-216 recommended skim coating to reduce potential problems in areas that would be subjected to direct and severe lighting conditions. In 1989 the Gypsum Association’s GA-216-89 contained an expanded recommendation regarding the use of primers and sealers; the recommended product was a “high quality, heavy bodied primer/sealer of a type recommended by the paint or texture manufacturer.” Current Research. When it occurs, the discoloration becomes a problem that is difficult and expensive to correct. Gypsum board producers, the paint industry, and paper manufacturers, have attempted to definitively ascertain the cause of this problem and thereby uncover a solution. Thus far the cause of the problem remains elusive; however, the phenomenon appears to be the result of a mixture of interactions. Most investigators agree that high humidity and prolonged drying times are major contributors to the appearance of discoloration. Tighter buildings designed for greater energy efficiency appear to cause elevated humidity during the drying periods, thereby prolonging the time which paints need to adequately dry. The amount of moisture in the air is also a function of the use of fossil-fuel fired space heaters during construction. These devices literally pump water vapor, a byproduct of combustion, into the air during the day. This airborne water vapor translates into elevated relative humidity which becomes even higher as the heaters are turned off for the night and ambient temperatures drop. The net result of using these heaters is to prolong drying times for the paint and texture materials.
A lack of understanding of the cause(s) and cure unfortunately occasionally generates ill will between the various trades and product manufacturers who have to deal with the problem when it occurs on a job site. Although the current intensity and frequency of paint discoloration seems to be a relatively recent development in decorating gypsum board, specific recommendations for priming and/or sealing have been common to the industry for many years. While the search for a suitable technical solution continues, the gypsum board industry’s best thinking on this matter is detailed below and is offered to aid those contractors and applicators who wish to minimize the possibility that discoloration will occur on their jobs. The gypsum board manufacturers, as well as all other parties involved in manufacturing and applying the materials used for decorating gypsum board, including the paint manufacturers and the organization representing the paint applicators, have made the recommendation for a quality sealer/primer coat for more than 40 years. Following the recommendation for applying a quality primer/sealer may not totally eliminate the discoloration problem, but the feeling is that a thorough priming and/or sealing of the surface with a high quality product prior to final painting seems to be the most widely accepted and effective solution at this time. The current recommendation of the gypsum industry appears on page 16 of GA-216-2000: “Gypsum board surfaces to be painted or textured shall be primed with a good quality, heavy bodied primer of a type specified by the paint or texture manufacturer.” Recommended Levels of Gypsum Board Finish (GA-214) was jointly published in 1990 (and subsequently revised) by the Painting and Decorating Contractors of America (PDCA), the Association of the Wall and Ceiling Industries – International (AWCI), Ceilings & Interior Systems Construction Association (CISCA), and the Gypsum Association. This cooperatively-developed special recommendation publication advises that a “good quality, white, latex primer/sealer formulated with high binder solids” be applied undiluted. In addition to following the formal recommendations of the associated drywall industries, it may be helpful if decorators are aware of other potential causes and take steps to mitigate those problem areas. For example, anecdotal evidence exists that possibly links part of the discoloration to natural light; therefore, precautions may be needed to keep direct sunlight from falling on freshly decorated walls until they are thoroughly dry. Much of the current evidence links the discoloration to simultaneous high humidity and temperature levels, especially where space heaters have been used that create additional moisture in the area. When drywall is decorated under high humidity and temperatures, drying times are extended and special efforts must be taken to permit more time between applications and to ensure that adequate drying is allowed to take place. In addition to allowing more drying time between coats of paint, finishers should provide conditions that are conducive to more rapid drying, such as positive ventilation, nonfossil-fuel heat, dehumidification, or using the buildings’ HVAC system during paint or texture application and drying. Summation. Persistent problems such as paint discoloration necessitate cooperative efforts from all parties involved in order to reach a satisfactory solution. The gypsum board industry, the paper producers, and the paint manufacturers will continue to search for a technical cause and solution to the problem. Until these research efforts pay dividends in the form of an explanation and suggestion as to how to further mitigate the potential for discoloration, drywall finishers and decorators should be aware of the potential problem and take steps to prevent the possibility of discoloration as much as possible by following the recommendations in this paper. The gypsum industry is committed to continuing its investigation into the problem and is working with the paint industry to definitively find the causes and to develop workable solutions.
Control Joints in Gypsum Board Construction
Control joints in gypsum board/frame construction are designed to reduce the potential for cracking in gypsum board walls and ceilings resulting from movement of the system from any one of several sources. The primary sources of movement are dimensional changes in framing related to changes in temperature and humidity, both of which affect the dimensional stability of either wood or metal framing and other structural components. Control joints are not the same as building construction joints which are designed to accommodate significant movement of adjacent structures. Since control joints are visible after installation, the precise placement of the joints should be specified by the designer, who best knows the overall aesthetic objective of the space. Control joints in gypsum board systems should be installed where indicated on the plans or specifications. Gypsum Association recommended specifications and manufacturer’s recommendations specify minimum requirements for the installation of control joints. Control joints in the gypsum board shall be specified where any of the following conditions exist.
- A partition, wall, or ceiling traverses a construction joint (expansion, seismic, or building control element) in the base building structure.
- Where a wall or partition runs in an uninterrupted straight plane exceeding 30 lineal feet. (NOTE that a full height door frame may be considered a control joint).
- Interior Ceilings With Perimeter Relief: Control joints shall be installed so that linear dimensions between control joints shall not exceed 50 ft and total area between control joints shall not exceed 2500 sq. ft. A control joint or intermediate blocking shall be installed where ceiling framing members change direction.
- Interior Ceilings Without Perimeter Relief: Control joints shall be installed so that linear dimensions between control joints shall not exceed 30 ft and total area between control joints shall not exceed 900 sq. ft. A control joint or intermediate blocking shall be installed where ceiling framing members change direction.
- Exterior Ceilings: Control joints shall be installed so that linear dimensions between control joints shall not exceed 30 ft and total area between control joints shall not exceed 900 sq. ft. A control joint or intermediate blocking shall be installed where ceiling framing members change direction.
Water-resistant Gypsum Backing Board on Ceilings
For many years, the gypsum industry recommended against the use of water-resistant gypsum backing board (MR board or “green” board) on ceilings. This position was based on a concern that the materials added to the gypsum core to provide water resistance created a gypsum board that was slightly less resistant to sagging under conditions of high humidity. In recent years, the use additives has been fine tuned to the point where this concern is less pronounced.
The industry has revisited this issue by conducting a series of tests to evaluate the sag resistance of modern water-resistant gypsum backing board. The results of these tests have found that modern water-resistant gypsum backing board can be applied to ceilings with confidence when the ceiling framing members are spaced closer together than the traditional 24 or 16 inches on center.
The sag resistance testing has led to the establishment of recommendations for the use of water-resistant gypsum backing board on ceilings which are published in Specifications: Application and Finishing of Gypsum Board (GA-216-2007), and which are stated as follows:
14.3.2 Water-resistant gypsum backing board shall be permitted to be used on ceilings where ceiling framing is spaced not more than 12 in. o.c. (305 mm) for ½ in. (12.7 mm) thick water-resistant gypsum backing board and not more than 16 in. o.c. (406 mm) for 5/8 in. (15.9 mm) thick water-resistant gypsum backing board.
Using Screws as an Alternative to Nails in Rated Wood Stud Walls
In many areas of the country there is a trend to move away from the use of nails toward the exclusive use of screws or a combination of screws and nails for installing gypsum board on wood framing. Screws allow faster application and more consistent quality in the finished wall or ceiling. This practice, however, has raised the question of the effect of using screws on fire rated systems. Screws are spaced farther apart than nails and they are usually not as long as nails.
Over the years, many fire resistance tests have been conducted on walls with gypsum board nailed to wood studs. There have also been many tests conducted where screws were used to attach the gypsum board to both wood and metal framing. Both attachment methods have proven to be effective in holding the gypsum board on the studs in fire resistance tests of wall systems of 1 and 2 hour duration.
Screw heads are larger than nail heads, providing a greater bearing surface to support the gypsum board. Additionally, the threaded shank of the screw, even though shorter than the nail shank, provides greater holding power than a nail.
A review of fire tests on both screw and nail attached systems has led a nationally recognized independent testing laboratory to conclude that Type W screws spaced a maximum of 12 inches on center can be substituted for nails spaced 7 or 8 inches on center for the attachment of gypsum board to wood studs in fire resistance rated wall systems rated at one hour or less. The conclusion of this review also applies to the use of a combination of screws and nails with nails located at each stud along horizontal joints and 7 inches on center at vertical joints, and screws 12 inches on center to intermediate framing. Blocking at horizontal joints is not required. Screws used for this application must penetrate the wood studs a minimum of 5/8″.
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Potential Winter Construction Problems
Precautions are needed on the construction site to avoid potential problems associated with cold and damp weather conditions. Neglecting caution during cold and damp weather can contribute to avoidable problems in gypsum board construction. Joint compound bond failure, delayed shrinkage, beading, nail popping, joint shadowing, and gypsum board sagging occur more often in jobs built in the winter than in any other time of the year, even though the difficulty may not be visible until after the spring thaw. Taking the following precautions in the winter, during construction, is usually far less costly than coming back in six months to repair the results.
Observing the following precautions during periods of cold and damp weather will reduce job problems in the months to follow.
- installing gypsum board to frozen, frost covered, or damp surfaces.
- temporary heat as needed to maintain room temperatures at or above 40°F (5°C) for mechanical attachment of gypsum board and 50°F (10°C) for adhesive application of gypsum board; for joint treatment, texturing, and decoration; and when mixing materials used for joint treatment or laminating layers of gypsum board.
- water-based materials, such as ready-mixed joint compounds and textures, from freezing.
- temporary heat for a minimum of 48 hours before, and continuously until applied materials are thoroughly dry.
- ventilation to assure normal drying conditions and to eliminate the unusually high humidity caused by certain types of temporary heating units. Do not allow local temporary heat to raise room temperatures higher than 95°F (35°C).
- setting type joint compound rather than ready-mixed compounds.
- the proper thicknesses of gypsum board for ceilings to avoid sagging when textures are to be applied.
- a latex primer to and allow to dry prior to any decorating. This could take up to 48 hours or longer when low temperatures and/or high humidity conditions are present.
- foil-backed gypsum board or vapor retarder faced mineral or glass fiber insulation where vapor retarders are required.
- a polyethylene vapor retarder is installed behind gypsum board on ceilings, install batt or blanket insulation BEFORE the gypsum board. If loose fill insulation is used, install the insulation IMMEDIATELY after the gypsum board is installed.
- cold temperatures and dampness that come with winter are not isolated to the northern climates. Remember that low temperatures and high humidity are ideal conditions for moisture condensation on cool surfaces. Following the above precautions during this time of year is as important in Tallahassee or Phoenix as it is in Seattle or Yellow Knife.