🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Building & Construction › Carpentry › Roofing: Systems, Materials, and Installation › Lesson 3
BLD-CARP03-04 · Lesson 3 · 14 clock hours · demo

Common Rafter Layout and Cutting

Objective. Students will lay out and cut a common rafter—including plumb cut, bird's mouth, and tail cut—for a given span and slope, with each cut verified within 1/8-inch tolerance.

What Is a Common Rafter?

A common rafter runs at a right angle from the wall plate to the ridge board. Every common rafter in a roof carries the same span, the same slope, and the same three cuts: the plumb cut (also called the ridge cut) at the top, the bird's mouth (seat cut) where the rafter rests on the wall plate, and the tail cut at the eave end. Laying out and cutting these three features accurately is one of the most fundamental skills in residential framing.

Understanding Slope and the Framing Square

Slope is expressed as rise-over-run: the number of inches the roof rises for every 12 inches of horizontal run. A 6-in-12 slope rises 6 inches for each 12 inches of run. You read slope from the architectural drawings before you pick up a framing square.

The framing square has a long blade (24 inches) and a shorter tongue (16 inches). To mark a plumb line, hold the square so the tongue reads the unit rise on the edge of the lumber and the blade reads 12 on the same edge; the tongue then lies along the plumb-cut line. To mark a level (horizontal) line, swap: the blade gives the level direction. Every rafter layout begins with this plumb line at the ridge end.

A speed square can be used to set and confirm angles quickly once you know the slope. It is especially useful for setting the circular saw bevel before cutting.

Laying Out the Plumb Cut

1. Select a straight, dry 2×8 and crown it—place the crowned (bowed) edge up; that becomes the top edge of the rafter.

2. At the upper end, position the framing square with the tongue at the unit rise and the blade at 12 on the top edge. Mark along the tongue: this is the plumb-cut line.

3. Measure the total rafter length (theoretical, from center of ridge to outside of wall plate) and mark that distance along the top edge from the plumb-cut line.

Laying Out the Bird's Mouth

The bird's mouth is a notch that lets the rafter sit flat on the top plate. It has two parts:

The depth of the plumb portion of the bird's mouth must not exceed one-third of the rafter depth—cutting deeper weakens the rafter. For a 2×8 (actual 7-1/4 inches), that limit is about 2-3/8 inches. The seat length typically matches the width of the top plate (3-1/2 inches for a 2×4 plate).

To mark the bird's mouth, position the framing square at the measured location on the rafter, tongue on the rise side and blade on the 12 side, and mark both the plumb line and the level (seat) line. The intersection of those two lines is the heel of the bird's mouth.

Laying Out the Tail Cut

From the bird's mouth, measure the overhang (run) along the top edge of the rafter. At that point, draw another plumb line using the framing square in the same orientation as the ridge plumb cut. This is the tail plumb cut. Some plans call for a plumb cut at the tail; others call for a square (vertical) cut—always check the drawing.

Making the Cuts Safely

Personal protective equipment (PPE) is required before any saw is energized:

Circular saw setup: Use a speed square to set the saw's bevel angle to the plumb-cut angle for your slope. Make a test plumb cut on scrap lumber and verify the angle with the speed square before cutting your rafter. Log the result in your field notebook. Support stock at both ends so the cut-off piece falls freely and does not pinch the blade. Keep the blade guard fully functional—never wedge it open.

Lockout/tagout (LOTO): Any time you adjust the blade guard, change blades, or clear a jam, unplug the saw or remove the battery. Follow your program's LOTO procedure without exception.

Test-Fitting the Rafter

After cutting, hold the rafter in position against the practice ridge board and wall plate:

Use a tape measure to check each dimension. If any measurement falls outside 1/8-inch tolerance, identify which cut is off, mark the correction, and recut. Minor adjustments are normal; taking careful layout measurements reduces recutting.

NCCER Connection

NCCER Carpentry Level 1 covers floor, wall, and ceiling framing; rafter layout appears in the roof framing module and builds directly on the square-use skills introduced in those earlier units. Accuracy in rafter layout affects structural integrity, water-shedding, and finish appearance—all checkpoints in NCCER performance testing.

Key terms9

Common rafterA rafter that runs perpendicular from the wall plate to the ridge board, carrying the same span and slope as every other common rafter in the same roof.
Plumb cutA cut made along a vertical (plumb) line on the rafter; used at the ridge end and sometimes at the tail. Its angle matches the roof slope.
Bird's mouthA notch near the lower end of a rafter consisting of a level seat cut and a plumb cut, allowing the rafter to bear squarely on the wall plate.
Seat cutThe horizontal (level) portion of the bird's mouth that rests on and is supported by the top plate.
Unit riseThe number of inches a roof rises for each 12 inches of horizontal run; the numerator in a slope expression such as 6-in-12.
RunThe horizontal distance a rafter covers, measured from the outside of the wall plate to the center of the ridge; equal to half the building span for a symmetrical gable roof.
OverhangThe portion of the rafter that extends beyond the outside face of the wall plate to form the eave.
Framing squareAn L-shaped steel layout tool with a 24-inch blade and 16-inch tongue used to mark plumb and level lines on rafters based on rise and run.
CrownThe natural bow or curve along the edge of a piece of dimensional lumber; rafters are installed crown-side up so the load tends to straighten them.

Worked examples4

1. A roof has a slope of 6-in-12. What angle does the plumb cut make with the vertical face of the rafter, and how do you set a speed square to mark it?
  1. Identify rise and run values: rise = 6, run = 12.
  2. Calculate the angle from vertical using the arctangent: arctan(6/12) = arctan(0.5) ≈ 26.6°. Round to 26.5° for practical use, which is the standard published value for a 6-in-12 slope.
  3. On the speed square, pivot it on the top edge of the rafter until the '6' mark on the COMMON RAFTER scale aligns with the top edge.
  4. Mark along the pivot (square) edge—this is the plumb-cut line at approximately 26.5° from vertical.
  5. Set the circular saw bevel to 26.5° (or the manufacturer's equivalent setting) and test on scrap before cutting the rafter.
Answer: The plumb cut is approximately 26.5° from vertical (63.5° from horizontal) for a 6-in-12 slope. Set the speed square to the '6' mark on the common rafter scale, or set the circular saw bevel to 26.5°.
2. A building has a span of 28 feet and a 6-in-12 slope. What is the theoretical rafter length (ridge to outside of plate, ignoring ridge thickness and tail)?
  1. Calculate the run: run = span ÷ 2 = 28 ÷ 2 = 14 feet.
  2. For a 6-in-12 slope, the rafter length per foot of run is found using the Pythagorean theorem: √(6² + 12²) = √(36 + 144) = √180 ≈ 13.416 inches per foot of run.
  3. Multiply by run in feet: 13.416 × 14 = 187.82 inches.
  4. Convert to feet and inches: 187.82 ÷ 12 = 15 feet 7-13/16 inches (approximately 15′ 7-7/8″).
Answer: The theoretical rafter length is approximately 15 feet 7-7/8 inches. (In practice, deduct half the ridge board thickness from the top and add the tail length based on the specified overhang.)
3. The plans call for a 12-inch overhang (horizontal measurement) on the same 6-in-12 roof. How long is the tail of the rafter along its top edge?
  1. The overhang run = 12 inches = 1 foot.
  2. Use the same rafter-length-per-foot factor: √(6² + 12²) = √180 ≈ 13.416 inches per foot of run.
  3. Tail length along top edge = 13.416 × 1 = 13.416 inches ≈ 13-7/16 inches.
  4. Mark this distance along the top edge from the heel of the bird's mouth to locate the tail plumb-cut line.
Answer: The tail measures approximately 13-7/16 inches along the top edge of the rafter for a 12-inch horizontal overhang on a 6-in-12 slope.
4. A 2×8 rafter (actual depth 7-1/4 inches) has a bird's mouth. What is the maximum allowable plumb-cut depth of the bird's mouth?
  1. The general framing rule limits the bird's mouth plumb-cut depth to one-third of the rafter's actual depth.
  2. One-third of 7.25 inches = 7.25 ÷ 3 = 2.417 inches.
  3. Round down to the nearest practical measurement: 2-3/8 inches (2.375 inches).
  4. Do not cut the bird's mouth plumb portion deeper than 2-3/8 inches to avoid weakening the rafter at the bearing point.
Answer: The maximum bird's mouth plumb-cut depth for a 2×8 rafter is approximately 2-3/8 inches (one-third of the 7-1/4-inch actual depth).

The math in this lesson2

Learners compute rafter length for a 28 ft span at 6-in-12, the overhang tail, and the bird's-mouth depth limit.
N-Q.1G-SRT.8
Brush up in the math halls at newSchoolWorks.com:
In class: Watch instructor demonstrate full rafter layout on a 2×8 using a framing square, marking plumb line, seat cut, and tail; then replicate the layout independently on an identical board. · Practice setting a circular saw to the correct bevel angle and making plumb cuts on scrap lumber before cutting the actual rafter, logging each cut result in the field notebook. · Test-fit the completed rafter against the practice wall-top-plate and ridge board, checking plumb cut contact, bird's mouth bearing, and overhang length; adjust and recut if outside tolerance.
Materials: 2×8 dimensional lumber (practice stock), framing square, speed square, tape measure, circular saw, hearing protection, safety glasses, pencil, field notebook; NCCER rafter layout reference
Check: Instructor measures plumb cut, bird's mouth depth, seat length, and tail length against the layout plan; all four dimensions must fall within 1/8 inch of specification to pass.

Practice check8

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

NCCER Carpentry - Roof Framing
1. When laying out a common rafter with a framing square, what numbers are held on the top edge of the rafter to mark a plumb cut for a 6-in-12 slope?
NCCER Carpentry - Roof Framing
2. Which part of the bird's mouth actually bears on and transfers rafter load to the top plate?
NCCER Carpentry - Tools and Equipment
3. Before making the plumb cut on a rafter with a circular saw, what is the correct first step regarding the saw angle?
NCCER Carpentry - Roof Framing
4. A 2×8 rafter has an actual depth of 7-1/4 inches. The maximum depth of the bird's mouth plumb cut should not exceed approximately:
OSHA 10 - Electrical and Tool Safety
5. When you are about to change the blade on a corded circular saw, what must you do first?
NCCER Carpentry - Roof Framing
6. A roof has a span of 24 feet and a 5-in-12 slope. What is the unit-run measurement used when laying out with a framing square?
NCCER Carpentry - Quality and Tolerances
7. During test-fitting, the plumb cut of a completed rafter shows a 3/16-inch gap against the ridge board. What is the correct action?
NCCER Carpentry - Materials and Lumber
8. Which of the following best describes the purpose of crowning lumber before using it as a rafter?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →