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   Author: MetalMag

IN NOVEMBER 2005, THE NATIONAL Frame Builders Association (NFBA) released "Accepted Practice for Post-Frame Building Construction: Framing Tolerances." The process to develop the document began at the March 4, 1999, meeting of the NFBA Technical and Research (T&R) Committee. At the meeting it was proposed NFBA move forward with the development of a second construction tolerances document-one that covers metal trim and corrugated panel installation. As with the framing tolerance document, it was apparent development of the new document could not begin until fundamental research was conducted; in this case, research that would determine just how accurately metal panels and trim were installed on the typical post-frame building.

Funding for field research was subsequently approved by the NFBA board with the University of Wisconsin-Madison (UW-Madison) agreeing to conduct the research. At its Oct. 30, 2002, meeting, the NFBA T&R Committee discussed specific information to be collected by the UW-Madison researchers.

Working under my direction, UW-Madison graduate student David Cockrum began a "quality assessment" of light-gauge metal cladding and trim installation during May 2003. This work was completed during June 2004. Fifty-two buildings located in southern Wisconsin were investigated, representing the work of seven independent companies. Note the scope of this study was limited to buildings in southern Wisconsin for purely economical reasons (to limit transportation costs).

Nineteen of the surveyed buildings were designed for an agricultural-related end use, 15 were constructed for commercial/industrial use and 18 were residential-related buildings. Maximum, minimum and average widths of the surveyed buildings were 104, 24 and 47.7 feet (31.6, 7.3 and 14.5 m), respectively. Maximum, minimum and average building lengths were 400, 27 and 85.9 feet (121.9, 8.2 and 26.1 m), respectively. Maximum, minimum and average eave heights were 22, 8.3 and 13.8 feet (6.7, 2.5 and 4.2 m), respectively. Finally, maximum, minimum and average floor areas were 32,000, 650 and 4660 square feet (2973, 60 and 433 m2), respectively. Approximately 80 percent of the buildings were less than one year old when surveyed.

The following items were assessed at the building sites: wall panel plumb, wall panel-to-roof panel alignment, corner trim squareness, corner trim-to-wall panel alignment, upper panel-to-wainscot alignment, roof panel end offset, roof panel overhang, wall panel base-to-trim spacing, wall panel end offset, horizontal alignment of wall fasteners, wall fastener drive depth, wall fastener drive angle, roof fasteners missing framing, irregular fastener patterns, scratches, scuffs and scrapes, dings/dents and crimps/kinks.

Most measurements were made from the ground or ladders. Some data was collected from a scaffold plank supported by ladder jacks. Roof surface characteristics were not assessed for two reasons. First, items not within normal view from the ground (roof fastener alignment) typically are not a subject of discussion or contention between builders and owners. Second, the cost to provide an OSHA-approved fall-protection system for roof access at each building site-a system that would not permanently mark/alter the building-was beyond the project budget.

Field data was analyzed during June and July 2004 and summarized in an ASAE paper presented at the 2004 ASAE Annual International Meeting in Ottawa, Canada (Bohnhoff, D.R. and D.K. Cockrum, 2004, Quality assessment of light-gauge metal cladding and trim installation, ASAE Paper No. 044113). In addition to containing a compilation of data, this paper details measurement methods and quantifies the accuracy of the methods.

During fall 2004, I wrote the initial draft of a document titled "Accepted Practices for Post-Frame Building Construction: Metal Panel and Trim Installation Tolerances." I modified this draft following a line-by-line review by the T&R Committee on Dec. 2, 2004. Major adjustments included the addition of two appendices; one about galvanic corrosion and the other titled "Panel and trim design/selection considerations."

The second version was reviewed by the T&R Committee on Feb. 23, 2005, during the NFBA annual meeting in Columbus, Ohio. The primary change resulting from this review was the relocation of material about panel fabrication requirements from the body of the report to the appendix.

The third version of the document was reviewed by the committee during a conference call on April 1, 2005. This review resulted in the addition of new definitions and tolerances for edge ripple, edge kink and rib kink.

The fourth draft was reviewed by the NFBA board at its May 14, 2005, meeting in Kansas City, Mo. The board approved the document subject to final changes by the T&R Committee with input from companies representing metal panel and fastener suppliers. The T&R Committee met with supplier representatives on June 9, 2005, in Chicago. This meeting resulted in minor editorial changes to the main body of the document, a new table in the appendix about galvanic corrosion and the addition of a table about base-metal thicknesses.

The document was reformatted for publication in early August 2005, at with time a disclaimer was added. A review of this final draft by the T&R Committee resulted in no objections, thus paving the way for document distribution by NFBA.

Like the framing tolerances document, the new construction tolerances document is written using the general format prescribed by the International Organization for Standardization (ISO). The first three sections contain the Purpose and Scope, Normative References and Definitions, respectively. This is followed by the meat of the document, Sections 4 through 7, which cover panel positioning, trim positioning, fasteners installation, and surface and edge blemishes, respectively. Section 7 is followed by the Commentary, which contains a detailed explanation of each section's contents. The Commentary is followed by four appendices and a reference list.

Following is a brief description of each of the document's major sections and its appendices.

Section 1: Purpose and Scope
The purpose and scope addresses the applicability of the document. It specifically states the document only applies to the installation of exterior metal panel and exterior metal trim with a nominal base-metal thickness less than 0.05 inches (18-gauge steel) and to exposed (through-panel) fasteners. This section also requires measurements taken for assessment of construction quality be made prior to building use/occupancy or within 90 days of substantial building completion, whichever comes first. This clause is included because the greater the elapsed time between construction and field assessment, the more difficult it is to separate deviations and damage associated with normal structural use and aging from those associated with initial component placement.

Section 2: Normative References
The provisions of any documents listed as a normative reference become provisions of the document in which they are listed. The new construction tolerance document lists only one normative reference-the "NFBA Accepted Practices for Post-Frame Building Construction: Framing Tolerances" document (the framing tolerances document). The document was established as a normative reference because it helps define post-frame building terminology, influences post-frame building component selection and contains tolerances appropriate to post-frame building construction. If provisions established in the framing tolerances document are not met, it becomes exceedingly difficult for builders to meet panel and trim installation requirements.

Section 3: Definitions
Section 3 contains 114 definitions associated with fabrication, installation, and durability of metal panels and trim. For example, definitions for scratch (shallow and deep), scuff, scuff area, scrape and scrape area are unique to the document and essential to the document's application. In general, a scratch is a surface blemish caused by a single sharp point. Scuffs and scrapes are surface blemishes resulting from another surface or edge being drawn over the surface.

Unlike deep scratches and scrapes, shallow scratches and scuffs do not reveal underlying metallic coating and/or base metal. To qualify as a shallow scratch or scuff, the blemish must be visible by a majority of normal-sighted individuals when viewed under natural noonday lighting from an at-grade position no closer than 15 feet (4.6 m) to the blemish in question. In practice, this requires a line be drawn all the way around the building 15 feet (4.6 m) from each exterior wall surface. If a majority of normal-sighted individuals spot the blemish on their own (without assistance from others) without ever entering the 15-foot (4.6-m) offset boundary, then the blemish meets the visibility criteria.

Three other surface blemishes that have been categorized, uniquely titled and defined in the document are edge rippling, edge kink and rib kink. Edge rippling is the waviness of the overlapping edge of a corrugated panel. An edge kink is a permanent crease in the overlapping edge of a corrugated panel, which is specifically located between the edge and first panel bend in from the edge. An edge kink is a severe form of edge rippling. A rib kink is a compression failure of a major panel rib that occurs when panel bending forces exceed panel bending strength. Photo 3 and 4 show edge kinks and rib kinks, respectively.

Section 4: Metal Panel Positioning
Included in this section are tolerances for plumbness of individual wall panels, plumbness of adjacent wall panels, panel fanning, end-to-end alignment of wall panels, end offset of adjacent panels (sawtooth), panel edge-to-trim spacing, panel end-to-trim spacing and roof panel overhang. The document does not include a tolerance requirement for the alignment of roof panel ribs with those of wall panels. This is because research shows roof-to-wall panel alignment is seldom controlled during panel installation on buildings with eave overhangs and/or wide eave trims (Bohnhoff and Cockrum, 2004).

Section 5: Metal Trim Positioning
Section 5 contains tolerances for the relative orientation and camber (edge curvature) of metal trim, trim edge-to-panel rib spacing and corner trim bend angle.

Section 6: Fastener Installation
Fastener installation tolerances control drive angle, sealing washer compression, framing penetration, and horizontal and vertical alignment of through-panel fasteners. The combination of drive angle, sealing washer compression and framing-penetration requirements help ensure a proper, long-lasting seal. If a fastener does not meet framing-penetration requirements (it misses all or a portion of the underlying frame), it can be removed and redriven at a different angle through the same hole (to meet the penetration requirement) as long as the new drive angle meets drive-angle requirements. If this does not work, the installer is allowed to plug the hole with a special corrective screw (a goof screw) or by driving a fastener through the hole and into a wood block or steel washer/backer plate that has been placed on the backside of the panel. Sealing a panel hole with sealant is not recommended.

Section 7: Surface and Edge Blemishes
Section 7 establishes limits for shallow and deep scratches, scuffs, scrapes, dents, rib kinks and edge kinks. It also addresses removal of metal chips, cutting with an abrasive blade and oil canning. It requires all deep scratches and scrapes on wall and roof panels and exterior trim be touched up using paint approved by the supplier/manufacturer of the panels/trim.

Appendix A: Measurement Equivalencies
This appendix helps individuals express tolerances in different ways. Specifically, it explains conversions between tolerances expressed as fractions of component length, percent slopes and angles in degrees.

Appendix B: Recommended Panel Fabrication Tolerances
If variances from specified panel dimensions are not controlled during panel fabrication, it can be difficult if not impossible for those erecting the building to meet installation tolerance for panels and trim. For this reason, builders may benefit by establishing panel fabrication tolerances. To guide builders in this effort, Appendix B contains recommended tolerances for fabricated panel length, end cut, cover width and edge camber.

Appendix C: Galvanic Corrosion
Galvanic corrosion is a common form of corrosion that occurs when dissimilar metals or metal alloys are brought into electrical contact by immersion in a conductive electrolyte. In the case of building materials, this conductive electrolyte generally is impure water (rainwater or groundwater). When electrically connected, one of the dissimilar metals becomes the anode and corrodes faster than it would by itself in the conductive electrolyte while the other metal becomes the cathode and corrodes slower than it would alone in the conductive electrolyte. Because minimization of galvanic corrosion plays a critical role in panel, trim and fastener selection, it is covered in detail in Appendix C. One feature of the appendix is Table C.2, which gives fastener recommendations for different panel/trim surface materials. This appendix also addresses galvanic action between fasteners and preservative-treated lumber.

Appendix D: Panel and Trim Design/Selection Considerations
The quality of metal panel and trim installation is influenced, in part, by overall building design and component selection. To this end, knowledge of metal panel and trim design/selection considerations is fundamental to discussions involving installation quality and is included in Appendix D for that purpose. Specific topics included in the appendix include base-metal type, base-metal thickness, metallic coatings, paint coatings, structural loads, panel profiles, diaphragm design, panel end laps, lap sealant, cool roofs, metal trim properties, vent and closure strips, and gutters.

"Accepted Practices for Post-Frame Building Construction: Metal Panel and Trim Installation Tolerances" began with a proposal. More than six years later, the document was releases to establish standards of professional conduct, enhance professional reputation, minimize costly litigation and maintain regulatory control within the profession.

The document can be purchased by calling NFBA at (800) 557-6957 or visiting Contact me at (608) 262-9546 or if you have any questions regarding the development of the document.

David R. Bohnhoff, Ph.D., P.E., is a professor of Biological Systems Engineering at the University of Wisconsin-Madison.

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