Bottom chords are typically designed for 10 psf. Drywall and insulation takes up most of that. Any unusual loads on the truss should be given to the truss company, which can easily account for that in the design. The Truss Plate Institute (TPI) manual is the truss design manual. The TPI manual relies on the “Building Designer/Engineer" for that information. The TPI manual list the following responsibilities:
Building Designer/Engineer - a design professional, individual or organization, having responsibility for overall building design. Within the scope of wood trusses, the building designer / engineer, shall specify the following:
(a) Design loads in accordance with various sections of the National and / or Provincial Building
(b) Truss profile and intended support locations.
(c) Vertical and horizontal deflection limits.
(d) Moisture environment for intended end use.
(e) Any special requirements to be considered in the truss design.
(f) Additional loads from mechanical, electrical units, which may induce extra load to various truss
members and their locations. As this standard does not cover the design for the complete structural system of a building, the building designer / engineer shall provide the following in the design and detailing of the building:
(a) Truss supports and anchorage accommodating horizontal, vertical or other reaction or
(b) Permanent truss bracing to resist wind, seismic and any other lateral forces acting parallel or
perpendicular to the plane of trusses.
(c) Method of connection or anchorage of mechanical, electrical units to various truss members.
IMO here is the reason for most truss problems. The truss company relies in this mysterious Building Designer/Engineer to give them the information they need to design the truss and to ensure the tie down and bracing system is in place. But in reality the building contractor orders the trusses directly and doesn’t know or care where the HVAC is going. The contractor doesn’t typically know how to read truss plans, so he/she doesn’t know the standard hurricane clips may not be adequate for all trusses within the roof framing system. The contractor probably doesn’t know some girder trusses need additional support all the way down to the foundation. The contractor doesn’t know the truss designer may have relied on and interior wall for support on long truss spans. Some body needs to get the truss designer, contractor, homeowner, engineer and home inspector in one room and hash this out. Everyone thinks someone else is taking care of the details.