Open-web floor trusses

Open web floor trusses can be useful for long spans and for threading mechanicals, but they add height to the structure and have to be engineered and fastened appropriately.

Band joists are not necessary, though engineered lumber may be used or a simple ribbon band of 2x material at the top of the outside edge.

They can be bottom chord bearing, top chord bearing or hung from hangers - but must be engineered for each particular fastening approach as well as for any intermediate supports or point loads.

Deep trusses may require a strongback for stiffening, and top chord bearing trusses must be tied together at the bottom to a fixed termination to prevent lateral swing. They are generally extremely strong in the stiffness and deflection but can vibrate if not properly anchored or stiffened.

And, like any light-framed truss, they collapse quickly in the event of a structure fire and can suffer plate corrosion if in contact with incompatible compounds (such as the ammonium sulfate in some cellulose insulation) or excess humidity.

The rule of thumb for web truss depth is the span less 8. A 12" truss would span 20', a 16" truss would span 24'.