Proper Bracing Of Valley Framing
Question:
It is critical that permanent top chord bracing is supplied by proper nailing of the valley truss bottom chords to the carrying truss top chords, through purlins or properly installed rated roof sheathing. If the sheathing is not carried through under the valley framing, then the permanent top chord bracing must be supplied in another manner. There are actually a number of different conditions where a similar issue arises: where the over framing runs perpendicular to the trusses supporting it (like valley framing and some truss-on-truss framing) and where the over framing runs parallel to the trusses supporting it (like multi-piece trusses or piggyback trusses). The design of the permanent structural bracing system to transfer both vertical and lateral loads within the roof system and between the roof system and the structure is the responsibility of the building designer. John Meeks, P.E, discusses most of these conditions in detail in the Commentary for Permanent Bracing of Metal Plate Connected Wood Trusses in SBCA’s Metal Plate Connected Wood Truss Handbook. However, he does not specifically address the condition where the valley framing is carried out with valley truss frames (not actually trusses, but traditionally called so). Must the roof sheathing be carried through underneath valley trusses?
Answer:
When there are large valleys, the area under valley framing can be considerable. Installers might feel that continuing the sheathing under the valley framing is not advisable because it is a waste of material or because they want to allow the free flow of air between roof areas. Regardless, the top chord of the supporting trusses beneath the valley set must be properly braced. Valley trusses are non-structural elements that are merely transferring loads onto carrying trusses below them, due to a change in the roof line/configuration. They require continuous bearing, usually by sitting on sheathing or by sitting on the top chords of the carrying trusses below them if carrying trusses are spaced 24 in. O.C. or less. Continuous bracing of the top chord of the carrying trusses can be accomplished by one of three methods:
- Properly installed rated sheathing applied before the valley trusses are installed. Sheathing supplies both the required lateral and diagonal bracing requirements. This method works for conventionally framed valleys.
- Purlins properly spaced and fastened as specified on the truss design drawing for both the carrying truss and the valley truss as well as required diagonal bracing. This method works for conventionally framed valleys.
- Valley trusses used in place of purlins spaced and fastened per the truss design drawing as well as required diagonal bracing. One must also follow proper temporary bracing procedures, including diagonal bracing, for the carrying trusses and the valley trusses.
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