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newSchoolWorks.com › VoTechWorks › Building & Construction › Construction (general) › Framing: Wood and Metal Stud Construction › Lesson 6
BLD-CONS02-06 · Lesson 6 · 18 clock hours · practice

Roof Framing: Common Rafters, Ridge, and Gable Ends

Objective. Students calculate common rafter length for a given span and roof pitch using the unit-rise method, cut a pattern rafter, and frame a simple gable-roof structure with ridge board, common rafters, and collar ties within ±1/4 inch of calculated layout.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Roof Framing: Common Rafters, Ridge, and Gable Ends

A gable roof is one of the most common roof forms in residential and light commercial construction. It consists of two sloping planes that meet at a central ridge. Your job as a framer is to calculate rafter length accurately, cut a pattern rafter that fits perfectly, and erect the assembly plumb, level, and to layout. Precision matters: errors compound across every rafter, and a roof that is out of tolerance creates problems for sheathing, roofing, and finish trades.

Key Roof Geometry Terms

Span is the full width of the building measured from outside wall to outside wall. Run is half the span — the horizontal distance a common rafter travels from the outside wall to the centerline of the ridge. For a 24-foot-wide building, the run is 12 feet.

Rise is the vertical distance from the top of the wall plate to the top of the ridge. Pitch is expressed as unit rise over 12 inches of run. A 6-in-12 pitch rises 6 inches for every 12 inches of horizontal run.

Unit length is the rafter length per foot of run and is found in the rafter table stamped on a standard framing square. For a 6-in-12 pitch, the unit length is 13.42 inches per foot of run.

Calculating Common Rafter Length

Use the unit-rise method:

1. Line length = Run (in feet) × Unit length (inches per foot of run)

2. Ridge deduction = Half the thickness of the ridge board, measured horizontally. For a 1½-inch-thick ridge board (dimensional 2×), deduct ¾ inch from the line length.

3. Overhang addition = Overhang run (in feet) × Unit length. Add this to the line length after the ridge deduction.

The result is the total rafter length from the plumb cut at the ridge to the tail cut at the fascia.

The Three Cuts on a Common Rafter

Plumb cut (ridge cut): A vertical cut at the top of the rafter that bears against the ridge board. Set your framing square with the unit rise (6) on the tongue and 12 on the blade; the tongue edge gives the plumb-cut line.

Bird's mouth (seat cut): A notch cut into the rafter bottom that sits on the top plate. It consists of a plumb cut (called the heel cut) and a level cut (called the seat cut). The level cut should bear fully on the plate without cutting more than one-third of the rafter depth.

Tail cut (fascia plumb cut): A vertical cut at the rafter tail that the fascia board nails to. It is laid out the same way as the ridge plumb cut.

Cutting a Pattern Rafter

Cut one rafter carefully, test-fit it against your ridge board mock-up on sawhorses, and check that the bird's mouth seats flat on the plate and the plumb cut contacts the ridge squarely. Adjust if needed, then use this pattern rafter to trace all remaining rafters. This prevents cumulative error from individual layout on each piece.

Erecting the Ridge and Common Rafters

Calculate ridge height: Rise = Run × Unit rise per foot = 12 ft × 6 in = 72 in = 6 feet above the top of the wall plate (measured to the underside of the ridge board for layout purposes, adjusted by the plumb-cut height). Set temporary ridge supports — adjustable posts or engineered shoring — at the calculated height. Plumb and brace the ridge so it does not shift during rafter installation.

Install rafters in opposing pairs from the center of the ridge outward, nailing through the plumb cut into the ridge with 16d nails and toenailing the bird's mouth to the top plate. Installing in pairs keeps the ridge from racking to one side. Space rafters at 16 inches on center (measured along the plate from a consistent starting point using a chalk line layout).

After all common rafters are installed, check the ridge for level along its length with a long level or taut string, and verify it is straight in plan view with a chalk line or string line.

Collar Ties

Collar ties are horizontal members that connect opposing rafters and resist the outward thrust of the roof load. Install them in the upper third of the rafter span (measured vertically from the ridge down). Use 1×6 stock, nail to each rafter with at least three 8d nails per end, and keep them level. Collar ties are not a substitute for ceiling joists or rafter ties at the wall plate.

Safety

Key terms9

SpanThe full horizontal width of a building measured from outside face to outside face of the opposing wall plates; the rafter run equals half the span.
RunThe horizontal distance a common rafter covers, equal to half the building span; used as the base measurement for rafter length calculations.
Unit lengthThe rafter length (in inches) for each foot of run at a given pitch; found in the rafter table on a framing square. For 6-in-12 pitch, unit length is 13.42 inches per foot.
Plumb cutA vertical cut on a rafter, laid out with the framing square tongue at the unit rise and blade at 12; used at the ridge end and the tail (fascia) end.
Bird's mouthA notch cut near the lower end of a rafter consisting of a heel (plumb) cut and a seat (level) cut that allows the rafter to bear flat on the top plate.
Ridge boardThe horizontal board at the peak of a gable roof against which the upper plumb cuts of opposing common rafters are nailed; typically one size larger than rafter stock.
Ridge deductionThe amount subtracted from calculated line length to account for the ridge board thickness; equals half the actual (dressed) thickness of the ridge board, measured horizontally.
Collar tieA horizontal framing member connecting opposing rafters in the upper third of the rafter span, resisting the outward thrust caused by roof loads.
Pattern rafterThe first carefully fitted and verified rafter used as a template to trace and cut all remaining rafters, ensuring consistent accuracy across the assembly.

Worked examples4

1. Calculate the LINE LENGTH of a common rafter for a building with a 24-foot span and a 6-in-12 pitch. (No overhang yet; no ridge deduction yet.)
  1. Step 1 — Find the run: Run = Span ÷ 2 = 24 ft ÷ 2 = 12 ft
  2. Step 2 — Look up unit length for 6-in-12 pitch on the framing square rafter table: 13.42 inches per foot of run.
  3. Step 3 — Multiply: Line length = 12 ft × 13.42 in/ft = 161.04 in
  4. Step 4 — Convert to feet and inches: 161.04 in ÷ 12 = 13 ft 5.04 in ≈ 13 ft 5 inches
Answer: Line length = 13 ft 5 inches (approximately; carry the decimal for subsequent deductions)
2. Apply the ridge deduction. The ridge board is a 2×10 (actual thickness 1½ inches). What is the rafter length after the ridge deduction?
  1. Step 1 — Ridge deduction = half the actual ridge board thickness = 1.5 in ÷ 2 = 0.75 in (¾ inch).
  2. Step 2 — This deduction is made horizontally (on the level/plumb-cut line), so it is already in the same plane as the line-length measurement.
  3. Step 3 — Subtract from line length: 161.04 in − 0.75 in = 160.29 in
  4. Step 4 — Convert: 160.29 in ÷ 12 = 13 ft 4.29 in ≈ 13 ft 4¼ inches
Answer: Rafter length after ridge deduction ≈ 13 ft 4¼ inches
3. Add a 12-inch (1-foot) horizontal overhang to the rafter. What is the total rafter length from plumb cut to tail cut?
  1. Step 1 — Overhang run = 1 ft.
  2. Step 2 — Unit length for 6-in-12 pitch = 13.42 in per foot of run.
  3. Step 3 — Overhang length along rafter = 1 ft × 13.42 in/ft = 13.42 in ≈ 13⁷⁄₁₆ in.
  4. Step 4 — Total rafter length = rafter length after ridge deduction + overhang length = 160.29 in + 13.42 in = 173.71 in
  5. Step 5 — Convert: 173.71 in ÷ 12 = 14 ft 5.71 in ≈ 14 ft 5¾ inches
Answer: Total rafter length (ridge plumb cut to tail plumb cut) ≈ 14 ft 5¾ inches
4. Where should collar ties be installed on rafters that have a total vertical rise of 72 inches (6 feet)?
  1. Step 1 — Code and best practice place collar ties in the upper third of the rafter span measured vertically from the ridge.
  2. Step 2 — Upper third of rise = 72 in ÷ 3 = 24 in below the ridge.
  3. Step 3 — Therefore, collar ties should be installed with their top edge no lower than 24 inches below the underside of the ridge board (i.e., within the top 24 inches of the 72-inch rise).
  4. Step 4 — Level the collar tie across opposing rafters and nail with at least three 8d nails per end.
Answer: Collar ties must be installed within the top 24 inches (upper third) of the vertical rise, measured down from the ridge.

The math in this lesson2

Learners compute common-rafter line length for a 24 ft span at 6-in-12, then apply ridge deduction and overhang.
N-Q.1G-SRT.8
Brush up in the math halls at newSchoolWorks.com:
In class: Given a 24-foot building width and a 6-in-12 pitch, use a framing square and rafter tables (or the unit-rise calculation: run × unit length per foot of run from the rafter table) to calculate total rafter length including ridge deduction and overhang; record all calculations in the lab log before cutting. · Cut a pattern rafter with plumb cut (ridge), seat cut (bird's mouth), and tail cut (fascia plumb); test-fit the pattern against the ridge board mock-up on sawhorses, adjust, then use it to trace and cut all remaining common rafters as a team. · Erect the ridge board at calculated height using temporary supports, nail common rafters in opposing pairs from center outward at 16-inch on-center layout, install collar ties at the upper third of the rafter span, and check ridge for level and straight along its length.
Materials: 2×8 rafter stock, 2×10 ridge board, framing square with rafter tables, speed square, circular saw, framing nailer, 16d nails, 1×6 collar tie stock, temporary ridge support (adjustable post or engineered support), 4-foot and 6-foot level, 100-foot tape, chalk line, safety glasses, hard hat, work gloves, NCCER Framing module rafter calculation reference
Check: Instructor compares student's written rafter-length calculation to master answer (pass = within 1/4 inch); measures three installed rafter spans from wall plate to ridge at both sides (pass = all within ±1/4 inch of design); lab log calculation sheet collected.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Construction Technology — Framing
1. For a building with a 24-foot span and a 6-in-12 pitch, what is the rafter run?
NCCER Construction Technology — Framing
2. Using the rafter table on a framing square, the unit length for a 6-in-12 pitch is 13.42 inches per foot of run. What is the line length of a common rafter with a 12-foot run?
NCCER Construction Technology — Framing
3. When making the ridge deduction for a 2×10 ridge board (actual thickness 1½ inches), how much is deducted from the rafter line length?
NCCER Framing module
4. What is the purpose of cutting a pattern rafter before cutting all remaining rafters?
NCCER Construction Technology — Framing
5. Why should common rafters be installed in opposing pairs rather than completing one side at a time?
NCCER Construction Technology — Framing
6. Collar ties should be installed in which portion of the rafter span?
OSHA 10 — Personal Protective Equipment
7. Which PPE items are required at all times during the roof framing lab activity described in this lesson?
NCCER Construction Technology — Framing
8. A rafter has a bird's mouth cut. Which statement about the seat (level) cut is correct?
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