A collar tie is a short board or strap that connects opposing rafters in the upper third of the attic. A rafter tie is a low tie, at or near the wall plates, that stops the rafters pushing the walls outward. The 2021 IRC sets a minimum size for each (1x4 nominal for collar ties, 2x4 for rafter ties), and a collar tie never replaces the low tie.
People mix the two up because both are small boards spanning between rafter pairs. They sit at different heights, resist different forces, and are nailed differently. This guide covers where each goes, the code minimums, the ridge strap alternative, and how to show them on a roof framing plan. The adopted code edition and local amendments govern.
Collar tie vs. rafter tie
| Collar tie | Rafter tie | |
|---|---|---|
| Location | Upper third of the attic space | At the bottom of the rafters, at the wall plates |
| Main job | Connects opposing rafters near the ridge | Holds the walls together against rafter thrust |
| Minimum size | 1 inch by 4 inches nominal | 2 inches by 4 inches nominal |
| Maximum spacing | 4 feet on center | 24 inches on center |
| Code section | R802.4.6 | R802.5.2.2 |
| Replaces ceiling joists? | No | Yes, where ceiling joists run the wrong way |
Summary of 2021 IRC Sections R802.4.6 and R802.5.2.2.
The simple test is height. A tie in the bottom third of the rafter height that is fastened to both rafters per Table R802.5.2(1) is a tie against thrust. A board near the ridge is a collar tie. Think of the rafter pair as an A-frame: the low tie is the string on the bow, while the collar tie is a brace near the apex that keeps the two rafters aligned.
What is a collar tie?
Section R802.4.6 says that where collar ties are used to connect opposing rafters, they go in the upper third of the attic space and are fastened per Table R602.3(1). The size minimum is 1 inch by 4 inches nominal, and spacing is not more than 4 feet on center.
Note the wording "where collar ties are used". The code gives you a way to connect opposing rafters at the ridge, and Section R802.4.2 lists the options: a collar tie or ridge strap under R802.4.6, or rafters meeting directly opposite each other at a gusset plate. R802.4.2 also limits the offset between opposing rafters at a ridge board to 1-1/2 inches.
Collar tie nailing
Table R602.3(1) gives the fastening for a collar tie to rafter, face nailed at each rafter. The listed options are:
- 4-10d box (3 in. x 0.128 in.) nails, or
- 3-10d common (3 in. x 0.148 in.) nails, or
- 4 nails of 3 in. x 0.131 in.
Those are the nails at each rafter end of the tie. Check the table in your adopted edition, since local amendments can change fastener schedules.
What collar ties actually do
Collar ties keep opposing rafters connected near the peak, which helps resist the ridge separating when wind uplift pulls the roof upward. They do not stop the walls spreading. A collar tie near the ridge sits where the rafters are close together, so it carries almost none of the outward thrust at the bottom. That is why the 2021 IRC treats thrust restraint as a separate requirement under Section R802.5.2, covered below.
What is a rafter tie?
Section R802.5.2.2 requires wood rafter ties of not less than 2 inches by 4 inches, installed per Table R802.5.2(1), at a maximum of 24 inches on center. The same section permits other approved rafter tie methods. Blocking under R802.5.2.3 must be not less than utility grade lumber.
The principle behind it is Section R802.2: the roof and ceiling assembly must provide continuous ties across the structure so thrust is not applied to the supporting walls. For the ridge board side of that argument, see ridge beam vs. ridge board.
Rafter ties vs. ceiling joists
Ceiling joists often do the job of rafter ties. Section R802.5.2 splits the cases:
- Ceiling joists parallel to the rafters, in the bottom third of the rafter height. They are fastened to the rafters per Table R802.5.2(1) and act as the tie.
- Ceiling joists above the bottom third of the rafter height. The ridge must be designed as a beam per Section R802.3.
- Ceiling joists not parallel to the rafters. Rafters must be tied across the structure with a rafter tie per Section R802.5.2.2, or the ridge must be designed as a beam.
Where a ceiling joist is the tie, it must be continuous across the building or properly joined. Section R802.5.2.1 requires ends to be lapped at least 3 inches or butted over bearing partitions or beams and toenailed to the bearing member. Where the joists provide the continuous tie, lapped joists are nailed together per Table R802.5.2(1) and butted joists need a connection of equivalent capacity. Joist sizes come from the ceiling joist span chart, a separate question from tie nailing.
Rafter tie heel joint nailing
The connection between the rafter and the tie at the wall (the heel joint) is where thrust is transferred, so the nail count depends on the load. Table R802.5.2(1) lists the number of 16d common nails per heel joint by ground snow load, rafter slope, rafter spacing, and roof span. The count rises with snow load, span, and spacing, and falls as the slope gets steeper.
2021 IRC Table R802.5.2(1), required number of 16d common nails per heel joint, 30 psf ground snow load, rafter spacing 16 inches on center, ties at the bottom of the attic space, 10 psf roof dead load.
| Rafter slope | 12 ft roof span | 24 ft roof span | 36 ft roof span |
|---|---|---|---|
| 3:12 | 4 | 8 | 12 |
| 4:12 | 3 | 6 | 9 |
| 5:12 | 3 | 5 | 7 |
| 7:12 | 3 | 4 | 5 |
| 9:12 | 3 | 3 | 4 |
| 12:12 | 3 | 3 | 3 |
The table has further columns for 20, 50, and 70 psf ground snow loads and rows for 12, 19.2, and 24 inch spacing. Read your own cell rather than this extract. Footnotes to the table that matter on a drawing:
- 10d substitution. 10d common nails may replace 16d common if the nail count is taken as 1.2 times the tabulated number, rounded up to the next full nail.
- Supported ridge. Heel joint connections are not required where the ridge is supported by a load-bearing wall, header, or ridge beam.
- Struts or purlins. Where vertical struts or purlins to a load-bearing wall give intermediate support, the tabulated requirement may be reduced proportionally to the reduction in span.
- Lap splices. Ceiling joist to ceiling joist lap splices need equivalent nailing patterns.
Raised rafter ties
The table assumes the ties are at the bottom of the attic. Where they are higher, Table R802.5.2(2) increases the heel joint requirement by a factor based on HC/HR, the height of the ties above the rafter support walls divided by the height of the ridge above those walls.
2021 IRC Table R802.5.2(2), heel joint connection adjustment factors.
| HC/HR | Adjustment factor |
|---|---|
| 1/3 | 1.5 |
| 1/4 | 1.33 |
| 1/5 | 1.25 |
| 1/6 | 1.2 |
| 1/10 or less | 1.11 |
Where HC/HR exceeds 1/3, the table note requires connections designed per accepted engineering practice. Raised ties also change rafter spans, which the rafter span chart covers through its raised-tie factors.
Ridge straps instead of collar ties
Section R802.4.6 allows ridge straps to replace collar ties. The strap must be:
- not less than 1-1/4 inch by 20 gage,
- nailed to the top edge of each rafter,
- with not fewer than three 10d common (3 in. x 0.148 in.) nails per rafter, and
- the closest nail not closer than 2-3/8 inches from the end of the rafter.
A strap across the ridge is a simple choice where an attic is finished or has a vaulted section and a board across the room is unwelcome. It is a substitute for the collar tie only. A ridge strap does not provide the low tie required by Section R802.5.2.
Collar ties on a cathedral or vaulted ceiling
With no ceiling joists and no low rafter ties, nothing resists the thrust, and collar ties do not fix that. The ridge has to be designed as a beam under Section R802.3, supported at each end by a wall or column, and the beam, posts, and footings need sizing by a designer. See ridge beam vs. ridge board for when that applies. A trussed roof avoids the question, because each truss carries its own bottom chord; rafters vs. trusses compares the two.
Showing ties on a roof framing plan
On a roof framing plan or section, label the tie type and location so a reviewer can find the code cell you used:
- Collar ties: note size and spacing, for example
1x4 COLLAR TIES @ 48" O.C. MAX, UPPER THIRD, and the fastening. - Rafter ties or ceiling joists: note which one is acting as the tie, its size and spacing, its height above the plate, and the heel joint nail count with the table and snow load used.
- Ridge straps: call out the strap size and nailing.
- Ridge type: state whether the ridge is a board or a beam, since that decides whether the heel joint nailing applies. The gable and hip roof framing guide shows where hips and valleys change the tie layout, and roof pitch and slope explains the slope input to the nailing table.
Framing Plan draws joist layouts, beams, and openings and exports PNG, PDF, and DXF. It does not check code or size members, and its editor works in millimeters with a sample profile of demo figures, so use it for a preliminary drawing and take tie sizes and nailing from the code tables or your designer. The tables here are prescriptive provisions of the 2021 IRC for the stated conditions only. Engineered products use manufacturer instructions, and the chapter is also available from the ICC code site. Open the framing studio to sketch the roof layout before it goes to your designer or plan reviewer.