7.9 Roof Framing Systems

Roof Framing Systems (Rafters, Trusses & Diaphragms)

Roof framing closes the structural box. In California, roofs must carry gravity loads and act as diaphragms that deliver seismic and wind forces into shear walls. Class B contractors must read roof framing plans, choose correct members, preserve engineered truss integrity, and complete nailing/strap schedules before dry-in.

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Rafter Framing Fundamentals

Common rafters span from ridge to wall plate (or beam). Size and spacing come from CRC span tables based on species, grade, spacing, and design loads. Bearing should be full and secure; birdsmouth cuts must not overcut the rafter seat.

Ridge board vs. structural ridge beam:

• A ridge board is typically a non-structural alignment member when opposing rafters are tied by ceiling joists or collar ties as required.
• A structural ridge beam is sized to carry rafter loads when a vaulted ceiling removes the ceiling joist tension tie. It needs posts/points of bearing to foundation—not “hanging in air.”

Ceiling joists commonly serve dual roles: supporting ceiling finishes and acting as rafter ties that resist outward thrust at the plates. Removing joists for a vault without substituting a ridge beam or engineered tie allows walls to spread and ridges to sag.

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Hips, Valleys & Roof Geometry

Hip rafters run from ridge to exterior corner, supporting hip jack rafters. Valley rafters run where roof planes intersect inward, supporting valley jacks and concentrating water and load.

Construction points:

• Provide solid bearing or hangers at designed connections—do not toenail critical hips/valleys as an afterthought.
• Valley areas need careful sheathing cuts and, later, proper valley underlayment/flashing (often woven or metal valley systems).
• Wide chimneys or penetrations may require crickets on the high side to divert water (see roofing provisions when width exceeds threshold such as 30 inches parallel to ridge).

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Metal-Plate-Connected Wood Trusses

Most production California roofs use engineered trusses:

• Install to the truss placement plan: right-side-up, correct spacing, bearing at designated points only.
• Install temporary and permanent bracing per the truss engineer’s drawings—webs and chords can buckle during and after erection without bracing.
• Never cut, notch, or bore truss members or metal plates for ducts, lights, or vents without written repair details from the truss engineer.
• Gable end frames and drag struts must connect into the wall bracing system for seismic load path.

Scenario: A Sacramento framer notches the bottom chord of a garage truss 2½ inches to pass a furnace flue. The inspector fails the roof framing and requires an engineered repair. Cost dwarfs the HVAC redesign that would have offset the flue between trusses.

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Roof Diaphragm & Seismic Basics

Under CRC/CBC lateral design concepts used throughout California’s high seismic regions:

• Roof sheathing (typically wood structural panels) acts as a diaphragm collecting lateral loads.
• Boundary nailing (edges and ridges/hips) is denser than field nailing; missing edge nails are a top framing-inspection failure.
• Diaphragm forces move into shear walls through continuous load paths—straps, collectors, and properly sheathed walls below.
• Large roof openings (stairs to roof decks, big skylights) interrupt diaphragms and often need framed headers, blocking, and detailing shown on structural sheets.

Gravity load reminders for California:

• Tile roofs impose high dead load; verify rafters/trusses were designed for tile, not just composition shingles.
• Solar PV add-ons add dead load and uplift; attachments must hit structural members per listing and local rules—not random sheathing screws only.
• Roof live load / snow load categories appear on structural notes for mountain jurisdictions (Tahoe, etc.); lowland coastal jobs still need correct wind/seismic detailing.

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Installation Quality Checklist Before Frame Inspection

1. Rafter/truss spacing and bearing match plans.
2. Hurricane/seismic ties and straps at plates installed where scheduled (hardware section expands this).
3. Ceiling joists lapped/nailed or hangers installed as designed; vaults resolved with ridge beams.
4. Roof sheathing gaps, edge distances, and nail schedules correct; H-clips if required.
5. Ridge, hip, and valley members supported; no unauthorized truss cuts.
6. Fire blocking / draft stops at concealed spaces as required before cover.

Roof framing errors rarely stay cosmetic. Spreading walls, bouncy ridges, and incomplete diaphragms are structural defects that California inspectors and later home inspectors both catch.

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Purlin, Strongback & Attic Load Paths

Where long rafter spans need intermediate support, purlins with struts to bearing walls redistribute gravity loads. Struts must land on walls or beams capable of carrying the load—not on ceiling joists alone unless designed. Strongbacks on truss systems straighten bottom chords and help distribute loads; install per truss drawings.

Attic mechanical platforms add concentrated dead load. Frame platforms to transfer load to walls or strengthened joists; do not hang heavy air handlers from a single bottom chord. Provide permanent attic access sized for equipment replacement so future Californians are not cutting trusses to remove a furnace.

Pre-Dry-In Weather Protection

California’s wet season can soak unprotected OSB roof decks. Schedule sheathing and underlayment promptly after framing approval milestones allow, or use temporary protection. Saturated panels lose nail-holding capacity and can cup—creating both structural and roofing problems before the covering goes on.