spring beam

A spring beam is the horizontal tension member that connects the feet of a truss, running the full span between opposing rafter pairs. It resists the outward thrust, or spread, that the sloped rafters try to push downward and outward. Without it, the rafter feet would thrust the walls apart; with it, the truss becomes a closed, self-supporting triangle. It is called the tie beam because it ties the structure together, and "spring" because it must flex slightly under load rather than sit rigid.

Spring beams appear in pitched roof trusses of all types: common rafters, collar ties, scissors trusses, and attic trusses. In a simple common rafter pair, the spring beam runs at the level of the wall plate or slightly above. In attic trusses used to create living space within the roof, the spring beam is positioned higher, creating the floor of the attic room. The depth and section of the spring beam must be sized to handle the total outward thrust from both rafters, which increases with roof pitch, rafter span, and live load.

Sizing and failure modes

Spring beam sizing is governed by tension stress in the wood grain and by horizontal deflection limits. A typical residential truss with a 30-foot rafter span and a 6:12 pitch can generate 10,000 to 15,000 pounds of outward thrust at each end, requiring a 2x10 or 2x12 member depending on wood species and grade. In timber frame construction, spring beams are sometimes doubled or made from solid sawn beams 8 inches or thicker. The most common failure occurs when the spring beam is omitted or when it is installed with insufficient fastening at the rafter feet, allowing creep and permanent deformation over decades.

The term "spring beam" is less common in modern technical literature than "tie beam" or simply "bottom chord," but it persists in carpentry and timber framing communities. In truss shop drawings and engineering documents, it is labeled as the bottom chord, tension member, or tie. The word "spring" refers not to a mechanical spring but to the act of springing apart, which the beam prevents. In older building trades, the spring beam was sometimes called the "binder" or "collar tie," though these terms more properly describe collar ties at the rafter apex.

Where the spring beam sits depends on the truss type and building code. In standard attic trusses for residential construction, it typically sits 8 to 12 feet above the wall plate, defining the ceiling height of the room below. In cathedral or scissor trusses with sloped ceilings, the spring beam is lower and can be visible from below, requiring finish-grade material. In industrial or agricultural clear-span structures, the spring beam may be a heavy steel angle or pipe to minimize deflection across very long spans.