The bottom chord is one of three key components to any truss. In addition to the top chord and webs, the bottom chord is used to transfer forces to the truss bearings. The bottom chord typically carries combined tension and bending stresses. For traditionally spaced trusses, the bottom chord is cut out of 2x4 or 2x6 dimension lumber but can also be cut out of 2x8, 2x10, 2x12, and even LVL material for roof trusses. Factors that determine the size of the bottom chord include on-center spacing, dead loads applied to the truss, span, and if there is any pitch applied to the bottom chord. An example of this is a vaulted ceiling application framed by scissors trusses. In some cases a live load can be applied to a bottom chord, either for storage purposes or if there will be movement within the truss, such is the case in an attic truss.
In floor truss applications, 2x4 or 2x3 material is typically used with the wider portion of the board resting on the bearing or what is known as a 4x2 configuration. Orienting the chord material in a 4x2 manner creates a very stable truss that allows installers the ability to walk easily across members without tipping the truss while installing additional members or floor deck sheathing.
In standard heel applications of roof trusses, the bottom chord is cut on each end at an angle consistent with the roof pitch of the truss. This creates additional surface area for the two members to transfer forces and allows for them to be plated together, creating a strong, resilient joint. On the outermost part of a bottom chord a small vertical cut, known as a butt cut, is created to give the truss a starting and ending point that can be aligned with the bearings below. The butt cut is typically ¼” on standard heel trusses but can be increased for a taller heel. When combined with a slider or stacked chord, a raised heel, sometimes known as an energy heel, can be created to raise the profile of the roof and increase the amount of insulation and ventilation over the bearing of the structure.
The bottom chord is intersected by the top chord and web members at various points throughout the truss, creating a joint secured by metal connector plates. A bottom chord can be multiple pieces of lumber secured by a metal plate known as a splice, either at a joint or mid panel. While a vast majority of trusses will have bottom chords of the same species and grade, changes in both lumber species and grade are possible at splice points. This is most often seen in attic trusses where the bottom chord is increased in the living space and additional loads are applied and then reduced near the bearings to reduce expense.