With scissor trusses that support a 40 psf total load, can the lower chord pitch be 2/3 of the upper chord pitch without causing drywall cracking or other problems?
The general rule of thumb for scissor trusses is that the bottom chord pitch is half of the top chord pitch. The bottom chord can be more steeply pitched than that but that usually means that the truss needs to be made deeper – usually by increasing the depth at the heel (where the truss sits on the wall). This is referred to as a high heel condition. I suspect that the drywall cracking you are concerned with may be due to the horizontal deflection of the trusses. Vaulted framing naturally responds to downward gravity loads by thrusting outwards. ANSI/TPI 1-1995 The National Design Specification for Metal Plate Connected Wood Truss Construction recommends limits to horizontal deflection. It states, “In lieu of specific provisions for lateral movement of trusses and supports, total horizontal deflection at the reactions for the design of trusses shall be limited to 1.25″ due to total load or 0.75″ due to live load, whichever controls.” If this seems excessive to you, you can request a s
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