Collar Tie Calculator

About the Collar Tie Calculator

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.

When This Page Helps

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.

How to Use the Inputs

1. Enter the building span (outside wall to outside wall).
2. Enter the roof pitch.
3. Enter the desired height above the top plate for the collar ties.
4. Enter the on-center spacing of the collar ties (max 48″).
5. Enter the building length.
6. Read the tie length and total number of ties needed.

Example Calculation

Tips & Best Practices

• Collar ties must be in the upper third of the rafter span—measure from the plate to the ridge to verify.
• Minimum collar tie size is 1×4 (nominal), but 2×4 is recommended for nailing strength.
• Each tie must be face-nailed to each rafter with three 10d nails minimum.
• Maximum spacing is 4 feet on center per IRC R802.3.1.
• Collar ties do NOT replace ceiling joists or rafter ties for resisting horizontal thrust.
• For cathedral ceilings without ceiling joists, a structural ridge beam is needed—collar ties alone are insufficient.

IRC Collar Tie Requirements

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.

Collar Ties in Vaulted Ceilings

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.

Upgrading Collar Ties

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.

Frequently Asked Questions

• What is the difference between a collar tie and a rafter tie?

  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.

• Do all rafters need collar ties?

  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.

• Can I use collar ties as ceiling joists?

  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.

• What happens without collar ties?

  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.

• Where exactly should collar ties be placed?

  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.

• Can I use metal straps instead of collar 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.