2021 IRC Chapter 8
This lesson covers the roof frame itself, rafters, the ridge, and the tie system that keeps a sloped roof from pushing walls outward, plus the two things a rehab or infill inspection checks in the attic once the frame is up: ventilation and access. The numbers below come from IRC Chapter 8, Sections R802, R806, and R807 of the 2021 code, checked against the code text. Local adoption governs, so always confirm the edition and any amendment a given city has adopted before citing a number in the field.
A conventionally framed sloped roof is really two rafters leaning against each other at the ridge. Left alone, that pair pushes outward at the bottom as much as it pushes up at the top, and that outward push, called thrust, is what a tie system exists to stop. At the top, Section R802.3 requires a ridge board not less than 1 inch nominal in thickness and not less in depth than the cut end of the rafter, so the full angled cut bears against it. A ridge board is not a beam; it is a nailing surface that keeps opposing rafters aligned and carries no vertical load of its own. Where ceiling joists or rafter ties do not provide a continuous tie across the structure, the ridge must instead be a beam, designed by accepted engineering practice and supported on each end by a wall or column, and it carries the rafters. Confusing a ridge board with a ridge beam on a cathedral-ceiling remodel is a common mistake in older rehab houses where a ceiling was removed without adding a beam.
Section R802.5.2 covers the connection where ceiling joists act as rafter ties, resisting thrust. That connection must sit in the bottom third of the rafter height, close to the rafter's bearing point, because a tie placed higher multiplies the force it carries and lets the wall tops deflect further before it engages. Under Section R802.5.2.1, ceiling joist ends over a bearing partition must be lapped at least 3 inches or butted; where the joists serve as the continuous tie, lapped joists are nailed together per Table R802.5.2(1) and butted joists need a connection of equal capacity, so the tension path across the building stays unbroken. Collar ties do a different job: installed under Section R802.4.6 in the upper third of the attic space, not less than 1x4 nominal and spaced not more than 4 feet on center (metal ridge straps are an accepted alternative), they resist wind uplift at the ridge, not thrust. Without a rafter tie in the lower third, or a structural ridge beam, the wall tops bow outward no matter how many collar ties sit near the peak; a sagging ridge with bowed exterior walls is the classic field symptom of that missing lower tie. Where ceiling joists run perpendicular to the rafters, they cannot act as ties at all, so rafter ties (wood ties at least 2x4, at most 24 inches on center, per R802.5.2.2) or a ridge designed as a beam are needed instead, worth checking on any hip roof or framing where joist direction changes partway across the house.
A purlin is a horizontal member that picks up mid-span rafter load, shortening the effective rafter span. It must be sized no smaller than the rafters it supports; undersized purlins sag and leave the roof plane wavy. Its braces matter just as much: they are 2x4 members that must slope not less than 45 degrees down to an actual bearing wall, spaced not more than 4 feet on center, with unbraced length capped at 8 feet. A brace landing on a ceiling joist over open space, instead of a wall built to carry that load, has only relocated the overload.
A sawn rafter is a bending member like a floor joist and follows the same proportional cutting limits under Section R802.7.1, which adopts the R502.8.1 limits: edge notches limited to one-sixth of the depth and prohibited in the middle third of the span, end notches up to one-fourth of the depth, and bored holes up to one-third of the depth with 2 inches of edge clearance, letting an inspector evaluate a bath-fan duct or wiring bore through a rafter without a separate table.
A manufactured roof truss is different: an engineered assembly where every member, including the bottom chord, is sized for a specific tension or compression role. Field-cutting a bottom chord to route ductwork, a routine rehab-attic finding, can sever the tie holding the whole truss together, and Section R802.10.4 bars cutting, notching, drilling, or splicing truss members without the approval of a registered design professional; no prescriptive fix applies. The truss drawings also specify permanent bracing for the webs and chords, which, unlike temporary erection bracing, stays part of the finished structure and becomes invisible once insulation is in, so it belongs on the frame-stage inspection.
Section R806.2 sets the baseline net free ventilating area at 1/150 of the vented space's area, reducible to 1/300 where ventilation splits between high and low openings: 40 to 50 percent of the required area in the upper portion (upper ventilators not more than 3 feet below the ridge, measured vertically), the balance in the bottom one-third of the attic space. In Climate Zones 6, 7, and 8, the reduced ratio also requires a Class I or II vapor retarder on the warm-in-winter side of the ceiling. Under Section R806.1, ventilation openings must have a least dimension between 1/16 inch and 1/4 inch, and larger openings need corrosion-resistant screening or similar material with openings in that range, small enough to exclude pests but large enough not to clog with dust. Cross ventilation means each separate attic space has its own intake and exhaust path; a hip section cut off by a valley, or a compartment above a dropped ceiling, needs its own vents, and one blocked bay can leave a section unvented even when the rest of the house looks fine.
An unvented attic assembly is not simply an attic with the vents left out; it is a designed alternative under Section R806.5, permitted only where every listed condition is met: the attic is completely within the building thermal envelope, no Class I vapor retarder is installed on the ceiling side, and the insulation is placed and rated as the section prescribes (for example, air-impermeable insulation in direct contact with the underside of the roof sheathing, with the rigid board R-values of Table R806.5 where required for condensation control). The details are prescribed, so an unvented attic is either built exactly to spec or it is a vented attic with the vents missing, a defect either way.
Section R807.1 requires an attic access opening in buildings with combustible ceiling or roof construction wherever the attic has 30 inches or more of vertical height over an area of 30 square feet or more. Where met, the rough-framed opening must be at least 22 by 30 inches, with at least 30 inches of headroom above it, in a hallway or other readily accessible spot, not behind fixed shelving or somewhere needing an unusual climb. That size is easy to confuse: 18 by 24 inches is the crawl-space floor-access number from a different section (R408.4), a common mix-up on a report. Verify the clear opening after the hatch, trim, and weatherstripping go in, not just the rough framing dimension on the plans, since finish materials routinely eat into the minimum.
In the field
On a rehab where blown insulation has been added to an existing attic, walk the eaves first. Insulation packed tight against the sheathing at the soffit vents is a common find, killing the intake side of the ventilation path even though the ridge vent above still looks fine. Ventilation depends on a genuine stack effect, cool air in low, warm air out high, and a blocked soffit just pulls the ridge vent's air from the conditioned house instead of outdoors. Section R806.3 requires at least a 1-inch space between the insulation and the roof sheathing at the vent, and baffles at the eave keep that path open once insulation is blown in.
A house in a mixed climate zone (not 6, 7, or 8) has a 2,400-square-foot attic, with 45 percent of the vent area high near the ridge and the balance low at the soffits, satisfying the 40-to-50-percent split Section R806.2 requires for the reduced ratio. Because that split is met, the required area drops from 1/150 to 1/300: 2,400 divided by 300 equals 8 square feet total, with 45 percent (3.6 square feet) high and the balance (4.4 square feet) low; the section allows 40 to 50 percent (3.2 to 4.0 square feet) high. Without that split, the same attic would need the full 1/150 ratio, 16 square feet, exactly double, which is why an inspector checking a 1/300 installation verifies the split before accepting the smaller total.
1. Under Section R802.3, a ridge board must be at least:
2. Where ceiling joists act as rafter ties and lap over an interior partition instead of running continuously, the minimum lap is:
3. Collar ties under Section R802.4.6 primarily resist:
4. An inspector finds no ceiling joists or rafter ties anywhere in a vaulted room's roof framing, and no structural ridge beam either. What is missing?
5. Rafter ties resist outward thrust most effectively when located:
6. Rafters bearing on a masonry wall need a minimum bearing length of:
7. A homeowner's HVAC installer cuts through the bottom chord of a manufactured roof truss to route a duct. The correct inspector response is:
8. Under Section R806.2, the baseline minimum net free ventilating area for an attic is:
9. A house in a mixed climate zone (not 6, 7, or 8) has a 3,000-square-foot attic, with 45 percent of the vent area high near the ridge and the balance low in the soffits, satisfying the required split. The minimum net free ventilation area is:
10. Under the reduced 1/300 ratio, the upper ventilators must be located within approximately how far, measured vertically, of the ridge or highest point of the attic space?
11. An inspector finds blown attic insulation packed tight against the roof sheathing at the eaves, burying the soffit vents. The correct finding is:
12. Under Section R807.1, an attic access opening is required where the attic has an area of 30 square feet or more and a vertical height of at least:
13. The minimum rough-opening size for a required attic access under Section R807.1 is:
14. An inspector finds purlin braces supporting a rafter's mid-span landing on a run of ceiling joists over open attic space, rather than on a bearing wall below. The concern is: