Calculate collar tie length, spacing, and placement height for rafter roofs. Ensure your collar ties meet IRC structural requirements.
Collar ties are horizontal members that connect opposing rafters in the upper third of the roof structure. They help prevent rafters from separating at the ridge under wind uplift or unbalanced loads. The IRC requires collar ties (or ridge straps) on every other rafter pair at a maximum of 4 feet on center.
This collar tie calculator computes the tie length based on where the ties intersect the rafters at the specified height above the top plate. It also calculates the number of ties needed for the building length and verifies that the tie height falls within the upper third of the rafter span as required by code.
Don't confuse collar ties with rafter ties (ceiling joists). Rafter ties resist the horizontal thrust at the wall plate and are a different structural element, typically located at or near the plate level.
This measurement supports better project estimation, enabling contractors and engineers to deliver accurate bids and avoid costly overruns during the construction process.
Collar ties are a code requirement that's often misunderstood. This calculator ensures your ties are the right length, at the correct height, and at proper spacing. Getting it wrong can lead to a failed inspection or, worse, a structural issue. Regular use of this calculation supports compliance with building codes and inspection requirements, helping projects proceed smoothly through the permitting and approval process.
Rafter span at tie height = Building Span − 2 × (Tie Height / tan(pitch angle)) Tie Length = Rafter span at tie height Number of ties = floor(Building Length / Spacing) + 1 Upper third check: Tie Height ≥ 2/3 × Rise
Result: 18'-0″ collar ties, 11 ties needed
With a 28-ft span and 6:12 pitch, the total rise is 7 ft. At 5 ft above the plate, horizontal inset = 5/tan(26.57°) = 10 ft total (5 ft each side). Tie length = 28 − 10 = 18 ft. At 48″ OC over 40 ft: floor(40×12/48)+1 = 11 ties.
Section R802.3.1 of the IRC requires collar ties (or ridge straps) in the upper third of the attic space, spaced at not more than 4 feet on center. They must be at least 1×4 nominal lumber and face-nailed with three 10d common nails at each end.
In cathedral or vaulted ceiling designs, collar ties can serve as the visible ceiling structure. However, they must be combined with a structural ridge beam or adequate rafter ties at the plate level to resist horizontal thrust. Without these, the walls will spread outward over time.
In high-wind or heavy-snow regions, upgrading collar ties from 1×4 to 2×6 provides additional resistance to uplift and unbalanced loading. Some engineers specify metal connectors at each end instead of simple face nailing for improved connection strength.
A rafter tie (ceiling joist) is located at or near the wall plate level and resists the outward horizontal thrust of the rafters. A collar tie is located in the upper third of the rafter span and primarily resists ridge separation from uplift loads.
The IRC requires collar ties or ridge straps on not less than every other rafter pair, spaced no more than 4 feet on center. So every pair needs them if rafters are at 4-foot OC, or every other pair at 2-foot OC would also satisfy.
Collar ties can support a flat ceiling if they're sized for the span, but they cannot perform the structural role of rafter ties. The two serve different structural purposes and are located at different heights.
Without collar ties (or ridge straps), the ridge connection relies solely on nail strength to keep the rafter tops together. Strong wind uplift or unbalanced snow loads could separate the rafters at the ridge.
In the upper third of the rafter measured vertically. For a roof with 9 feet of rise, the collar ties should be at least 6 feet above the top plate. Placing them lower than the upper third converts them into rafter ties.
Yes. The IRC allows metal ridge straps as an alternative to collar ties. They must be rated for the required tension load and installed per the manufacturer's instructions with the specified fasteners.