Deck Beams and Joists

Deck beams and joists need to be sized, spaced, and supported properly to support the loads that they are subjected to. The tables in 2021 IRC R507.5 list maximum beam spans for the different wood species members based on various ground snow loads. Table R507.6 helps to determine the maximum joist span, considering live load, wood species, joist spacing, joist size, and whether there’s a cantilever or not. Cantilevers are included but are maxed out at L/4. Joist spacing should be no more than 16″ o/c for decking that runs perpendicular to joists and no more than 12″ o.c. if the decking runs diagonal to them. DCA 6, Table 2 is the comparable section that addresses maximum joist spans and overhangs. These member sizes and spans are developed using established code minimum distributed loads of 40 psf live and 10 psf dead.

Of course joists and beams are gravity load–carrying members and need to be properly supported at a minimum of two contact points. Joists, when attached back to the ledger at one end, per IRC R507.7, must be positively attached with 1.5″ minimum bearing on a proper joist hanger fastened to the ledger; and at the other end, where they run over the beams, they should also have 1.5″ minimum bearing on the wood and be attached to resist uplift and out-of-plane forces using code-compliant hurricane ties. The uplift requirement can be found in IRC R505.7 and R502.9, where Section R507.5.1 addresses the requirement noting restraint to prevent rotation. Note that DCA 6, Figure 6 allows for either a hurricane clip or a mechanical fastener for this condition.

For beams, per R507.6, a minimum1.5″ bearing on the wood post is also required; and when bearing on concrete or masonry, a minimum of 3″ of bearing is required for the full width of the beam. When there is a multispan beam, each ply must have full bearing on the post. Joists have the same requirement.

Simpson Strong-Tie has many solutions for achieving these connections:

1. HUS/LUS standard joist hangers
2. HUC/LUC concealed joist hangers
3. H1A hurricane tie — holds both faces of the joist as it runs over the beam
4. 5A hurricane tie — holds only one face of the joist as it runs over the beam

Other miscellaneous deck joist considerations to keep in mind:

1. DCA 6 requires blocking at support locations over beams, and where blocking is not present, joists are not to exceed spans that are greater than 8′. The depth of the blocking should equal the depth of the joist.
2. At the end of the beams, utilize concealed flange hangers so as to ensure proper installation.
3. Joists must seat tight against the ledger/rim board. Gaps must be less than 1/8″. For greater gaps, loads are reduced per Simpson Strong-Tie Testing. Depending on the gap, it can be up to 30% for download capacity and up to 40% for uplift.
4. Joist hangers need to have a depth at least 60% of the ledger’s depth. See DCA 6 Table 3A for minimum joist hanger vertical capacity.
5. Do not use clip angles or nonbearing brackets to support joists at the ledger.
6. NADRA, though not necessarily code required, recommends slightly larger member sizes with a minimum 2×10 joist size supported by a 2×10 hanger. These hangers should be installed with code-listed structural screws, and, if the joist hanger is fastened in double shear, the fastener needs to be 2.5″ minimum length.
7. As always, when building a deck, match the corrosion resistance of the fastener to the joist hanger. ZMAX® or hot-dip galvanized hangers (ASTM A653) should be used only with properly galvanized nails or screws (ASTM A153). Stainless-steel connectors should only be used with stainless-steel fasteners.

Lateral Connection Requirements

While a properly screwed or bolted code-compliant ledger-to-rim board connection is suitable for supporting gravity loads [read the third blog in the series], building codes (IRC, DCA 6, and NADRA) require a connection that can also resist lateral loads. In these situations, tension ties are necessary to tie the joists of the deck directly back to the primary structure.

It’s important to understand that lateral loads can be imposed on your deck not only by seismic forces and wind forces but also by other factors. For example, the weight of enough people moving on the deck can sometimes exceed the forces of nature. The out-of-plane movement that results from these lateral loads can cause the entire deck to pull away from the structure it is attached to. In 2009, the first IRC code requirement specific to decks was introduced to address this specific condition. The detail required a minimum of two 1,500 lb. –capacity holdowns be used to connect the joists of the deck back to the joists or blocking inside the house. This same detail is still shown in the 2021 IRC Figure R507.9.2(1). With each new edition of the code, the deck section has expanded and an additional solution for the lateral connection is included.

The 2021 IRC Section R507.9.2, as well as IRC Figure R507.9.2(1), discusses the requirement for the two 1,500 pound holdowns within 24″ of each end of the deck, while IRC Figure R507.9.2(2) details 750 lb. holdowns at four locations evenly distributed including one within 24″ of each end of the ledger. Originally, when two 1,500 lb. holdowns were the only detail provided in the code, homeowners ran into challenges when they didn’t have access to the interior floor joists. Just imagine a finished kitchen with the cabinetry already installed. Because this was a deterrent for homeowners and builders, the industry worked to come up with an alternative solution that didn’t require access to the interior floor joists. We’ll discuss the code-listed options that Simpson Strong-Tie introduced for such an alternative in just a moment.

The reference to deck holdowns in DCA 6 is described on pages 17 and 18 and in Figures 22 and 23, where only the two 1,500 lb. holdowns are depicted. DCA 6 also provides details for offsets to joists for the 1,500 lb. holdowns, including attachments to I-joists and attachments to blocking. NADRA also addresses the deck holdowns, in their Section 8k, where they elaborate on the recommendation a bit more by prescribing that for decks up to 250 square feet, two 750 lb. tension ties shall be placed on end joists and two additional 750 lb. tension ties shall be placed on joists within 4′ of the end joists and connected to the primary structure. If the deck exceeds 250 square feet, NADRA recommends that two more tension ties shall be placed equidistant from each other per the manufacturer’s details.

Simpson Strong-Tie has two primary connectors that can achieve the just-mentioned code requirements. The DTT2 can be used to satisfy the IRC provision for a 1,500 lb. lateral-load connection at two locations on the deck. Our technical bulletin T-C-DECKLAT provides detailing to help achieve this connection for various applications. This document shows how to connect directly back to joists of the house but also addresses applications that require the maximum 1.5″ offset and scenarios when you have to attach to blocking.

The DTT1Z is the Simpson offering that can be used to achieve the two 750 lb. tension tie requirement. This holdown may be engaged into the double top plates, studs, headers, and other structurally sound members of the supporting structure, including into concrete. These various installation conditions are described in detail in T-C-DECKLAT, the above-referenced technical bulletin. The DTT1Z is fastened to the underside or the face of the deck joist and is then attached to the supporting structure using a Strong-Drive® SDWH™ Timber-Hex HDG screw with a minimum 3″ of thread engagement into that primary structure. For installation into concrete, a fully threaded 3/8″ mechanically galvanized Titen HD® heavy-duty screw anchor may be used with 2.5″ minimum anchor embedment.

Conclusion

As we have discussed in the previous blog posts in this series, a proper continuous load path that connects all of your load-carrying deck members either back to the house through the ledger or all the way down to the foundation is extremely important. In the current post, we described the load-carrying aspects and code requirements of the joists and beams and also how the various codes provide guidance on restraining our decks for lateral-load conditions. Understanding your deck’s critical connections and making sure they comply with building codes and safety standards  will give you the peace of knowing that your outdoor space is a safe and secure structure.