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Collar ties join opposing rafters high in the attic. The 2021 IRC (R802.4.6) puts them in the upper third of the attic space, requires at least a 1 × 4 nominal board and spaces them no more than 4 ft on centre; Table R602.3(1) calls for 3-10d common or 4-10d box nails at each end. The ridge rises (width ÷ 2) × pitch ÷ 12 above the wall plate, so the lowest tie the code allows is two-thirds of the way up, and because each rafter climbs in a straight line the tie there is one-third of the building width. Enter the width, pitch and, if you like, your own tie height, roof length and rafter spacing, and the Collar Tie Calculator returns the ridge height, the lowest allowed tie height, the tie length and end cut, which rafter pairs get a tie and the nails. On a 24 ft wide house at 6/12 the ridge is 6 ft up, collar ties go at least 4 ft above the plate and are 8 ft long; a 40 ft roof with rafters at 16 in needs 11 ties.
Collar ties are horizontal boards that join opposing rafters high in the attic, mainly to hold the roof together at the ridge under wind uplift. The 2021 IRC (R802.4.6) puts them in the upper third of the attic space, requires at least 1 × 4 nominal lumber and spaces them no more than 4 ft on centre; 2021 IRC Table R602.3(1) calls for 3-10d common, 4-10d box or 4-3 in × 0.131 in face nails at each end. Rafter ties in the lower third (R802.5.2) are a separate requirement.
| Width | Pitch | Ridge height | Lowest tie | Tie length |
|---|---|---|---|---|
| 24 ft | 4/12 | 4 ft 0 in | 2 ft 8 in | 8 ft 0 in |
| 24 ft | 6/12 | 6 ft 0 in | 4 ft 0 in | 8 ft 0 in |
| 24 ft | 8/12 | 8 ft 0 in | 5 ft 4 in | 8 ft 0 in |
| 24 ft | 12/12 | 12 ft 0 in | 8 ft 0 in | 8 ft 0 in |
| 28 ft | 6/12 | 7 ft 0 in | 4 ft 8 in | 9 ft 4 in |
| 28 ft | 8/12 | 9 ft 4 in | 6 ft 2.69 in | 9 ft 4 in |
| 32 ft | 6/12 | 8 ft 0 in | 5 ft 4 in | 10 ft 8 in |
| 32 ft | 10/12 | 13 ft 4 in | 8 ft 10.69 in | 10 ft 8 in |
In the upper third of the attic space, measured from the wall plate up to the ridge. The 2021 IRC (R802.4.6) requires it. On a 24 ft wide house at 6/12 the ridge is 6 ft above the plate, so collar ties go no lower than 4 ft above it.
No more than 4 ft on centre under IRC R802.4.6 — every third rafter pair at 16 in centres, every second pair at 24 in centres. A 40 ft roof with rafters at 16 in has 31 pairs and needs 11 collar ties.
At least 1 × 4 nominal under IRC R802.4.6. Many framers use 2 × 4 or 2 × 6 because they nail better and double as a ceiling frame, but the code minimum is a 1 × 4.
Face nail each end to the rafter with 3-10d common (3 in × 0.148 in), 4-10d box (3 in × 0.128 in) or 4-3 in × 0.131 in nails, per 2021 IRC Table R602.3(1). A 1 1/4 in 20-gauge ridge strap is the listed alternative.
Collar ties sit in the upper third and mainly keep the rafters together at the ridge under wind uplift. Rafter ties (or ceiling joists) sit in the lower third, near the plates, and stop the rafters pushing the walls apart (IRC R802.5.2). Collar ties do not replace rafter ties — raising ties to get a vaulted ceiling needs a structural ridge beam.
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