A rafter tie is a horizontal framing member connecting the lower ends of opposing rafters, typically near the wall top plates, resisting the outward thrust that a sloped roof naturally exerts on the walls it rests upon, and often serving double duty as a ceiling joist in conventional roof framing.
This structural function is critical in most conventional roof designs, since without adequate thrust resistance, the outward force from a loaded roof can gradually push the supporting walls apart over time.
Building codes specify precise requirements for rafter tie placement, spacing, and connection strength, reflecting just how essential this often-overlooked element is to a roof’s overall structural stability.
What Does a Rafter Tie Involve?
- Horizontal framing connecting opposing rafter bases
- Placement near the wall top plates
- Adequate connection strength to resist tension forces
- Often doubling as the structure’s ceiling joists
Why Are Rafter Ties Important?
By resisting the outward spreading force rafters naturally exert under load, rafter ties keep a building’s walls from gradually pushing apart, a structural function so fundamental that its absence or inadequate connection is a leading cause of sagging ridges and spreading walls in older or poorly built roof structures. This is precisely why removing ceiling joists that also serve as rafter ties, a common temptation during attic conversions, requires an engineered alternative like a structural ridge beam to safely compensate.
Connection quality at each end of a rafter tie matters as much as its size, since the member works in tension, meaning a rafter tie that’s adequately sized but poorly connected can still fail to resist the thrust it’s meant to counter.
Rafter Tie vs. Collar Tie
A rafter tie is positioned low, near the wall plates, actively resisting outward thrust. A collar tie is positioned higher, near the ridge, addressing a different aspect of roof structural behavior related to ridge sag rather than wall spreading.