For more than 40 years, truss partition separation, often loosely referred to as “truss uplift,” has been a practical (and annoying) issue for homeowners and homebuilders. While it was first investigated at the field level and researched in the laboratory in the 1970s, an October 2020 JLC Online post, “Crown Detail Hides Truss Movement,” reminded me of the issue and its history.
Can Truss Movement Contribute?
As a PhD graduate student at Purdue University in the 1970s, I had the opportunity to be involved in possibly the first structural analysis of this phenomenon with the type of truss software used today by plate companies and component manufacturers (CMs). The late Professor Don Percival at the University of Illinois had observed trusses that had separated from a partition in the winter months up to ¾-inch and shared this information with the late Dr. Stan Suddarth. In some cases involving major separation, Don observed ice/frost on the top chord and bottom of the roof sheathing. Knowing it was during the heating season and having the presence of ice/frost on the top chord and sheathing, we theorized that the very “odd condition” was due to expansion of the top chord and shrinkage of the insulated and warm bottom chord, due to it being dried or kept dry during the heating season.
It was then my job to simulate different levels of chord expansion and shrinkage in Stan’s pioneering computer program (known as the PPSA). By making a range of assumptions which dictated the amount of chord expansion and shrinkage that was possible, for one case the program predicted “arching” up to ¾-inch which matched a reported maximum amount. The maximum arching was predicted by assuming 100% juvenile wood in one chord, coupled with assuming a maximum differential in moisture content of the top and bottom chords. The structural model assumptions that predicted the ¾-inch arching represented a “worst case scenario”, possible but not likely. While the longitudinal shrinkage rate of lumber is low for “mature” wood, it can be up to 10 times more for juvenile wood. The innermost growth rings are referred to as juvenile wood, whereas the outer rings are referred to as mature wood.
Causative Factors of Truss Partition Separation
Seasonal gaps at truss and partitions can be caused by a multitude of factors, from the differential movement of footings to the most simple, such as venting a bathroom fan or clothes dryer into the attic. Substantial educational and research information has been published on the subject of truss partition separation, such as the classic by wood truss legends Percival and Suddarth. This report, Ceiling-Floor Partition Separation Studies, by Prof. Don Percival, Dr. Stan Suddarth and Quentin (Louie) Comus (1982), contains comprehensive laboratory testing data of six different truss types (King-post, Fink, Howe, Mono, WW, Scissor), four typical truss lumber species groups (S. Pine, DFL, SPF, and HF), and varied moisture content levels of the top and bottom chords over a period of three years.
Probably the most striking information from the Percival–Suddarth research stemmed from the field data from actual cases of truss partition separation. The researchers collected relevant data from 731 reports (as well as personal observations) on homes that had experienced truss partition separation in 26 states over a period of about 10 years. In their 1982 report, the authors concluded the following: “It should be pointed out that all instances cannot be verified as the result of truss 'arching', nor can the phenomenon be attributed to one single cause.”
Their conclusion makes sense for single family residences as it is well known that framing lumber (roofs, wall assemblies, floors, girders) dimensions change in length and width due to the changing temperature and humidity history. And equally important, foundation sections differentially move up and down during seasonal changes based on changes in soil moisture content and the amount of expansive soil material present. Given a certain partition gap such as 3/8-inch, I believe it would be very difficult to investigate a (recurring) case and conclude that the gap observed was caused by some combination of structural elements, such as 50% X, 35% Y, and 15% Z. In some limited cases, it might be possible to reasonably conclude that the bulk of the problem was caused by a single factor, such as venting a dryer into the attic.
Addressing the Truss/Partition Area During Construction
Twenty-five years ago, the Structural Building Components Association (previously known as the Wood Truss Council of America) initially published a two-sided informational pamphlet on how to proactively address the potential for seasonal movement of framing and foundations. This pamphlet has since been republished by SBCA in the form of an SBCA Research Report Summary Sheet, “Partition Separation Prevention and Solutions,” which can be found on SBCA’s website at: https://docs.sbcindustry.com/sample/11. On the second page, this pamphlet has four drawings/details on how homebuilders and general contractors can install roof truss framing and drywall whereby unsightly gaps at non-load bearing partitions would be concealed. Without these details in the hands of the framing and drywall subcontractors, it is likely the trades will perform their work contrary to wood truss industry recommendations, potentially resulting in visible gaps at partitions and subsequent complaints.
[See PDF or View in Full Issue for image: Structural Building Components Association Copyrighted 2004–2020 details for preventing an unsightly gap at gypsum corner of a non-load bearing wall.]
It is not surprising that the complaints might eventually come to the Component Manufacturer (CM), since it would appear to a layman that the truss is (oddly) moving upwards. Being that a truss is involved, I would expect the CM would be viewed by builders and others as the most knowledgeable person on the subject of truss behavior. As such, I believe that non-loading bearing wall “connections” should be addressed by the CM in typical truss package documents similar to how load bearing walls are typically addressed.
Stabilizing Non-Load Bearing Walls
Naturally and due to the consequences of failure, the industry has focused on providing construction information for gravity and uplift forces at “load bearing” walls. Even though not required by the truss design standard ANSI/TPI 1, I have seen truss package documents that included toe-nail tables and required hurricane hangers. Knowing that the 2015 IBC (Section 1607.14) has a minimum horizontal load requirement of 5 psf, the residential (and commercial) framing contractor is faced with using their experience and local practice for stabilizing interior “non-load bearing” walls to the engineered wood trusses using a couple of toe-nails.
A Proactive Approach
For CMs that supply residential trusses in areas that are prone to truss partition separation issues, I suggest the two-page SBCA pamphlet containing the four industry details for non-load bearing walls be included in the “truss package documents,” along with any other construction requirements or recommendations. In my experience involving disputes, the contents of the truss package documents often matter. It is important for the general contractor and subcontractors involved to have access to the information for each project and that the delivery of the same is documented.
Additional Details for CMs to Consider
Including two details from BCSI (2019) is another option to consider for your “truss package documents” aimed at helping the framing and drywall contractors construct the truss–wall partition connection with the objective of preventing unsightly gaps at non-load bearing walls. BCSI Figures B8-15 and B8-16 from the bottom of page 68 are shown below [See PDF or View in Full Issue for image: The two BCSI 2019 details and accompanying text indicate that a “Floating Connection” is required to address the potential for unsightly in-service gaps between the ceiling and wall gypsum.]
The BCSI details have two advantages over the originally published SBCA details. At the field level and for building inspectors, BCSI is widely recognized as a “truss industry standard.” And perhaps the more important aspect of using the page 68 BCSI details is that BCSI is a “referenced standard” in the IRC. The fact that a document is a “referenced standard” in a model code, such as the IRC, elevates the status of the document in being a potential requirement by a local jurisdiction and potentially useful for designers and contractors that voluntarily adopt specific provisions in defense of their design or work.
Some CMs may not be aware of the reference to BCSI in the 2015 IRC. The language from the wood truss bracing section follows:
“R502.11.2 Bracing. Trusses shall be braced to prevent rotation and provide lateral stability in accordance with the requirements specified in the construction documents for the building and on the individual truss design drawings. In the absence of specific bracing requirements, trusses shall be braced in accordance with accepted industry practices, such as, the SBCA Building Component Safety Information (BCSI) Guide to Good Practice for Handling, Installing & Bracing of Metal Plate Connected Wood Trusses.”
This section establishes that, for contractors who use the BCSI details to address the potential of truss partition separation, their framing and gypsum installation work is recognized by the IRC as being a code compliant option. Without using the BCSI details, the burden of the contractors in defending their installation work as an “accepted industry practice” falls on the contractors involved.
As a side point, the BCSI book (101p.) provides a treasure of practical bracing design and safety information for contractors, engineers, and the building code community. I have been using copies of the book for several years in our Virginia Tech continuing education programs and other programs involving the building code community.
Concluding Remarks
While searching for literature on truss partition separation, I found the April 2020 thoughtful article by Evan Protexter at the SBCA, “Don’t Immediately Offer to Repair!”. Regarding the suggested list of questions when a CM gets a “call asserting that truss uplift is causing partition separation,” a different sequence of questions may be appropriate if the CM’s policy is to include the two-page pamphlet with the four SBCA recommended truss partition installation details or the two BCSI details in their truss delivery package. I believe the benefit of including one or both of these documents makes it easier to address complaints since a discussion could logically focus on whether or not the long-standing truss industry recommended details were implemented.
Additional literature from the 1980s that remains relevant today includes the following:
Interested readers are welcome to contact me for additional published information on the subject.
Frank Woeste, P.E., Ph.D., is Professor Emeritus, Virginia Tech, and serves as the consumer representative to the ALSC National Grading Rule Committee and is a member of the committee for the revision of ANSI/TPI 1–2014, National Design Standard for Metal Plate Connected Wood Truss Construction. Frank frequently consults with the public, design professionals, contractors, and building code officials on various aspects of engineered wood construction and residential construction.