Structural safety of wood decks and deck guards

Considerations for safe deck guards
A guard is a system of interconnected parts that protects occupants from falling off the deck for whatever reason. Guards are required on residential decks more than 762 mm (30 in.) above ground level. Per IRC, the guard must be 914 mm (36 in.) above the deck surface or, in the case of a bench-guard combination, the guard height must be 914 mm above the bench seating surface.

Wooden guards typically consist of many parts fastened with bolts, screws, or nails. Guard posts are typically stress-rated 4x4s bolted to the deck band joist. Two rails between the posts are usually 2×4 cross sections—one at the top of the post, and the other positioned less than 102 mm (4 in.) above the deck surface. The rails transmit the applied loads to the posts. Pickets between the rails transmit loads into the rails. A typical guard is shown in Figure 5. While not addressed in this article, numerous non-structural requirements for guards are extremely important to child and life safety, and are covered by IRC.

The well-known building code requirement for guards is 890 N (200 lb) concentrated load applied in any direction to the top of the guard system. As stated earlier, a critical connection is the guard-post-to-deck structure. Laboratory tests of 4×4 posts loaded at 914 mm above the deck surface demonstrated the large force that is produced at the base of the post. Some commonly observed post connection details that rely on lag screws alone have been tested at Virginia Tech. For one tested case, the guard post separated from the deck band joist at a load level approximately 25 percent of the code requirement.

Due to the ‘lever’ action, the band joist can also separate from the joist ends because it is inadequately attached with nails or screws installed into the end grain of the joists.⁵ This connection detail is weak because the fasteners used to attach the band to the joists are loaded in withdrawal from end grain. The guard post is through-bolted to the deck band joist, so it appears to be a strong connection—however, it can fail at very low loads compared to what is required by the code.

Laboratory tests of 4×4 stress-rated guard posts (Figure 6) demonstrated steel connectors can adequately transmit loads produced by the code-required concentrated load on the top of the guard into the deck joists.

Figure 7 shows a schematic of a post-to-deck attachment using one or two connectors. For guardrails running perpendicular to the joists, the post can be attached directly to the connector if the post location is adjacent to a joist. Otherwise, two connectors are used to attach the band to joists, and the guard post is attached to the band with bolts between the connectors.

When installing guards, one must also consider how rails are attached to the posts and the pickets to the rails. A preferred method is to attach the rails to the inside face of the posts with screws or threaded nails. The pickets are attached to the rails with screws or threaded nails. In no case should guard components be connected with smooth-shank nails, as the connection design strength is reduced by 75 percent due to in-service moisture changes.

Guard posts should not be notched. In the past, bottom-notched posts were commonly employed to allow the post notch to sit on the deck surface, as shown in Figure 8. This practice severely reduces the post’s bending strength, and notching can worsen as cracks travel up from the corner of the notch.⁶

For all fasteners and connectors used in exterior environments, corrosion is an issue since it reduces connection strength. Therefore, at a minimum, code-recognized and approved corrosion-resistant metals or coatings must be employed in guard construction. Stainless steel fasteners and connectors are recommended by AWC DCA 6, Prescriptive Residential Deck Construction Guide, for guard systems exposed to saltwater or coastlines.⁷

 

 

Notes
¹ For more, see the authors’ co-written article with D. Carradine and D. Bender in the May 2008 issue of Structure Magazine, “Lessons Learned: Residential Deck Ledger Connection Testing and Design.” Visit www.structuremag.org/Archives/2008-5/C-LessonsLearned-DeckLedger_Carradine-May08.pdf. (back to top)
² See the authors’ co-written article with R. Caudill, T. Platt, and Q. Smith, “Load-tested Deck Ledger Connections,” in Journal of Light Construction (vol. 22, no. 6). Visit www.jlconline.com/Images/Practical%20Engineering_%20Load-Tested%20Deck%20Ledger%20Connections_tcm96-1098165.pdf. (back to top)
³ See the article by Loferski et al, “Brown-rot Decay of ACQ and CA-B Treated Lumber,” in Forest Products Journal (vol. 57, no. 6). (back to top)
⁴ Ibid. (back to top)
⁵ See the authors’ co-written article with D. Albright and Caudill, “Strong Rail-post Connections for Wooden Decks,” in Journal of Light Construction (vol. 23, no. 5). Visit www.jlconline.com/lumber/strong-rail-post-connections-for-wooden-decks.aspx. See also the co-written article with Albright from the July 2007 Structure Magazine, entitled “Tested Guardrail Post Connections for Residential Decks: Lessons Learned.” Visit www.structuremag.org/Archives/2007-7/C-LL_Wood_Post_Connections_by_Loferski.pdf. (back to top)
⁶ Ibid. (back to top)
⁷ See Woeste’s article, “Safe and Durable Coastal Decks,” from Coastal Contractor (vol. 5, no. 2). Visit www.coastalcontractor.net/pdf/2008/0803/0803safe.pdf. (back to top)

Joe Loferski, PhD, is a professor of sustainable biomaterials at Virginia Tech. He has an international reputation and experience in the areas of performance of wood and wood composites in buildings, along with the preservation of historic wood buildings. Loferski can be reached at jloferski@vt.edu.

Frank Woeste, PE, PhD, is an adjunct professor of sustainable biomaterials at Virginia Tech. He is a wood construction and engineering consultant, and a past contributor to The Construction Specifier. Woeste can be contacted via e-mail at fwoeste@vt.edu.