How to Cut Common Rafters and Build a Simple Gable Roof

Cutting rafters comes down to two things: the angle of the cuts and the length of the board. Once you understand run, rise, and pitch, the rest is layout and consistency.

Key terms in plain English

• Gable roof: two sloped planes meeting at a ridge. Think Snoopy’s doghouse.

• Common rafter: any rafter that runs from the wall top plate to the ridge on a straight gable.

• Run: horizontal distance the rafter covers.

• Rise: vertical gain across that run.

• Pitch: rise over 12 inches of run, written like 8/12 or “8-in-12.”

• Plumb cut: vertical cut at the ridge and tail.

• Level cut: horizontal cut (part of the birdsmouth).

• Birdsmouth: the notch that lets the rafter sit on the wall. It has a plumb cut and a level “seat” cut.

• HAP (Height Above Plate): the point on the rafter directly above the wall’s outside face where length is measured along the rafter.

The big picture

1. Pick your roof pitch.

2. Find the run for one side (half the building span).

3. Calculate rafter length along the top edge.

4. Lay out plumb and level cuts with a framing square.

5. Add the birdsmouth and tail overhang.

6. Cut one perfect common rafter and use it as your pattern.

Step 1: Determine pitch, span, and run

• Pitch example: 8/12 means every 12 inches of horizontal run gains 8 inches of rise.

• Span is the whole building width. Run is half the span.

Example

• Span: 8 ft

• Run: 4 ft (48 in)

• Pitch: 8/12

Step 2: Three ways to get rafter length

You need the length along the top edge from the HAP to the centerline of the ridge.

A) Math (Pythagorean or pitch factor)

For any pitch, the slope factor per foot of run is:

1+(rise12)2\sqrt{1 + \left(\frac{\text{rise}}{12}\right)^2}1+(12rise​)2​

For 8/12:

1+(8/12)2=1+0.4444=1.4444≈1.20185\sqrt{1 + (8/12)^2} = \sqrt{1 + 0.4444} = \sqrt{1.4444} \approx 1.201851+(8/12)2​=1+0.4444​=1.4444​≈1.20185

Rafter length = run (ft) × slope factor (ft/ft).

With a 4 ft run:

4×1.20185=4.8074 ft=57.689 in≈571116 to 5734 in4 \times 1.20185 = 4.8074\ \text{ft} = 57.689\ \text{in} \approx 57\frac{11}{16}\text{ to }57\frac{3}{4}\text{ in}4×1.20185=4.8074 ft=57.689 in≈571611​ to 5743​ in

This matches what you’ll see on square tables and rafter books for an 8/12 roof.

B) Framing square tables

Flip the framing square to the “Common Rafter Length per Foot of Run” table.

• At 8/12, the table reads 14.42 in per foot of run.

• 4 ft of run = 4 × 14.42 = 57.68 in.

C) Step-off method (with a framing square)

At the rafter’s top edge:

• Set the square so 12 sits on the top edge (run) and 8 sits on the cheek (rise).

• Mark a plumb line. Slide the square to that line. Repeat for each foot of run.

• After four step-offs you’ll be at the top length. Measure along the top edge from your HAP.

Note on accuracy: the step-off works, but small placement errors can add up on long rafters. The math or table is faster and avoids cumulative error.

Step 3: Adjust for ridge thickness and tail

Lengths in tables usually run to the centerline of the ridge. If your ridge is 2x material (1-1/2 in thick), subtract 3/4 in along the rafter length or shift the HAP setup accordingly. Then add your tail overhang by run.

Two clean approaches:

• Approach 1: Adjust length laterCalculate to ridge centerline, then shorten along the rafter by 3/4 in.

• Approach 2: Adjust run firstDeduct 3/4 in from your run before calculating length, then add your tail run (say 12 in). This gives you a “net run” that yields the full cut length in one go. Either path is fine—pick one and stick with it.

Step 4: Lay out the common rafter with a framing square

Use one system on the square—either all outside scales or all inside scales. Don’t mix them.

1. Ridge plumb cut

   • Place 12 on the top edge and 8 on the cheek.

   • Strike the plumb cut. This end bears against the ridge.

2. Mark the HAP and length

   • From the ridge plumb cut, measure down the top edge the calculated length to the HAP point.

3. Birdsmouth layout

   • From the HAP plumb line, slide the square to lay out the seat cut to match your wall thickness.

   • Typical: 2×4 wall with 1/2 in sheathing = 4 in seat.

   • Align the square’s 4 in mark with the bottom of the rafter while the blade stays on your HAP plumb line, then strike the level cut.

   • Keep the birdsmouth depth within code: not more than one-third the rafter’s depth.

4. Tail overhang and fascia

   • From the HAP plumb line, mark the tail run horizontally (often 12 in).

   • Strike a plumb for the tail end.

   • If you’re installing a flat soffit and 1×6 fascia, “bob” the tail to get the fascia height you want. A common setup is a 4 in vertical drop from the tail plumb to the level bob.

Step 5: Cut one, test, then pattern

• Cut your one best common rafter.

• Test fit at the wall and the ridge.

• Check the ridge height, seat bearing, and tail line.

• When it sits perfectly, trace it to make your set. Consistency beats speed.

Worked example (matches the tutorial)

• Span: 8 ft → Run 4 ft

• Pitch: 8/12

• Slope factor: 1.20185

• Length to ridge centerline: 4 × 1.20185 = 4.8074 ft = 57.689 in

• Deduct half the ridge thickness (3/4 in) along the rafter:57.689 − 0.75 = 56.939 in ≈ 56-15/16 in to the near face of the ridge.

• Add a 12 in tail run, then layout tail plumb and bob as needed for fascia/soffit.

If you prefer the square’s 14.42 in/ft value:4 × 14.42 = 57.68 in, same result within rounding.

Birdsmouth rules that save headaches

• Seat cut wide enough for full bearing on the top plate.

• Don’t exceed one-third the rafter depth for the notch.

• Keep your HAP consistent on every rafter.

• Use a sharp pencil and keep your square tight to the edge.

Nail placement and installation quick hits

• Toenail or use metal hangers as required by local code.

• End cuts should be clean and square to your layout lines.

• Sheathing ties everything together. If a rafter is slightly proud or shy, adjust with shims before you deck.

Troubleshooting

• Ridge won’t sit flat: your plumb cut is off or the ridge height is wrong. Re-check pitch on the square.

• Birdsmouth rocks on the plate: seat cut isn’t level or HAP shifted. Re-establish the HAP and redraw.

• Fascia line waves: tails weren’t bobbed to a line. Snap a line, recut tails to that line.

Safety basics

• Stable staging and ladders.

• Eye and hearing protection.

• Gloves when handling framing lumber and sheathing.

• Don’t cut toward the hand that’s holding the square.

Quick reference: pitch factors (common rafters)

Pitch	Factor (in/ft of run)	Slope factor (ft/ft)
4/12	12.65	1.0541
6/12	13.42	1.1180
8/12	14.42	1.2019
10/12	15.62	1.3017
12/12	16.97	1.4142

• Multiply factor × run (ft) to get length in inches.

• Or multiply slope factor × run (ft) to get length in feet.

Cut-list template (fill in on site)

• Pitch: ____ /12

• Span: ______ ft → Run: ______ ft

• Ridge thickness: ______ in → Half: ______ in

• Length to ridge CL: ______ in

• Deduct for ridge half: ______ in

• Net rafter length to near face ridge: ______ in

• Tail run: ______ in

• Birdsmouth seat: ______ in

• HAP: consistent per layout

FAQ

Do I need a special calculator?

No. The square table or the slope-factor formula gets you there. Construction calculators are nice, not required.

Step-off or table?

For short runs either is fine. For long runs, the table or math avoids drift.

How deep can I cut the birdsmouth?

Stay within one-third the rafter depth and follow your local code.

What if my ridge isn’t centered?

Calculate separate runs to each wall, then lay out each side. Same process, different run numbers.