Scabbing Lumber to Joists for Offset Lateral Deck Ties
The International Residential Code (IRC) requires that a deck connected to the side of a residence be tied laterally at two places to the framing of the main structure. These `lateral ties’ are the most efficient where placed at or near the opposite ends of the deck. The purpose of the ties is to keep the deck attached to the main structure when the deck is subject to lateral (sideways) forces such as from wind or earthquake ground motions, and from incidental motions from use. In cases where the main structure floor framing and the deck framing are at approximately the same level, and where the deck joists are parallel to the main structure floor joists, the lateral ties from the deck are connected straight to the floor joists that they are `supposedly’ lined up (with each other), using pairs of Tension Tie or Holdown devices. (See attached sketch.)
The Simpson Strong-Tie DTT2 (or DTT2Z) is a commercially available device manufactured specifically for this purpose. The `DTT’ stands for Deck Tension Tie; the `2′ for being rated for an allowable tension load of about 2000 pounds (lb); and the `Z’ for where it is corrosion-protected the Simpson ZMAX.
A typical lateral tie system for a deck, then, consists of pairs of deck tension ties at each tie location (4 devices total).
The deck tie pairs work perfectly where the deck joists align perfectly inside and outside the structure at the tie locations. Where they do not, additional joists may be required, or, if they are not very far out of alignment, `scabbing over’ to the nearby joist may make up the difference. The purpose of this article is to illustrate how to structurally scab on over to that nearest joist.
Consider the situation illustrated in the accompanying sketch. The deck joist and floor joist are `off’ by about 1-1/2 inches (in.). The `scabbing over’ will be accomplished by structurally attaching a relatively short piece of dimension lumber to one of the joists, making up the difference. The `scabbing’ could be done inside or out (deck joist or floor joist), depending on the sequence of construction; for illustration it is shown on the deck joist.
The IRC requires that the tension tie (or two-device system) be rated for an allowable load of at least 1500 lb. In general, then, the `scabbing over’ should be able to transfer 1500 lb from the device to the receiving joist. In an application where a particular device has been specified,unless otherwise noted by the designer, the transfer should be designed for the full rated load of the device. The DTT2/DTT2Z is rated for 1825 lb (installed in Douglas fir). This article, therefore, will focus on transferring 1825 lb.
The DTT2/DTT2Z is intended to be installed to the wide face of a member at least 1-1/2 inch (in.) thick. Instead of this being the joist itself, we will fasten the device to a 1-1/2 in. piece of dimension lumber that is nailed (beforehand) to the nearby joist. The dimension lumber piece must be long enough to: 1) receive enough nails to attach the scab piece to the joist; 2) receive the DTT; and, 3) be large enough so that the nailing and the attachment of the DTT device doesn’t `destroy’ the scab piece (too many nails too close together, or interference of nails and the device screws). The focus of this article is the determination of the required number of nails, scab piece to joist. For this we will use the National Design Specification for Wood Construction (NDS).
Consider using 10d galvanized box nails to make the connection (`scab over’). Table L4 of the NDS provides the important dimensions of 0.128 in. diameter and 3 in. length. Using Douglas fir lumber (for both joist and scab piece), the lateral design value for the 10d nail, from Table 11N, going through the 1-1/2 in. thick scab (`side’) piece, is 93 lb. This value is only good where the nail penetrates at least 10 nail diameters into the receiving piece, otherwise the 93 must be reduced. In our example the nail is 3 in. long, thus after going through the scab piece, the nail will penetrate 1-1/2 in. into the (1-1/2 in. thick) joist. The minimum penetration for this size nail is 10 times 0.128 in. = 1.28 in. Since the minimum penetration is achieved, the full 93 lb may be used. But, it must also be adjusted for the in-place or `service’ conditions of the deck.
Adjustment factors for nails are listed in Table 10.3.1 of the NDS and provided in the pages following. In our particular example we are assuming dry service and normal temperatures, thus the only applicable adjustment is for `duration of load’. The deck tie is intended to resist wind an earthquake actions, for which the Load Duration factor is given to be 1.6. (Nails are assigned higher strength in the relatively short duration loads of earthquake and wind action than they are for load of longer duration.) In our example each nail, therefore, has an allowable load of 93 lb x 1.6 = 149 lb. For the case of the DTT2, 1825 lb divided by 149 lb per nail gives … 12.2 nails required (13 nails).
So, here is our solution (for scabbing over 1-1/2 in. for the DTT2, assuming Douglas fir dimension lumber for both joist and scab piece): 1) select scab piece and locate on joist as to provide sufficient room for all fasteners without damage to scab piece and joist; 2) attach scab piece to joist with 13 – 10d galvanized box nails; 3) fasten deck tension device to scab piece per manufacturer instructions.
The above solution is particular to scabbing Douglas fir dimensions lumber to Douglas fir dimension lumber, and where the tie device normally installs to a member 1-1/2 in. thick, and for which the `scab distance’ happens to be 1-1/2 in. Tension ties for heavier loads often require thicker receiving members. And joist offsets will, by no means, always be 1-1/2 in. As such, there are a whole host of other situations that are potentially more complicated, but for which the above provides a starting place.
References
International Residential Code, 2009 and 2012, International Code Council, Washington, D.C., in particular Sections 502.2.2 (2009) and 507.2.3 (2012).
Wood Construction Connectors, 2011-2012, C-2011, Simpson Strong-Tie Company, Pleasanton, CA.
National Design Specification for Wood Construction, 2005, American Wood Council, Washington, D.C.
