Detailing Specific Cladding Requirements for Mid-Rise Wood-Framed Buildings Warren R. French, RBEC, F-IIBEC, PE French Engineering, LLC 8900 Eastloch Dr., Ste. 130, Spring, TX, 77379 281-440-8284 • warren.french@frenchengineering.com IIBEC 2020 Virtual International Conve ntion & Trade Show | June 12-14, 2020 French | 109 Warren R. French is a registered professional engineer in 20 states, as well as a Registered Building Enclosure Consultant (RBEC) through IIBEC. He has extensive experience with design peer reviews, as well as numerous forensic investigations pertaining to exterior cladding systems for hundreds of mid-rise, wood-framed, multifamily residential projects. 110 | French IIBEC 2020 Virtual International Conve ntion & Trade Show | June 12-14, 2020 ABSTRACT SPEAKER Multistory wood-framed buildings are ubiquitous in modern construction throughout North America, particularly for multifamily residential construction, but also for light commercial construction. Specific cladding problems can occur if special attention is not given to appropriately detail these building enclosure interfaces. This paper and presentation discuss the causes for these problems, the lack of existing detail development that is currently found within the industry, and the need for such detailing for various cladding types. The current state of the industry does not formally recognize or address the issue of shrinkage and elastic shortening of the vertical members within these structures. INTRODUCTION Virtually everyone recognizes that the design and detailing requirements for single- story or two-story buildings and those required for multistory buildings are quite different from one another. This is particularly so for wood-framed multistory buildings. One aspect of the design for multistory, wood-framed buildings that may be different is the cumulative effects of wood shrinkage and elastic column shortening of the vertical wood elements such as columns, studs, and perimeter members. This shrinkage and shortening of the structural framing unavoidably results in changes to the building height either during or shortly after erection and construction of the building frame. Requirements for accommodating changes in building height due to moisture shrinkage and elastic column shortening have been codified within specific construction requirements for various aspects of certain building components and subsystems, while it appears that other portions of the building and required construction have not formally addressed this issue. This is particularly so when the construction sequence involves completing (or nearly so) the exterior vertical cladding and roofing in order to “dry-in” the building, then following that dry-in with the subsequent dead loads of utilities, such as mechanical and plumbing systems, as well as dead loads from interior finishes, such as drywall, cabinets, fixtures, ceilings, flooring materials, etc. See Figure 1. Ultimately, these finished spaces will be filled with furniture and the live loads of people living or working within these buildings. While the structures are more than likely engineered to safely accommodate all of these loads, it should be noted that the structural response would not be without some effects on the framing, whether temporary or permanent. Statement of Problem Our investigations of the exterior cladding systems on numerous multistory, wood-framed buildings have revealed that proper details for accommodating changes of the building height within new cladding systems have been lacking and/or completely overlooked on a number of newly constructed facilities. We have observed problems for cladding systems composed of portland cement plaster (stucco), exterior insulation and finish systems (EIFS), metal panels, and even fiber cement lap siding. This paper will identify and describe the problem and suggest detailing and design methods to accommodate these issues. Multistory, wood-framed buildings are ubiquitous throughout North America in modern construction— particularly for multifamily residential construction, but also for light commercial construction. Specific cladding problems can occur if special attention is not given to appropriately detailing these building envelope interfaces. This paper discusses the causes for these problems, the lack of existing detail development that is currently found within the industry, and the need for such detailing related to various cladding types. BACKGROUND AND RESOURCES Prior Resources There are some guides available within the industry to address the issue of shrinkage and elastic shortening of the vertical members within these structures. The most specific resource is a technical bulletin entitled “Accommodating Shrinkage in Multi-Story Wood-Frame Structures,” which is available from WoodWorks™ as part of the Wood Products Council.1 There is also a 2015 webinar available on the WoodWorks™ website that covers some of these same issues. The purpose of this paper is not to duplicate those resources, but rather to supplement them with respect to exterior cladding systems. Accordingly, we will not be presenting the method by which the wood shrinkage can Detailing Specific Cladding Requirements for Mid-Rise Wood-Framed Buildings IIBEC 2020 Virtual International Conve ntion & Trade Show | June 12-14, 2020 French | 111 Figure 1 – Typical exterior wall section. be estimated or calculated, but will start with the concept that those structural effects can be developed. The question then becomes how to accommodate them or mitigate their effects with respect to the cladding system. Even the technical paper by WoodWorks™ simply states that for wood-framed buildings of three or more stories, “cumulative shrinkage can be significant and have an impact on the function and performance of finishes, openings, mechanical/electrical/plumbing (MEP) systems, and structural connections.” However, conspicuously absent from that list is the exterior building enclosure or cladding system. Likewise, some mention of wood shrinkage occurs in the 2015 International Building Code (IBC), stating that an analysis of shrinkage must be conducted for wood walls and bearing partitions that are greater than two floors in height such that the evaluation “shows that shrinkage will not have adverse effects on the structure or any plumbing, electrical, or mechanical systems, or other equipment installed therein due to excessive shrinkage or differential movements caused by shrinkage.”2 Again, there is no mention of the exterior building enclosure or cladding system. Previously, the Western Wood Products Association (WWPA) released a technical bulletin in 2002 that specifically dealt with shrinkage calculations for multistory wood frame buildings, which may be the basis for some of the later papers.3 Wood Science Wood shrinkage occurs due to the fact that wood is hygroscopic (has the ability to absorb and release moisture) and results in potential dimensional changes corresponding to the moisture content (MC) and the fiber saturation point (FSP). However, detrimental wood shrinkage primarily occurs perpendicular to the grain, affecting its width and depth. Longitudinal shrinkage is generally thought to be negligible and is often neglected in the calculations of vertical shrinkage. Accordingly, wood shrinkage is concentrated at the wood plates, sills, floor and roof joists, and rim boards. See Figure 2. Depending on the materials used and the details of the assembly, typical shrinkage within light-frame wood construction can range from 0.05 to 0.50 in. per level.4 Our experience indicates that common construction assemblies and configurations of platform framing methods will easily result in vertical movements of 0.25 to 0.30 in. per floor level, when considering both shrinkage and elastic column shortening, with shrinkage making up the great majority of that movement. Although wood shrinkage is observable in both cross-sectional dimensions, the amount of shrinkage is not isotropic and generally shrinks more in the direction tangential to the growth rings. Calculations can theoretically take into account exact grain orientation and species-dependent shrinkage coefficients; however, it is impossible to predict the grain orientation of the various assembly members in the field. Therefore, an average of the two coefficients is typically used, and a simplified analysis may take this shrinkage to be one percent in cross-sectional dimension for every four percent moisture content change.5 The three variables with the most influence on the magnitude of the shrinkage are: 1. Initial or installed MC 2. In-service MC, or equilibrium moisture content (EMC) 3. Cumulative thickness of cross-grain wood elements. Obviously, for any project, these variables must be estimated and reasonable values used for the calculations in order to achieve an estimate of movement that is close to that to be experienced in the field. Accordingly, an evaluation should be made regarding whether kiln-dried wood was specified and utilized, whether the wood materials had been subjected to additional moisture or high humidity during construction, and what a reasonable equilibrium content will be after the structure is dried-in and the HVAC system has had an opportunity to remove construction moisture to certain levels. It is not uncommon to have in-service equilibrium moisture contents from 7 to 15 percent. The reader is referred to the WoodWorks™ paper or the National Design Standard (NDS) for the method and information on how to conduct these calculations. DIFFERENTIAL MOVEMENT When considering the effects of shrinkage within the wood framing, it is necessary to also consider the differential movement between the framing and other 112 | French IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 Figure 2 – Typical framing section at floor line. building materials and components. These other materials may exhibit significantly different shrinkage and swelling characteristics, such as: 1. Expansion due to moisture and/or thermal changes (brick veneer) 2. Overall thermal expansion and contraction (steel components, PVC piping, etc.) 3. Materials with little to no significant shrinkage Among the materials that we would consider to be primary in this evaluation are the cladding system, as well as how it is supported, how it is constructed, and how it is connected to, or interfaces with, the wood-framed structure. We will look at several situations where various cladding materials and/or systems were adversely affected by a lack of accommodation for differential movement. Brick Masonry It is generally well known that clay brick masonry will irreversibly increase in size with respect to time and moisture absorption, with some portion of that size increase occurring for several years after production of the brick masonry. The Brick Industry Association (BIA) has produced Technical Notes on the subject of volume changes within clay masonry, and has also suggested means for accommodating expansion from thermal expansion as well as from moisture volume changes.6 When the exterior cladding of a wood-framed structure is brick veneer, then a certain amount of differential movement must be accommodated within the interface of these materials. Generally, this differential movement can be ameliorated, particularly for two- or three-story buildings, by the use of a soldier course under a soffit that has been provided with extra head room. However, this becomes more problematic for buildings greater than three stories, since the differential movement is greater and it is difficult to achieve that much extra head room. The same is true at window and door openings, where head and sill flashings will require particular attention to detail in order to accommodate this movement. Specifically, if the brick veneer wall is supported on the concrete foundation and the brick veneer at the various floors is supported at openings by loose-laid lintels, this assembly creates a relatively rigid structure with respect to gravity loads, which will not shrink or deflect, and will generally increase in height over time. Compare that to a wood-framed structure that shrinks over one inch in height, and the differential is sufficient to fail sealants, bend flashings, deform window frames, and crush building materials. See Figure 3. Another particularly prevalent problem is the interface between masonry and framing where the brick veneer is only partial height and transitions to a different cladding material at one of the upper floors. Usually, these transitions will be provided with a flashing to discharge incidental water from the upper cladding. However, if installed too soon or too rigidly to accommodate the differential movement, the flashing will “settle” with the wood framing, resulting in a reverse slope that directs water back toward the building. This can cause significant problems if the flashing is not constructed with end dams at openings and changes in direction, as well as where running laps may not be fully sealed or are damaged due to the movement. See Figure 4. IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 French | 113 Figure 3 – Deformation of windowsill. Figure 4 – Displacement of brick rowlock course and sheet metal flashing. Metal Panels Although not as common traditionally as some other cladding types, decorative metal panels are becoming used more and more as entire cladding systems, or else as one of several materials used in combination on one structure. Metal panels are typically provided in certain panel sizes, and they may be installed either horizontally or vertically. Metal panels are usually attached directly to the structure, or they may be attached to intervening purlins or furring strips of various materials. Metal panel cladding systems will usually incorporate various flashings and sheet metal trim pieces at openings, transitions, and terminations. All of these components and materials must be installed properly with an adequate air barrier and/or weather-resistive barrier in order to achieve a satisfactory cladding system assembly. Our observations have shown that the use of such cladding systems on multistory wood-framed buildings also requires adequate detailing to accommodate the framing shrinkage. Particular attention should be given to trim pieces that span across floor lines, such that the differential movement over time may be accommodated. Similar to sheet metal designed and installed 114 | French IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 Figure 6 – Deformation of vertical sheet metal trim at metal cladding. Figure 5 – Deformation of vertical sheet metal trim at metal cladding. Figure 7 – Displacement of sheet metal head flashing at lap siding. Figure 8 – Lateral displacement of sheathing behind lap siding at floor lines. to accommodate thermal expansion and contraction, the trim pieces should not be popriveted through the lap. See Figures 5 and 6. Lap Siding One of the most popular types of cladding in recent years for multistory woodframed buildings consists of lap siding in various configurations, particularly lap siding composed of cement boards. Accommodation of the thermal movement within the cement board siding is typically achieved by the discreet sizes of the boards and traditional details at openings and terminations. In addition, it would generally be thought that vertical differential movement between the siding and the framing would be accommodated by the small vertical dimensions of the panels and the overlap of the individual boards. In general, this may be true; however, our experience with certain projects indicates that the siding aesthetics and performance may be adversely affected by out-of-plane displacement of the underlying sheathing at floor lines, which is caused by the buckling of the exterior sheathing due to vertical shrinkage. Once again, particular attention should be given to detailing the sheathing attachment and butt joint spacings at the locations of rim boards and floor lines, such that the differential movement caused by the framing shrinkage may be accommodated. See Figures 7 and 8. Plaster and EIFS Another prevalent and popular cladding material used on multistory woodframed buildings—particularly for multifamily residential projects—is portland cement plaster used by itself or in combination with accent bands and trim composed of EIFS materials. In order to achieve “dry-in” of the building as soon as possible, the exterior cladding is often installed as soon as possible after the framing and sheathing has been installed, but this means the cladding (in this case, rather brittle portland cement plaster), will be in place sometime before all of the shrinkage within the framing has taken place. Add to this shrinkage the elastic column shortening, and the amount of stress that the plaster will experience after installation can be significant. While control joint accessories provide a limited amount of movement capability, it is our experience that a true horizontal expansion joint will be required to accommodate the total differential deflection in order to avoid adverse effects to the plaster, its accessories, and associated components. See Figures 9 through 17. The range of IIBEC 2020 Virtual International Conve ntion & Trade Show | June 12-14, 2020 French | 115 Figure 10 – Buckling of corner plastering accessory. Figure 11 – Buckling of plaster corner aid accessory. Figure 12 – Compression of control joint outstanding legs; sheathing displacement. Figure 9 – Buckling of corner plastering accessory. damage that can occur includes horizontal displacement of the plaster and accent components, vertical displacement and “crushing” of horizontal plastering accessories, buckling of vertical plastering accessories, and cracking of the plaster membrane. These adverse effects can be avoided with proper detailing that anticipates and accommodates the shrinkage and differential movement. CONCLUSIONS Shrinkage of the wood framing in multistory buildings is a well-known phenomenon, which can have significant effects on the exterior cladding systems installed on these structures—particularly if these enclosures are installed early on in the construction. While the plumbing code specifically recommends accommodation of the wood framing shrinkage for certain types of piping and other plumbing installations, the only other mention of such shrinkage in the building codes is a general reference to performing an “analysis” to show the shrinkage will not have “adverse effects on the structure or any plumbing, electrical, or mechanical systems, or other equipment.” However, the codes do not specifically mention the effects of shrinkage on the exterior cladding systems, which is an oversight. In addition, architectural designs generally do not provide details related to the cladding system, for such differential movement and horizontal expansion joints are often simply ignored or omitted. Likewise, structural designs make certain 116 | French IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 Figure 13 – Buckling and separation of plaster at control joint in plane of wall. Figure 15 – Buckling of plaster corner bead at outside corner occurring at balcony. Figure 14 – Buckling and separation of plaster at control joint in plane of wall. that the gravity loads, live loads, lateral wind loads, and seismic loads (if required) are properly accommodated, but they often do not address the anticipated shrinkage and elastic column shortening with respect to its effect on the cladding system. In our opinion, consultants specializing in the exterior building enclosure should make this phenomenon known to their clients during the design or remediation of multistory wood-framed buildings and recommend that details be developed and implemented for these systems during construction. Such details should concentrate on the accommodation of vertical movement at the floor lines using horizontal expansion joints that have the capability to absorb this movement without detriment. RECOMMENDATIONS Brick Masonry The steps necessary to achieve proper accommodation of differential movement within wood-framed buildings having a brick masonry cladding are specific to that cladding type. Besides making provisions within the masonry itself, the following items should be considered: 1. Provide a gap between the masonry and the wood framing at the top of the wall that is sufficiently sized for anticipated movement. The gap should be two times the anticipated movement if it occurs between the masonry and a rigid structure (assuming closure with a sealant having a compression capacity of 50 percent). 2. At openings and transitions, provide double flashings that are shingled properly, or else two-piece flashings that can accommodate the anticipated movement. 3. Although it adds dead load to the various members affected, consider supporting the brick veneer on the wood-framed structure. Although this type of configuration is more common with concrete and steel framing systems, it can be implemented with wood, as long as the structural engineer is apprised of Figure 16 – Lateral displacement of sheathing behind plaster and EIFS accent band. IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 French | 117 Figure 17 – Lateral displacement of sheathing behind plaster and EIFS accent band. Shrinkage of the wood framing in multistory buildings is a well-known phenomenon, which can have significant effects on the exterior cladding systems installed on these structures— particularly if these enclosures are installed early on in the construction. the plan and designs the individual members and the support frames for the loads. 4. Consideration could be given to delaying the sealant applications as long as possible, in order to give the structure time to manifest the shrinkage and the differential movement. However, the joint opening would still need to be designed for a majority of the anticipated shrinkage, since delay of the sealant applications cannot wait until absolute completion of the shrinkage process, which can take a year or more, depending on conditions. Metal Panels Metal panels installed vertically may be hard-lapped between floors but must be interrupted at floor lines with a horizontal joint that can accommodate movement. This can be achieved using a through-wall flashing that is fabricated to allow differential movement without distorting the sheet metal. Metal panels that are installed horizontally should lap properly between floors but likewise be provided with a horizontal expansion joint. Any vertical metal trim should be lapped at floor lines but only fastened outside the lap in order to allow differential movement between the trim attached at each floor. Lap Siding The multiple courses of small-width cement board lap siding materials that occur at each floor level are generally adequate to accommodate frame shrinkage without horizontal expansion joints. However, a design decision could be made to include expansion joints at each floor line and could be beneficial for the cladding system. Particular attention should be given to accommodating differential movement and potential horizontal displacement of the sheathing at the floor lines. In this case, the provisions for shrinkage and movement should make certain that the sheathing at the rim boards and perimeter trusses can accommodate the shrinkage and allows space for the movement. Plaster and EIFS Overall, plaster should be designed and installed in accordance with applicable reference standards, such as ASTM C926 and C1063. However, while these standards and guides are necessary for a good plaster wall assembly, they are not sufficient for addressing all of the conditions and configurations occurring within typical multistory wood-framed construction. The author has recommended through-wall flashings at floor lines for plaster cladding installed on multistory buildings of all types of construction for many years.7 For multistory wood-framed buildings, this recommendation takes on special significance and should be followed with special care due to the ubiquitous shrinkage experienced by these structures. Horizontal joints do not necessarily need to include a visible vertical leg as part of the through-wall flashing, but sealant joints will need to include proper spacing and sizing to be effective. See Figure 18. If the differential movement is to be accommodated primarily by sealant joints, the joint size should take into account the anticipated joint movement with a factor of two for sealants with a compression rating of 50 percent. Our experience indicates that this will be a joint size of at least 5/8 to ¾ in. Minimizing Shrinkage of Wood Framing All of the wood associations provide very similar, specific recommendations on how to minimize shrinkage, which are: 1. Avoid storing material where it is exposed to rain or standing water. 2. Keep unused framing materials covered. 3. Inspect building enclosure layers, such as weather-resistive barriers, for proper installation. 4. “Dry-in” the structure as soon as possible, including installation of windows in order to close up these openings. 5. Immediately remove any standing water from floor framing after rain showers. Although not as common as platform framing methods, consideration could be given to potentially using balloon-style framing methods. The balloon framing method avoids many, if not all, of the transection wood members such that the amount of shrinkage associated with these members in minimized. The only shrinkage that then occurs is within the vertical framing members (longitudinal), which is generally negligible. 118 | French IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 Figure 18 – Through-wall flashing/horizontal expansion joint at floor line. FINAL THOUGHTS Wood shrinkage and elastic column shortening can be accommodated through design, but the first step is recognition of the problem and developing appropriate details with the capacity to allow for the differential movement between the framing and the cladding system. It is hoped that the information in this paper provides the necessary identification of the potential problems to provide this first step. REFERENCES 1. Richard McClain and Doug Steimle. “Accommodating Shrinkage in Multi-story Wood-Frame Structures.” WoodWorks™. WW-WSP-10: Washington, D.C. 2017. 2. 2015 International Building Code (IBC). Section 2304.3.3., 2015. 3. “Shrinkage Calculations for Multistory Wood Frame Construction.” Tech Notes No. 10. Western Wood Products Association (WWPA): Portland, OR. November 2002. 4. Op. Cit. WoodWorks™. pg. 1. 5. “Volume Changes – Analysis and Effects of Movement.” Technical Note 18. Brick Industry Association: Reston, Virginia. October 2006. 6. “Accommodating Expansion of Brickwork.” Technical Note 18A. Brick Industry Association: Reston, Virginia. November 2006. 7. Warren R. French. “Evaluation and Remediation of the Building Envelope for Existing High-Rise Buildings.” Moisture Control in Buildings: The Key Factor in Mold Prevention – 2nd Edition; MNL 18-2nd. Heinz R. Trechsel and Mark T. Bloomberg, Eds. ASTM International: West Conshohocken, PA. Chapter 20, pg. 407. 2009. IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 French | 119
