Strengthening the connection at the top of the wall revolves around 2 main steps:
- Reinforcing the attachment of the sheathing to the top of the trusses or rafters.
- Attaching a series of horizontal braces to the underside of the top truss chords (or rafters if the roof is stick-built) and attaching the end of these braces to the gable end retrofit studs.
Reinforcing the Attachment of the Roof Sheathing
If the roof covering is being replaced, the sheathing at the gable end can easily be re-nailed to current standards for Wind-borne Debris Regions. According to the 2004 FL Building Code, roof sheathing for roofs in Basic Wind Speed zones 110 to 140 mph should be nailed as follows:
- 8d common nails for panels from 15/32" to 19/32"
- Nails should be spaced 6” on center at panel edges and intermediate supports
- Nails should be spaced 4” on center at panel edges and intermediate supports at locations marked #3 (the corners and the peaks of the gable ends) in the following diagram.
source: 2004 Florida Building Code
(Note: roof angles of 10 degrees are approximately equivalent to slopes of 2 in 12. Roof angles of 45 degrees are equal to slopes of 12 in 12. For roof sheathing nailing requirements on roofs with slopes lower than 10 degrees or steeper than 45 degrees, consult the 2004 Florida Building Code.)
To meet building code requirements, nails for pneumatic guns should be full-head, not clipped-head type and the shank diameter must be equivalent to that of an 8d common nail (0.131").
Joints between sheathing panel edges within 6’ of the gable end should be supported by 2x4 or 2x6 blocking beneath. If the sheathing has been removed in the re-roofing process, the blocking can be installed as part of the gable end bracing improvements. Otherwise the blocking must be installed from below (after clipping off the ends of any shingle nails on either side of the joint being supported). These blocks should be installed with sub-floor type construction adhesive between the block and the sheathing and fastened to the adjacent truss chords or rafters with 10d nails minimum.
source: FL Division of Emergency Management |
New sheathing within 6’ of gable ends should be installed with a ¼” bead of adhesive on each truss chord or rafter. If existing sheathing is not being removed, a ¼” bead of sub-floor adhesive should be applied to each side of each truss chord or rafter supporting the panels.
source: FL Division of Emergency Management |
At the gable end truss, apply a ¼” bead of construction adhesive to the side of the top chord and another ¼” bead to the underside of the sheathing. Nail a 2x2 strip to the side of the gable end truss chord that sandwiches this adhesive to the sheathing and the truss (see picture below).
Installing Horizontal Braces Below
the Top Chords or Rafters
source: FL Division of Emergency Management |
Horizontal braces made of 2x material should be attached to the underneath of the top truss chords (or rafters if roof is stick-built). These braces translate the wind-induced loads placed on the gable ends back into the framing beneath the roof diaphragm and engage the plywood sheathing in helping to secure the gable end.
The following specifications are derived from design guidelines adopted by the Florida Building Commission in October, 2007 for inclusion in the Florida Existing Building Code (FEBC). The guidelines are part of a separate FEBC section devoted to retrofitting existing structures to become more resistant to natural disasters such as hurricanes, floods and wildfires. An entire subsection of these new requirements is devoted to "Retrofitting Gable End Walls." The 3 major components in this gable end retrofit process include the following:
- Horizontal Braces (including Compression Blocks) at the top of the gable end wall.
- Horizontal Braces and Compression Blocks at the bottom of the gable end wall.
- Retrofit Studs to reinforce each existing gable end stud greater than 3 feet in height.
The following perspective drawing shows the location of these 3 major components for a truss-type gable end. The components are the same for a conventionally framed or stick-built gable end.
The remainder of this section will review the actual installation guidelines for Horizontal Braces and Compression Blocks at the top of the gable end. The next section will look at the Horizontal Braces and Compression Blocks at the bottom of the gable end. Attaching the Retrofit Studs to the top and bottom Horizontal Braces and to the existing studs will be covered in the section titled, "Reinforcing Gable End Sheathing and Studs."
Summary of Design Guidelines for Horizontal Braces at the
Top of Gable End Walls
It's important to recognize that the following guidelines are more than just recommendations. Since they have been adopted into the FL Building Code for Existing Buildings they are minimum requirements. Installing materials or following procedures with higher performance criteria (such as longer braces or fasteners or fasteners with thicker shanks) is not a problem. However, failing to meet the minimum standards could result in a failed inspection by a Building Official.
The first guideline covers the material for the horizontal braces and compression blocks. In both cases, conventional 2x4 #2 Spruce-Pine-Fir (SPF) framing lumber is sufficient. In regards to brace spacing, there must be a horizontal brace at each existing gable end stud greater than 3' in height. With respect to brace length, each horizontal brace must span the first 3 top truss chords or rafters from the gable end PLUS 2 1/2". The extra 2 1/2" extension is to prevent splitting at the end fastened to the last truss chord or rafter. The length of the compression block is one of several components that varies with the maximum height of the gable end and the Basic Wind Speed zone where the house is located.
The drawing below is a section detail that shows a top horizontal brace and compression block attached to a truss roof assembly where the trusses are spaced 24" on center. Note the 2 1/2" extension past the third truss chord (at the left).
The next drawing shows a top horizontal brace and compression block attached to a stick-built roof assembly where the rafters are spaced 16" on center. Note that the horizontal brace is shorter in length but still spans the first 3 rafters in from the gable end (plus the 2 1/2" extension).
The new Florida Existing Building Code guidelines for Gable End reinforcement contain Table A104.2, "Gable End Retrofit Element Sizing and Fastening." This table lists the various combinations of Maximum Gable End Stud height and Basic Wind Speeds for 2 Exposure Categories (B and C). The complete table shows different values for 9 separate components, the eighth of which is "Minimum Length of Compression Block," and the ninth "Minimum number of fasteners to connect Compression Block to Horizontal Brace." (For more on the difference between Exposure Category B and C see the note at the end of this section).
Table 1504.2
Gable End Retrofit Element Sizing and Fastening
According to this table, the required length of the compression block attached to each horizontal brace varies from a minimum of 11 1/4" to a maximum 17 1/2", and the required number of 3" fasteners varies from 6 to 12. Throughout these guidelines, #8 x 3" screws (deck screws, wood screws or sheet metal screws but NOT fine threaded drywall screws) and 10d common nails (full headed with minimum head dia. 0.30" and shank dia. 0.148") are permitted interchangeably. Wherever screws or nails are used in a retrofit assembly, they must be held back 2 1/2" from the end and 1/2" in from the edge of any framing member.
The next picture is a plan detail of a truss type gable end that shows placement and fastening details for horizontal braces, compression blocks and retrofit studs. Notice the staggered fastener pattern of the 3 screws attaching the horizontal brace to the "existing framing member." Three such 3" fasteners (#8 screws or 10d common nails) are required at each point where a horizontal brace connects to a truss chord or rafter. Note the 1/2" hold back from the edges of the brace. On the compression block, note the 2 1/2" hold back of the fasteners from each end of the block.
The drawing above shows details for a minimum combination of gable end height and wind speed. The 11 1/4" compression block is the smallest required, as is the 2x4 retrofit stud. Notice how the horizontal brace is "offset" 1 1/2" to the right of the existing stud. This is to accomodate the thickness of the retrofit stud and allows the retrofit stud and the stud anchoring strap to align just below and be attached to the horizontal brace above. The compression block is positioned to align with the existing stud. Neither the compression block nor the horizontal brace is fastened directly to the existing stud. This connection is accomplished by the 3" fasteners spaced 6" on center which attach the retrofit stud to the existing stud AND by the flat metal straps which anchor the retrofit stud to the horizontal brace.
The next picture shows this same retrofit assembly (brace, compression block and retrofit stud) for a stick-built gable end. Note the 1 1/2" offset of the horizontal brace and the alignment of the compression block behind the existing stud. All other details are the same as the previous drawing.
Impediments to Installing Top Horizontal Braces
The guidelines for installing the top horizontal braces are pretty straight-forward on paper but certain obstacles may be present in the field. While things like electrical and communication wires, water lines, HVAC ducts and the like are more of an issue for bottom brace installation, truss web members can interfere with both top and bottom brace location. When truss web members interfere with attaching a brace to the top truss chords, an acceptable alternative is to attach the horizontal brace to the web members themselves. The same number and type of fasteners must be installed and the required fastener setbacks from ends and edges must be maintained. The compression block must be aligned behind the existing gable end stud and the brace must be offset so that the strap attaching the retrofit stud to the brace can be installed properly. Steps to overcome additional obstacles at the bottom will be reviewed in the next section.
Note on Wind Exposure Categories in the Florida Building Code
In section 1609.4 of the 2004 Florida Building Code - Building, Exposure category B is defined as "Urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger." The codes states further that "Exposure B shall be assumed unless the site meets the definition of another type of exposure." Exposure category C is defined as "Open terrain with scattered obstructions, including surface undulations or other irregularities, having heights generally less than 30 feet extending more than 1500 feet from the building site in any quadrant." |
