7.2 Core Workflows and Decision Points
Key Takeaways
- Where ceiling joists are not parallel to and tied to rafters, rafter ties are required to resist outward thrust (R802.4.5).
- Collar ties (or a ridge strap) go in the UPPER third of the attic; rafter ties act in the LOWER third near the ceiling line.
- A ridge board must be at least 1 inch nominal and not less in depth than the cut end of the rafter; a structural ridge beam is required only when ties are omitted.
- Notches in rafters/joists max 1/6 the depth, never in the middle third; bored holes max 1/3 the depth and 2 in. from edges (R802.7).
- Rafter-to-plate connections must resist gravity AND wind uplift per Tables R802.11/R602.3(1), not gravity alone.
Ridge, rafter ties, and collar ties
A conventional gable roof develops outward thrust at the eaves: gravity on the sloped rafters tries to spread the walls apart. The IRC controls this with the ridge connection and ties.
- Ridge board (R802.3): Where rafters meet at a ridge, they bear against a ridge board not less than 1-inch nominal thickness and not less in depth than the cut end of the rafter. A ridge board is a non-structural alignment member; the thrust is resisted at the bottom.
- Ceiling joists as ties: When ceiling joists run parallel to the rafters and are lapped or strapped to them at the top plate, the joists are the rafter ties and resist thrust directly.
- Rafter ties (R802.4.5): When ceiling joists are not parallel to the rafters (or are raised), separate rafter ties are required, installed near the plate line in the lower third of the attic height. Spacing of rafter ties may not exceed 4 feet on center.
- Collar ties (R802.4.6): Collar ties resist ridge separation/uplift and are installed in the upper third of the attic height, spaced not more than 4 feet on center, using a minimum 1x4 nominal member.
- Structural ridge beam: If rafter ties (or ceiling-joist ties) are omitted — common with cathedral and vaulted ceilings — the ridge must be a structural ridge beam designed to carry the rafter reactions, with posts to a bearing point.
Exam trap: candidates swap the two. Rafter tie = lower third, resists wall spread. Collar tie = upper third, resists ridge uplift. A wrong-third answer is the classic distractor here.
Rafter and ceiling-joist connections
The rafter-to-ceiling-joist and rafter-to-plate connections carry both gravity and wind uplift, so simple bearing is never sufficient. The required number of nails in the heel-joint connection of a rafter to its ceiling joist is set by Table R802.5.2 (formerly R802.5.1(9)) as a function of ground snow load, roof span, and rafter spacing.
The rafter-to-top-plate connection is fastened per the fastening schedule in Table R602.3(1); in higher-wind areas this toenail is replaced by a metal hurricane/rafter tie. The inspector's rule: identify the design wind speed and ground snow load first, then count the fasteners or confirm the rated connector.
Cutting, notching, and boring (R802.7)
Rafters, ceiling joists, and roof members are dimension lumber and follow the same notch/bore limits as floor joists. Memorize the fractions — they are heavily tested:
| Action | Limit | Location rule |
|---|---|---|
| Notch depth | ≤ 1/6 of member depth | Not in the middle 1/3 of the span |
| Notch at member end | ≤ 1/4 of member depth | At the bearing end only |
| Notch length | ≤ 1/3 of member depth | — |
| Bored hole diameter | ≤ 1/3 of member depth | ≥ 2 in. from top/bottom edge and from any other hole |
| Cantilever notch | Remaining ≥ 4-in. nominal | Cantilever ≤ 24 in. |
Engineered products are different: manufactured trusses and I-joists may NOT be cut, notched, or bored except as the manufacturer's documents or a registered design professional permits. Field-altering a truss is an automatic failure and a frequent exam scenario.
Decision workflow
For a roof-frame question, run a fixed sequence: (1) Is the member sawn lumber or engineered? (2) What resists thrust — ceiling joists, rafter ties, or a structural ridge beam? (3) Is the tie/collar in the correct third? (4) Does the connection resist both gravity and the design uplift? (5) Are any cuts within the notch/bore limits? The answer that respects the load path and the correct code table — not the fastest field shortcut — is the one to choose.
Purlins, struts, and intermediate support
Where a rafter span exceeds the table maximum, the IRC permits purlins to reduce the unsupported span. A purlin is sized not less than the rafter it supports and is braced to a bearing wall or beam with struts (braces) not less than 2x4 nominal, installed at no more than 45 degrees from horizontal, spaced not more than 4 feet on center. This lets a builder use a 2x8 on a long span by breaking it into two shorter table-compliant spans.
Inspectors check that struts land on a load-bearing wall — a strut bearing on a non-structural partition or ceiling joist simply transfers the problem and is a deficiency. This is also why the load-path question matters: every roof reaction must reach a wall or beam that itself reaches the foundation.
Uplift and the continuous wind path
Wind does not only push down; it lifts. The IRC requires a continuous load path for uplift from the roof to the foundation. The rafter-to-wall connection (toenails per Table R602.3(1) or metal ties) transfers uplift to the wall, then the top-plate-to-stud and wall-to-foundation anchorage carry the same force down. The required uplift connection grows with the design wind speed and the roof's tributary area; in coastal and high-wind regions a simple three-nail toenail is replaced by a rated hurricane tie at every rafter.
The inspector reads the uplift requirement from Table R802.11 (uplift connection forces) and confirms the specified connector is installed at every bearing — one missing tie breaks the chain.
On a gable roof where the ceiling joists run perpendicular to the rafters, the framer omits ceiling-joist ties and adds collar ties 18 inches below the ridge. What is the code concern?
A 2x10 rafter (actual depth 9-1/4 in.) needs a bored hole for wiring. What is the maximum allowable hole diameter and minimum edge clearance per R802.7?
Which statement about altering a manufactured roof truss in the field is correct?