Rafter Layout Basics: Pitch, Run, Rise, and Cut Logic

This chapter focuses on the geometry and cut logic used to lay out a common rafter accurately, using simple relationships (rise, run, pitch) and repeatable marking steps. The goal is to understand what each mark represents on the rafter and how those marks relate to the ridge and wall plates.

Core geometry terms (kept practical)

Run, rise, and pitch

• Run: the horizontal distance the rafter covers from the outside of the wall plate to the ridge line (typically measured to the centerline of the ridge for common rafters).
• Rise: the vertical height gained over that run.
• Unit run / unit rise: the “per-foot” relationship used for layout. Example: a 6:12 roof has unit rise = 6 and unit run = 12.
• Pitch: commonly expressed as “X-in-12.” It is the unit rise over 12 inches of unit run (e.g., 6-in-12, 8-in-12).

Think of pitch as a repeatable triangle: every 12 inches of horizontal travel, the roof goes up X inches. You don’t need heavy math to lay out a rafter because the framing square can “carry” that triangle onto the board.

Common rafter vs. hip/valley rafters (conceptual)

• Common rafter: runs perpendicular to the ridge and is the baseline rafter you template from.
• Hip rafter: runs from a building corner up to the ridge, forming the outside edge of a hip roof plane. It is longer than a common rafter because it travels diagonally in plan.
• Valley rafter: runs from an inside corner up to the ridge, forming the inside intersection of two roof planes; also diagonal in plan.

For layout basics, focus on the common rafter: once you can mark plumb cuts, seat cuts, and tails consistently, the same cut logic carries over conceptually to hips/valleys (with different lengths and angles).

Cut vocabulary: what each cut does

Plumb cut (top cut)

The plumb cut is the angled cut at the top of the rafter where it meets the ridge. It is “plumb” because the cut line is vertical when the rafter is installed in its roof position.

Seat cut and birdsmouth

The birdsmouth is the notch that lets the rafter sit on the wall plate. It is typically made of two lines:

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• Seat cut: the horizontal line that bears on the top of the wall plate.
• Heel (plumb) cut: the vertical line at the inside edge of the wall plate; it aligns the rafter to the wall line and creates the “heel.”

In practice, you lay out the birdsmouth so the rafter has enough bearing on the plate and you do not remove too much wood from the rafter.

Heel height

Heel height is the vertical distance from the top of the wall plate up to the top edge of the rafter at the birdsmouth (at the heel). It matters because it affects insulation space, fascia alignment, and how the rafter meets the ridge height you’re targeting.

Tail length and overhang

• Overhang: the horizontal projection beyond the outside face of the wall (eave projection).
• Tail: the portion of the rafter that extends past the birdsmouth to form the overhang and support fascia/soffit.
• Tail cut: often a plumb cut at the end of the tail (for fascia), sometimes combined with a level cut depending on detail.

Before you mark: set yourself up for repeatability

Pick and keep a consistent reference edge

Rafters are laid out from a consistent edge—commonly the top edge (the edge that will face the roof sheathing). Mark a small “X” on the face and an arrow on the top edge so you always hook your square the same way.

Crown orientation checkpoint

Most dimensional lumber has a slight curve (crown). A common field practice is to install rafters with the crown up (so loads tend to flatten rather than sag). Before any layout, sight each rafter and mark the crown direction; keep it consistent across the roof.

Template mindset

Plan to make one rafter as a template (or pattern). Once verified, use it to mark the rest. This reduces cumulative measuring errors and keeps tails and birdsmouth locations identical.

Laying out a common rafter with a framing square (step-by-step)

The framing square method uses the pitch numbers directly on the square: one leg represents unit run (12), the other represents unit rise (your pitch number). This “steps” the roof triangle along the rafter.

Tools and simple materials

• Framing square (or speed square for marking angles)
• Pencil/marker, tape measure
• Square gauges (optional but helpful for repeat stepping)
• Rafter stock (straightest pieces reserved for patterns)

Step 1: Mark the top plumb cut

1. Place the framing square on the rafter so the 12 mark on the tongue (or body, depending on square style) aligns with the top edge, and the pitch number (unit rise) aligns on the other leg. Example for 6:12: align 12 on one leg and 6 on the other.
2. Hold the square firmly so both reference points touch the top edge of the rafter.
3. Draw the plumb line along the square’s tongue edge. This is your top cut line.

Checkpoint: Label it “TOP PLUMB” and mark which side is waste so you don’t flip the cut later.

Step 2: Establish the layout line for run (measure along the top edge)

Common rafter layout typically measures the building run to the ridge line and transfers that distance along the rafter’s top edge using the square’s stepping method or by measuring the rafter length if already known. A simple, low-math approach is stepping:

1. From the top plumb cut line, keep the same pitch setting on the square (e.g., 6 and 12).
2. “Step” the square down the rafter: slide it so the same two reference numbers land on the top edge again, then mark a small tick at each step.
3. Count steps until you reach the required horizontal run in 12-inch increments (each step represents 12 inches of run).
4. If the run is not an even number of feet, make the final partial step by aligning the square to the remaining inches of run (still keeping the rise/run relationship consistent).

What you’re doing: Each step transfers one unit of run (12 inches) and its matching rise onto the rafter, building the rafter length without calculating the hypotenuse.

Step 3: Locate the birdsmouth (seat position) at the wall plate

Once you’ve reached the point that corresponds to the outside edge of the wall plate (the “plate line” location along the rafter), you’ll lay out the birdsmouth. The birdsmouth is not placed randomly; it must land so the rafter bears properly on the plate and aligns with the wall.

1. At the point representing the outside of plate location, draw a plumb line using the same square pitch orientation. This line represents the outside face/edge reference for the seat location (depending on your plan detail).
2. From that plumb line, measure inward the actual plate width (commonly 3 1/2 inches for a 2x4 top plate, 5 1/2 inches for a 2x6), along a line that will become the seat cut location.
3. Draw the seat cut as a level line (horizontal when installed). On the rafter, this is drawn square to the plumb line at the birdsmouth location.
4. Draw the heel cut (the vertical part of the birdsmouth) at the inside edge of the plate. This is typically a plumb line that meets the seat cut, forming the notch.

Checkpoint: Clearly label “SEAT” and “HEEL.” Confusing these lines is a common cause of rafters that don’t sit flat or push the wall.

Conceptual safety constraints: minimum bearing and notching limits

• Minimum bearing: ensure the seat cut provides adequate contact area on the top plate. Too little bearing concentrates load and can crush wood fibers or lead to movement.
• Notching limits: avoid cutting the birdsmouth so deep that the remaining rafter section is weakened. A practical rule of thumb used in the field is to keep the notch shallow relative to rafter depth and to follow local code requirements for maximum notch depth.
• Don’t “chase” a fit by deepening the birdsmouth: if the rafter doesn’t sit correctly, re-check your layout references (top edge, plumb line location, plate width) before removing more material.

Step 4: Lay out the tail and overhang

After the birdsmouth is defined, the remaining length beyond the outside of the wall becomes the tail. Overhang is usually specified as a horizontal projection, but you can lay it out cleanly on the rafter:

1. From the outside-of-wall reference at the birdsmouth, mark the overhang location along the rafter using your plan’s overhang dimension (many crews measure along the rafter for consistency once the pitch is set, but confirm the intended method on your detail).
2. At the tail end point, draw a plumb cut for the fascia line (same pitch orientation as the top plumb cut).
3. If your detail includes a level cut (for a soffit return or specific fascia/soffit geometry), mark it from the plumb cut per the detail rather than guessing.

Checkpoint: If multiple rafters must align to a straight fascia, the tail plumb cut location must be identical across rafters—this is where a template rafter pays off.

Using a rafter (speed) square for quick marking

A speed square can quickly mark the plumb cuts for a given pitch:

• Set the square’s pivot at the rafter edge and rotate until the pitch mark (e.g., 6, 8) aligns with the top edge.
• Draw the plumb line along the square’s edge.

Speed squares are excellent for repeating the same plumb angle (top cut and tail cut). For stepping lengths, a framing square is often more straightforward, or you can measure from a verified template.

Accuracy checkpoints that prevent “mystery gaps”

1) Verify ridge location vs. rafter layout

• Confirm whether your run is measured to the centerline of ridge or to a face of ridge, and whether any ridge thickness adjustment is required by your detail.
• Dry-check conceptually: if the ridge is thicker than assumed, rafters can land short/long or create a ridge gap.

2) Keep crown orientation consistent

All rafters should be oriented the same way (commonly crown up). Mixing orientations can create a wavy roof plane even if every cut is “correct.”

3) Use identical templates and stop re-measuring

• Cut one rafter carefully, then test-fit (or compare against known reference points).
• Use it as a pattern to trace plumb cuts, birdsmouth, and tail cuts onto the rest.
• If a rafter is off, fix the template logic first—don’t “custom fit” every piece.

4) Confirm seat bearing and birdsmouth depth before cutting multiples

• Check that the seat cut lands fully on the plate and that the heel cut aligns with the wall line.
• Ensure the notch does not remove excessive material from the rafter.

5) Mark from the same edge, same face, same direction

Many layout errors come from flipping the board or switching reference edges mid-process. Use consistent reference marks: “TOP EDGE,” “OUTSIDE,” and arrows showing direction toward ridge.

Mini worked example (logic-focused, not math-heavy)

Scenario: You need a common rafter for a 6:12 roof. You will lay out the top plumb cut, step the run, place the birdsmouth on the plate, then mark the tail.

• Set square to 6 and 12 and draw the top plumb cut.
• Step the square down the rafter the number of times needed to match the plan’s run (each step = 12 inches of run). Mark the final step location.
• At the wall plate location, draw a plumb line, then lay out the seat cut for the plate width and draw the heel cut to complete the birdsmouth.
• From the outside wall reference, mark the tail/overhang and draw the tail plumb cut.

This sequence keeps the geometry consistent: the same pitch governs top cut, birdsmouth plumb reference, and tail cut, while the seat cut and plate width control how the rafter bears on the wall.