The 2009 IRC, which is the building code in my area, requires that a deck connected to and supported by its primary structure be "designed for vertical and lateral loads as applicable" (2009 IRC, R502.2.2).
The IRC's vertical load requirement is clear: 50 pounds (40 live plus 10 dead) per square foot. Using a deck's dimensions, you can easily calculate its vertical load on the connection between the deck ledger and the house band, and then refer to the code to find connection details and attachment configurations.
However, the IRC's lateral load provision—intended to prevent a deck from being pulled away from its supporting structure—is anything but clear, as most deck builders know.
The lateral-load anchor “permitted” by the current IRC requires access to the interior house framing. In an existing house, it’s impossible to verify that the subfloor attachment meets requirements without removing finished flooring.
Credit: International Code Council
Instead of quantifying general lateral load requirements, the code offers a single solution with a specific design capacity: You are "permitted" to install the mechanism drawn in IRC Figure 502.2.2.3—two Simpson Strong-Tie DTT2s (strongtie.com) connected by a threaded rod, with one fastened to a deck joist and the other to a house joist. Two of these mechanisms, each resisting 1,500 pounds, must be installed, regardless of the deck's size or shape.
Note that the IRC does not require this detail, nor does it set a lateral load requirement of 1,500 pounds. That load is merely what the allowed "hold-down device" should resist.
Unfortunately, installing this device poses numerous difficulties, especially after a house is built. Not only is it labor-intensive, it's arguably unnecessary for many decks (see Bad Solution to a Non-Existent Problem, page 40). In fact, lateral load requirements are slated for revision in upcoming versions of the IRC (see Structure).
More Than One Option
If you dislike the lateral-load anchor currently permitted by the IRC, what are your options? The code allows alternatives, but offers none. In this article, I'll describe a few solutions for resisting lateral loads that my deck-building company has successfully employed on our projects.
In my area, and probably in yours too, enforcement of the lateral brace provision is inconsistent. Of the 21 towns where I build decks, only one requires the code-specified bracket or an engineer-stamped solution. Four of the towns want "something reasonable," and the rest ignore the provision altogether. So I've organized my alternative solutions into two categories: "engineered solutions," which have been formally tested or stamped by an engineer; and non-engineered "informal solutions," which are versatile details we've been able to use when the inspector doesn't require an engineer's stamp—but that may or may not be approved by your building inspector or engineer.
This engineered brace provides 1,500 pounds of resistance to lateral loads and can be installed from the exterior. The diagonal brace is installed at a 45-degree angle and measures 5 feet from long point to long point, requiring that the deck be more than 3 1/2 feet above grade.
Credit: Jim Finlay
Diagonal foundation brace. The first alternative lateral-load anchor for a deck I ever designed consisted of a double 2x10 diagonal brace anchored to the deck framing with 1/2-inch through-bolts and to the foundation wall with a beefy 6-inch by 4-inch by 12-inch-long L-bracket fabricated from 3/8-inch steel. My engineer approved it—and I expect yours would too. It's well-suited to larger decks. Unlike the code-approved solution, this anchor can be installed without accessing the building's interior. Not only that, it allows the deck to be installed 6 or 7 inches below the elevation of the house floor, a common detail in wet or snowy climates.
Materials for one diagonal brace cost about $72 (since two are needed, total cost is $144). Installation labor is roughly one hour per brace, depending on the age and density of the concrete foundation. Once installed, each brace will resist 1,500 pounds of lateral force, just like the "permitted" hold-down device.
Credit: Jim Finlay
The author designed a second custom L-bracket for use without a 2-by brace. It’s fabricated from 1/4-inch-thick steel plate, with each leg measuring 3 inches by 9 inches. The brackets are hot-dipped galvanized for corrosion protection, and when installed, are isolated from treated framing with self-adhering flashing.
L-bracket. To further simplify anchor installation, I've also designed an L-bracket that doesn't require the diagonal 2-by brace. My local steel fabricator cuts 3-inch by 18-inch strips from standard 1/4-inch-thick steel plate and bends them 90 degrees into L-brackets with two 9-inch legs. After drilling the holes, he has the brackets hot-dipped galvanized. Because of the set-up time involved, the more I buy, the lower the unit cost; so I order three dozen at a time, which brings my cost down to about $20 per bracket.
When installed as shown above, the author’s custom-fabricated 1/4-inch-thick steel brackets provide 1,500 pounds of resistance to lateral loads.
Credit: Jim Finlay
To connect a bracket to a doubled joist, I use six 4 1/2-inch HeadLOK (fastenmaster.com) screws. The screw manufacturer has formally tested this configuration in shear, parallel to the grain of wet #2 pressure-treated SYP, for a double 2x8 joist. In testing, the connection exceeded 1,500 pounds in shear, even after factoring in a 3x safety margin. Though the brackets are galvanized, we isolate them from treated joists with a piece of Bituthene or Vycor (graceresidential.com) or a similar self-adhering membrane.
There are several options for bolting the bracket to the concrete. For example, a galvanized 1/2-inch-diameter by 4 1/4-inch-long Red Head Trubolt wedge-type expansion anchor bolt (itwredhead.com) embedded 3 3/8 inches into concrete can safely resist more than 1,900 pounds of withdrawal force, or more than 2,900 pounds in shear. Other concrete fastening options include sleeve anchors (available in stainless steel), strike anchors (available in yellow-dichromate-plated steel only), and epoxy-bolt systems.
The size of the anchor depends on the density of the concrete. Since that is virtually impossible to test, I generally assume (in the absence of visual deterioration) that foundation concrete has a compressive strength of 2,500 psi, the weakest allowed by code (2009 IRC, Table R402.2).
Sometimes blocking is required to avoid installing concrete anchors too close to a foundation edge. In those cases, the author uses 6-inch instead of 4 1/2-inch-long HeadLOK screws to attach the bracket to the doubled joists. The bracket in the photo was fabricated from 1/2-inch-thick angle iron (the steel was manufactured as a 16-foot-long angle bar with two 8-inch flanges). While the steel is much thicker than necessary, it was readily available, and the fabricator could just cut off 3-inch-wide pieces of the angled steel.
Credit: Jim Finlay
It's also critical not to install the anchor bolts any closer to the edge of the concrete than the manufacturer recommends. When the upper hole of the L-bracket would be too close to the top edge of a foundation (within 3 3/4 inches for 1/2-inch-diameter Red Head Trubolts, for example), I use a 2x4 PT spacer to lower the bracket, and I upgrade the joist-attachment screws to 6-inch HeadLOKs.
Materials for this anchor cost about $41, including the $20 bracket and the second joist. Installation usually takes less than 30 minutes. As a bonus, this L-bracket provides vertical support in addition to lateral strength.
Note that the diagonal-brace and L-bracket details connect to poured concrete foundations, which are common in my area. Hollow concrete-block foundations, however, pose a challenge, because their thin walls offer considerably less withdrawal strength. For instance, Simpson Strong-Tie's ETSP plastic screen tubes provide only 300 pounds of withdrawal strength and require 8-inch spacing. I suppose that threaded rods could be used as through-bolts, with a 2x6 PT block mounted inside as a big "washer," but that, of course, would require access to the inside of the house.
Credit: Jim Finlay
When the deck framing is aligned with the house framing, a simple metal strap can provide sufficient strength to meet lateral load requirements. On existing construction, the siding and sheathing must be temporarily removed to gain access to the framing.
Side strap. When the deck is aligned with the edge of a house, significant lateral loads can be resisted with a simple strap that connects the deck's outside joist to the house frame. We've used Simpson's 1 1/4-inch by 30-inch MSTA30 straps nailed to the deck and to the second story of a house, a solution that has been stamped by my engineer.
When installed as shown above, Simpson’s MSTA30 strap ties offer 1,820 pounds of allowable tension loads in SPF framing, more than enough to meet the IRC’s 1,500-pound lateral load requirement for permitted devices.
Credit: Jim Finlay
To ensure that the strap is fastened to solid house framing, some siding and sheathing has to be temporarily removed during installation. But once installed with 11 hanger nails in the house frame and another 11 in the deck's double joist, this connection resists more than 1,800 pounds of tension in SPF framing, according to the manufacturer's specifications.
The cost of materials for this solution is modest, just a few dollars for the strap and nails. Of course on existing construction, labor can run two hours or more, depending on the house siding.