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To cut a conventional common rafter, first establish the approved roof plan’s span, pitch, ridge thickness, bearing points and overhang convention. Use those references to lay out the ridge plumb cut, birdsmouth seat and heel, and tail on one trial rafter. Check that template for bearing, ridge alignment and tail projection before transferring the marks to the remaining stock. The geometry below explains the process; it does not replace span tables, engineering, permits or the building code for a particular roof.
What this method covers—and what it does not
Common rafters only
A common rafter runs from a wall plate to a ridge at right angles to the ridge. Hip, valley and jack rafters require different line-length and cheek-cut geometry. A mono-slope roof has no centered ridge and uses a different reference system. Trusses, I-joists, structural composite lumber, glulam and cross-laminated timber also cannot be treated as ordinary sawn rafters.
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Why project dimensions cannot be supplied here
A safe rafter size and cut list depend on the actual span or run, pitch, spacing, lumber species and grade, roof dead and live loads, snow and wind exposure, support conditions, ridge design, ties and local amendments. Obtain the approved framing plan or use the applicable span table before cutting. A geometry example is useful for learning, but its numbers are not a recommendation for your building.
Gather the reference dimensions first
Confirm the datum used on the plans. A “span” may be measured between framing faces, plate edges or other reference lines. The common-rafter run in a centered-ridge roof is normally one-half of the horizontal span, but only when the ridge is centered and both sides share the load equally.
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| Input | What it means | Why it matters |
|---|---|---|
| Horizontal span and run | The plan’s wall-to-wall horizontal distance; run is the common-rafter half-span in a centered layout. | Sets the horizontal leg of the roof triangle and affects line length. |
| Pitch | Rise for each 12 units of horizontal run, such as 6:12. | Sets the rise, plumb-cut angle and slope length. It is not the same as degrees. |
| Ridge thickness | The actual thickness of the ridge board or the specified ridge reference. | The workbook method deducts half the ridge thickness from the rafter line length so the rafter meets the ridge correctly. |
| Bearing line | The exact line where the rafter bears on the wall plate or other support. | Locates the birdsmouth and determines whether full bearing remains after cutting. |
| Overhang convention | Either a horizontal tail projection or a distance measured along the roof slope, as stated in the plan. | Those two distances are not interchangeable; converting one to the other changes the tail mark. |
| Rafter spacing and design loads | On-center spacing, species, grade and applicable loads. | These control the member selection and must be checked independently of the cut geometry. |
Calculate the common-rafter lines
1. Convert pitch to rise
For a pitch written as rise:12, calculate the rise from the run:
rise = run × (pitch rise ÷ 12)
A 6:12 roof rises 6 inches for every 12 inches of horizontal run. Calling 6:12 “6 degrees” is incorrect; the pitch ratio and the angle in degrees describe the same slope in different ways.
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The roof triangle gives the rafter line length from the ridge reference to the bearing reference:
line length = √(run² + rise²)
Then apply the plan’s ridge convention. The Oklahoma CareerTech carpentry workbook describes deducting half the ridge thickness from the rafter length. If the plan defines the ridge or bearing datum differently, follow that documented reference rather than moving the deduction by guesswork.
3. Add the tail using the stated convention
If the overhang is a horizontal projection, convert it to a sloping length with the roof’s slope factor:
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slope factor = line length ÷ run
sloping tail length = horizontal overhang × slope factor
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If the plan already gives an overhang measured along the slope, do not convert it again. Fascia, soffit and lookout details may also change where the visible tail is cut, so use the architectural and framing drawings together.
An illustrative calculation—not a cut list
Assume, solely to demonstrate the arithmetic, a centered 24-foot span, a 6:12 pitch, a 1.5-inch ridge board and a 12-inch horizontal overhang. The run is 12 feet and the rise is 6 feet. The triangle line is approximately 13.42 feet (161 inches). Deducting half the 1.5-inch ridge gives approximately 160.25 inches to the bearing reference. A 12-inch horizontal tail projects approximately 13.42 inches along the slope at this pitch. The resulting marks still require the approved bearing, notch, fascia and support details; these example numbers must not be used to frame a real roof.
Lay out one rafter with a framing square
1. Establish a long, straight reference edge
Choose sound, straight stock of the species and grade specified by the plan. Mark the ridge end and the wall-bearing direction on the board. Measure from the same reference edge throughout; switching edges or faces midway can mirror the cut and move the bearing line.
2. Mark the ridge plumb cut
Set the framing square to the specified pitch and place its tongue and body on the reference edge. Mark the plumb line at the ridge location. The plumb cut is the vertical cut that meets the ridge; its angle follows the pitch setting, not a degree value copied from a different roof. Extend the line across the board so the saw cut is easy to follow.
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3. Locate the wall-bearing line
Measure the calculated line length from the ridge reference, including the ridge-thickness deduction required by the plan. Mark the point where the rafter’s heel will meet the wall plate. Check that the measurement is along the rafter line, not horizontally along the board or from the wrong side of the ridge cut.
4. Lay out the birdsmouth seat
A birdsmouth has two parts: a level seat that rests on the wall plate and a plumb heel cut that drops to that seat. Use the framing square to mark the level seat from the bearing line, keeping the intended bearing surface on the support. The seat should be wide enough to provide the required bearing without cutting away the member’s support area.
5. Mark the heel and tail
From the bearing line, mark the plan’s overhang along the slope or convert a horizontal projection as described above. Draw the tail plumb cut. Confirm that the tail leaves room for fascia, soffit, insulation and any required lookout or ladder framing. Do not move the birdsmouth simply to make the tail look equal; bearing and the approved dimensions take priority.
6. Transfer the layout clearly
Label the waste side of every cut. Darken the final lines only after checking that the square is seated against the same edge and that the ridge, bearing and tail marks agree with the plan. A second person can read the marks while you verify the measurements, especially on long stock.
Cut the birdsmouth and other lines safely
Support the rafter on stable blocking so the board cannot roll or pinch the blade. Keep hands outside the blade path and follow the saw manufacturer’s instructions. OSHA’s construction woodworking rule, 29 CFR 1926.304(d), requires portable power-driven circular saws to have guards above and below the base plate or shoe; the lower guard must return automatically when the saw is withdrawn. Never remove, wedge open or defeat a functioning guard.
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Make the plumb and tail cuts with the saw held square to the marked lines. For the birdsmouth, the workbook describes cutting only part of the notch with a circular saw. Stop before the blade over-runs the heel or seat line; a hand saw can finish a notch corner when appropriate. Do not cut past the intersection, because an over-cut can remove critical wood at the bearing area. Remove the waste carefully and keep the seat flat and continuous.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Make a checked template before duplicating rafters
- Place the trial rafter on the wall plate and against the ridge or a full-size ridge mock-up.
- Check that the seat bears flat, the heel remains on the support and the required bearing length is present.
- Check that the ridge plumb cut meets the ridge without forcing the rafter sideways or opening a gap.
- Check the opposite rafter’s alignment, the roof overhang and the tail relationship to fascia and soffit lines.
- Correct the layout on this template, then use it to mark the remaining rafters. Recheck the first few installed members rather than assuming every board has identical crown, width and straightness.
Template work catches a wrong datum, reversed square setting, ridge deduction or overhang conversion before the error is repeated across the roof.
Code, bearing and notch limits
Use the IRC as a model-code reference, not automatic approval
The 2021 International Residential Code (IRC) is a model code. Your jurisdiction may not have adopted it, may use another edition or may have amendments. Its roof-framing provisions measure rafter spans along the horizontal projection and direct sizing to applicable tables. Verify the locally adopted text and the approved plans before ordering lumber or cutting.
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The cited 2021 IRC requires rafter ends to bear at least 1.5 inches on wood or metal and 3 inches on masonry or concrete. That is a minimum bearing statement, not a substitute for checking the actual wall, plate, connector and load path. A seat that looks deep can still leave too little length on the support.
Do not treat a one-third notch rule as universal
Code provisions for sawn rafters reference the limitations in R502.8.1. The permitted notch depends on the member’s location and geometry, the code edition, local adoption and the approved design. Calculator pages sometimes repeat a one-third shorthand; it is not sufficient authority to promise compliance. If the proposed birdsmouth approaches a limit, obtain a design professional’s direction instead of enlarging or relocating it by eye.
Engineered members have separate rules
The 2021 IRC separately restricts alterations to trusses, I-joists, structural composite lumber, glulam and cross-laminated timber. Cutting, drilling or notching is prohibited unless the manufacturer permits it or a registered design professional has considered the effects in the design. Never apply this sawn-lumber birdsmouth procedure to an engineered member or truss.
Common problems and corrective checks
| Symptom | Likely cause | Check or correction |
|---|---|---|
| Ridge ends do not meet cleanly | Pitch setting, ridge datum or plumb-cut direction is wrong. | Reset the square to the specified pitch, confirm which ridge face is the reference and recut only after checking the template. |
| Seat rocks on the plate | The seat is not level, the heel line is misplaced or the lumber is bowed. | Verify the bearing line and square layout; keep the seat flat and use straight, approved stock. |
| Too little wood remains at the heel | The notch was deepened to force a fit or a generic depth rule was used. | Stop cutting and check R502.8.1 or the engineer’s detail. Do not repair a prohibited notch by guesswork. |
| Overhangs vary | Some tails were measured horizontally and others along the slope, or the template shifted. | Choose the plan’s one convention, mark a clear reference line and transfer from the verified template. |
| Rafter is short at the wall | Half the span was not used for a centered ridge, or ridge thickness was deducted from the wrong datum. | Reconfirm span faces, run, ridge design and line-length formula before cutting new stock. |
| Saw binds or the cut wanders | Unsupported stock, a pinched kerf or a damaged/incorrect blade. | Support the board, keep the guard working, disconnect power before adjustments and follow the tool manual. |
Pre-cut checklist
- Approved framing plan, locally adopted code and any engineer or manufacturer details are available.
- Span, run, pitch, ridge thickness, bearing line, spacing, species, grade and load assumptions are confirmed.
- Overhang is identified as horizontal or sloping, with fascia and soffit details coordinated.
- One common-rafter template is laid out with a framing square and labeled waste sides.
- Birdsmouth seat and heel preserve the required bearing; no notch limit is being guessed.
- Stock is straight, supported and secured for cutting.
- The circular saw has functioning upper and lower guards, and the operator follows the manufacturer’s safety instructions.
- The trial rafter has been fitted and checked at the ridge, wall plate and tail before duplication.
Authoritative references
For the instructional sequence, the Oklahoma CareerTech Carpentry Skills Workbook covers rafter line-length selection, ridge-thickness deduction, framing-square birdsmouth layout and partial circular-saw cutting. The International Code Council’s 2021 IRC Chapter 8 supplies model-code provisions for roof framing, span measurement, bearing, sawn-lumber notch references and engineered-member alterations. OSHA 29 CFR 1926.304(d) sets the cited portable circular-saw guard requirement. A secondary guide such as CutOnce’s “How to Cut Rafters” can help explain the calculation sequence, but it is not a substitute for code or design approval.
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