Roof Framing Basics (Pitch, Common Rafters & Trusses)

Roof Framing Basics (Pitch, Common Rafters & Trusses)

Quick Answer: Pitch is rise/run (for example 6/12). From building span you get each side’s run, then rise, then rafter line length. Common rafters meet a ridge board; hips/valleys add complexity; trusses replace stick framing but must be braced and never cut in the webs. Speed squares and rafter tables turn pitch into cut angles and unit lengths.

Roof covering modules assume a framed plane. Commercial Carpenter practice banks and advanced roofing awareness (27403 territory) expect rafter vocabulary even when the job uses engineered trusses. This section builds that literacy without turning into a full rafter-book apprenticeship.

Pitch, Slope, and the 12-Inch Run Convention

In U.S. carpentry, pitch (often spoken as “six-twelve”) means inches of rise per 12 inches of horizontal run.

Slope may also be stated as a ratio or percent grade; on residential drawings you will most often see X/12.

Why 12? Rafter tables and speed squares are built around a unit run of 12 inches. Once you know pitch, the unit rise is the first number, and the unit line length is the hypotenuse of a right triangle with legs equal to unit rise and 12.

Span, Run, Rise, and Line Length

Define the four terms carefully—exam items love to swap them.

• Span: Overall horizontal distance covered by the roof framing between outer supports (for a simple gable, often outside-to-outside of the wall plates, depending on how the drawing defines it—read the plan).
• Run: Horizontal distance from the outside of the plate (or theoretical building line used in layout) to the centerline of the ridge for a common rafter. On a symmetrical gable, run ≈ span / 2 (adjusted for ridge thickness in precise layout).
• Rise: Vertical distance from the plate top (or selected reference) up to the ridge height for that run.
• Line length: The length along the rafter from plate reference to ridge reference—the hypotenuse—before adding overhang (tail) length.

Worked Example: 6/12 Pitch, 24-Foot Span (Teaching Model)

Assume a symmetrical gable, ignore ridge thickness for a first-pass classroom estimate:

1. Span = 24'-0" (wall to wall as defined for this example).
2. Run per side = 24' / 2 = 12'-0" = 144 inches.
3. Pitch 6/12 → rise per foot of run = 6 inches. For 12 feet of run: rise = 12 × 6" = 72" = 6'-0".
4. Line length from the right triangle: legs 144" (run) and 72" (rise).
   Hypotenuse = √(144² + 72²) = √(20736 + 5184) = √25920 ≈ 161.0" ≈ 13'-5" (about).

Using unit method: unit rise 6, unit run 12, unit line length = √(6²+12²) = √(36+144) = √180 ≈ 13.416" per foot of run. For 12 feet of run: 12 × 13.416 ≈ 161"—same result.

Overhang: If the design needs a 16" horizontal projection beyond the wall, add the corresponding rafter tail length (not always equal to 16" along the rafter—use the same pitch triangle for the tail).

Exam takeaway: You must know the relationships and be able to work a simple rise/run problem. Precise birdsmouth layout also accounts for ridge board thickness, seat cuts, and plumb cuts—field layout uses rafter squares or calculators with those corrections.

Common Rafters and the Ridge Board

A common rafter is a full-length rafter in the main roof plane from plate to ridge (not a hip, valley, or jack).

Cuts (conceptual):

• Plumb cut at the ridge (vertical face against the ridge board).
• Birdsmouth at the plate: a level seat cut and plumb heel cut so the rafter sits solidly on the plate without overcutting the seat (overcut weakens the rafter tail connection).
• Tail cut at the eave for fascia alignment (plumb, square, or decorative).

The ridge board is the horizontal member at the peak that common rafters bear against. It is not always a structural beam; in many wood roofs it aligns and ties rafters while ceiling joists or rafter ties resist outward thrust. Some designs use a structural ridge beam that carries load and changes the thrust behavior—read the drawings.

Rafter ties / ceiling joists near the plate line keep walls from spreading under roof load. Cutting them for attic access without an engineered fix is a serious structural error.

Hips, Valleys, and Jacks (Awareness)

• Hip rafter: Runs from an outside corner up to the ridge (or hip-ridge intersection) at the intersection of two roof planes; longer than commons; compound cheek cuts.
• Valley rafter: Lies in the internal intersection where two planes meet and water collects; also longer/compound.
• Jack rafters: Shorter rafters that run from plate to hip, or ridge to valley, in diminishing lengths.

You do not need full hip-valley mastery for every assessment item, but you must recognize why hip roofs need more layout skill and why valleys are structural and waterproofing critical points.

Speed Square and Rafter Tables (Conceptual Use)

A speed square (triangular square) has degree and common-pitch scales. For a known pitch (say 6/12), you can:

• Set the pivot and align the 6 mark on the common scale to mark a plumb cut.
• Use the tool to mark square cuts and guide circular-saw cuts.

Rafter tables (printed on framing squares or in manuals) list, for each pitch:

• Length of common rafter per foot of run.
• Hip/valley length per foot of run.
• Differences in jack rafter lengths for standard spacings.
• Side cut angles for hips/valleys.

Conceptual exam skill: Given pitch and run, use unit length × run to get line length; use the square to transfer plumb and level cuts. You are tested on knowing what the tools provide, not on memorizing every table row.

Trusses: Handling, Bracing, and the Never-Cut Rule

Wood trusses are engineered assemblies of chords and webs joined with metal connector plates. They arrive tagged for position (for example, gable end vs interior).

Handling:

• Lift with approved spreader bars or methods that do not bend trusses sideways excessively.
• Do not support a long truss only at the center peak during storage—follow manufacturer storage guides (usually horizontal with supports under joints).
• Install right-side-up; inverted trusses fail.

Erection and temporary bracing:

• Set trusses plumb and on layout marks.
• Install temporary longitudinal and cross bracing immediately per BCSI (Building Component Safety Information) style guides and the truss engineering—do not leave a “forest” of free-standing trusses.
• Permanent bracing (lateral web bracing, chord bracing) is part of the design—omit it and the roof can buckle under load or wind.

Never cut webs or chords to run ducts, hatches, or pipes. Field-cutting a web destroys the engineered load path. If a penetration is required, the truss engineer must design a repair or a different truss. Notching chords for wiring is equally restricted—follow the plate and engineering documents.

Gable end trusses may include stud pockets for overhangs or require ladder framing and lookouts—still no freestyle web removal.

Roof Sheathing on Rafters and Trusses

Sheathing (OSB/plywood roof panels) stiffens the roof diaphragm and supports the underlayment and covering.

• Stagger panel end joints; land edges on framing or use clips where allowed for unsupported edges.
• Follow nailing schedules (edge vs field spacing)—especially for wind and shear design.
• Leave specified expansion gaps if the manufacturer requires them.
• Hips, valleys, and ridges need solid backing; thin air gaps under valleys invite soft spots and nail pops.

Sheathing often serves as permanent bracing for truss systems once properly fastened—another reason temporary bracing stays until sheathing (and specified permanent braces) are complete.

Stick Frame vs Truss Decision Literacy

Field Scenario

A crew sets trusses on a 28-foot-wide commercial wood wing. One worker starts cutting a web to clear a large exhaust duct. Correct response: stop, restore stability, and request an engineered repair or redesigned mechanical route. Meanwhile another worker chalks a 6/12 plumb cut on a stick-framed porch rafter using a speed square at the 6 common mark—valid layout for that common rafter porch, independent of the main truss roof.

Exam Traps

• Swapping span with run (run is roughly half of span on a symmetrical gable).
• Calling pitch “6 inches total rise” instead of per 12 inches of run.
• Treating the ridge board as optional decoration with no ties or beam design.
• Cutting truss webs “just a little.”
• Sheathing only every other bay to save panels (destroys diaphragm and bracing assumptions).

Master pitch language, the span–run–rise–line length chain, common rafter/ridge roles, hip-valley awareness, truss brace-and-don’t-cut rules, sheathing discipline, and what speed squares/rafter tables are for—and roof framing literacy will support both practice-bank items and on-the-job coordination with roofing modules 27202/27203/27403.