Designing Overhangs on Gable Ends

It seems that each major hurricane tends to teach those of us in the construction industry some lesson. With Hurricane Andrew, the lessons were the importance of protection from windborne debris, and the importance of proper construction of gable ends.

There are two main areas where gable ends can fail. One is a failure of the hinge at the connection between the top plate of the wall and the gable end framing, if the gable end is not balloon-framed with continuous studs. This is now addressed in the International Residential Code. Since 2009, Section R602.3 has required that “Studs shall be continuous from support at the sole plate to a support at the top plate to resist loads perpendicular to the wall. The support shall be a foundation or floor, ceiling or roof diaphragm or shall be designed in accordance with accepted engineering practice.”

For existing construction, the International Existing Building Code specifies a method for retrofitting gable ends in Appendix C.  For new construction, Simpson Strong-Tie shows a couple of solutions for bracing top plates of gable ends in our High Wind–Resistant Construction Application Guide on Page 48.

Figure 1, Gable Wall Bracing Methods

Figure 1, Gable Wall Bracing Methods

The other common wind-related failure at gable ends is uplift of the roof decking at the overhang. This can be from two causes: inadequate nailing of the sheathing to supporting framing, or inadequate connections of the framing at the rake edge that supports the roof. As far as this author can tell, this area of light construction is not covered in the International Residential Code for wood framing, but it is covered for cold-formed steel framing, where Section R804.3.2.1.2 contains requirements for “Rake overhangs.” The two methods shown are the cantilever outlooker (Option 1) and the ladder outlooker (Option 2).

Figure 2, IRC Gable Overhand Details

Figure 2, IRC Gable Overhand Details

Figure 3, Gable End Wind Damage

Figure 3, Gable End Wind Damage

In the photo above, it appears that the cantilevered outlooker method was used, and that there was a failure of the outlooker connections at the gable end and the first full truss. If you look closely, the end nails from the full-height truss that were in the end of the outlookers can be seen in a couple of places.

If a truss roof is used with this method, the gable truss is manufactured 3½” shorter than the others. Then a 2×4 outlooker is placed over the dropped gable, and butted into the side of the adjacent full-height truss. Then the barge or fly rafter is attached to the end of the cantilevered outlooker. At the overhang, wind can cause uplift on both the bottom and top surface. The uplift at the end of the outlooker imparts an uplift force at the gable truss, which must be resisted by a tension connection such as a hurricane tie, and a downward force at the connection to the full-height truss.

Figure 4, Cantilevered Outlooker Method

Figure 4, Cantilevered Outlooker Method

The other method commonly used to support the sheathing and the barge rafter is the ladder method. With this technique, lookout blocks are used to connect the barge or fly rafter back to the gable framing. One way this can be constructed is as a full ladder, with parallel fly rafter and ledger with block framing in between. Either this assembly can be constructed on the ground and then raised and fastened in place, or it can be built in place at the overhang. Or there are also examples where a ledger is not used, and the block framing is just connected directly to the top chord of the gable truss or gable rafter. This method is less wind-resistant, and in literature is limited to a 12″ overhang.

Figure 5, Ladder Outlooker Block Method

Figure 5, Ladder Outlooker Block Method

If the gable overhang is to resist wind loads properly, it must either be designed, or constructed in accordance with some pre-engineered prescriptive detail. Figure 4 shown above was originally published in a Simpson Strong-Tie Technical Bulletin, the High Wind Framing Connection Guide. But this Guide is no longer published. As shown earlier in Figure 2, there are some prescriptive details in the IRC for cold-formed steel construction. These are limited to an overhang length of 12″ and apply for up to 139 miles-per-hour ultimate wind speed. For wood-framed construction, comparable details are contained in the American Wood Council Wood Frame Construction Manual. For the cantilevered outlooker method, connection design loads are published for various wind speeds. Cantilevered outlookers are permitted to extend out up to 24 inches, while the ladder outlookers are only permitted to extend out 12 inches. See below for excerpted figures and tables from the Wood Frame Construction Manual, courtesy of the American Wood Council.

Figure 6, WFCM Gable Overhang Design (courtesy, American Wood Council, Leesburg, VA)

Figure 6, WFCM Gable Overhang Design (courtesy, American Wood Council, Leesburg, VA)

In addition to the framing design, the connection of the roof decking at this location is critical. If you’re building to traditional construction methods, with 6″ nail spacing at panel edges and 12″ nail spacing at interior supports, the close nail spacing ends up at the nonstructural outer member, while the nailing at the actual roof edge over the gable is only 12″ on center. As shown in the details above, newer documents do indicate the importance of spacing the nails over the gable end at the closest spacing, both because these are subject to the highest withdrawal loads and because this is the edge of the diaphragm for transfer of lateral loads.

The Journal of Light Construction has a discussion of the unbraced gable end overhang on one of their Forums.

The Florida Division of Emergency Management provides some information on wind resistance of gable overhangs and some possible means of retrofitting them here.

Have you seen or designed with different methods for framing gable overhangs?

Here Come 2015 IBC Changes!

All of us here at Simpson Strong-Tie hope you had a happy and successful 2014. It seems that the folks at the International Code Council had a good year. True to their plan, the 2015 editions of the International Codes were published during the summer so that they are ready for adoption in 2015.

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SE blog 2Simpson Strong-Tie was tracking a number of issues during the development of the 2015 International Building Code and International Residential Code. Here is a summary of some of the significant changes that users will see in the 2015 International Building Code (IBC).

One significant change affecting Simpson Strong-Tie was the removal of the requirements for evaluation of joist hangers and similar devices from Chapter 17, and the revision of Sections 2303.5 and 2304.10.3 to reference ASTM D 7147 as the test standard for joist hangers.

Since the primary reference standard for design in Chapter 16, ASCE 7-10 has not changed; there were not a lot of significant changes in that chapter. The definitions of “Diaphragm, rigid” and “Diaphragm, flexible” were deleted from Chapter 2, and a sentence was added to 1604.4 stating when a diaphragm can be considered rigid, along with a reference to ASCE 7 for determining when designs must account for increased forces from torsion due to eccentricity in the lateral force resisting system.

In Chapter 19, significant improvements were made to the sections that modify ACI 318 so that the IBC and the standard are coordinated, correcting the problems in the 2012 IBC.  In addition, Sections 1908 (ASD design of anchorage to concrete) and 1909 (strength design of anchorage to concrete) were deleted to remove any conflict with ACI 318 anchor design methods.

In Chapter 23, a new section was added to address cross-laminated timber, requiring that they be manufactured and identified as required in APA PRG 320. The wood framing fastening schedule was completely reorganized to make it easier to use and the requirements for protection of wood from decay and termites were rewritten. Section 2308 on Conventional Light-Frame Construction was completely reorganized with significant revisions to the wall bracing section. As discussed in an earlier blog post, the holdown requirement for the portal frame with holdowns (now called PFH bracing method in the 2015 IBC) has been reduced from a required capacity of 4,200 pounds to 3,500 pounds.

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For designers, some of the most significant changes are in Chapter 35, which lists referenced standards. Some major standards that were updated for this edition of the IBC include ACI318-14, ACI530/530.1-13, several AISI standards (S100-12, S200-12, S214-12, and S220-11), several new and revised ASCE standards (8-14, 24-13, 29-14, 49-07, and 55-10), almost all the AWC standards (WFCM-2015, NDS-2015, STJR-2015, PWF-2015 and SDPWS-2015), AWS D1.4/D1.4M-2011, most NFPA standards (too many to list), PTI DC-10.5-12, SBCA FS 100-12 and TPI 1-14.

Kudos to the American Wood Council. They have posted view-only versions of all their referenced standards online, so designers do not have to buy new editions every time the code changes. AISI also enables one to download PDFs of the framing standards at www.aisistandards.org.

Finally, a couple of ICC Standards were updated to new versions that are referenced in the IBC: ICC-500-14, ICC/NSSA Standard on the Design and Construction of Storm Shelters; and ICC 600-14, Standard for Residential Construction in High-Wind Regions.

A future blog post will cover significant changes in the 2015 IRC. Please share your comments below.