Rafter Cutting Troubleshooting

Jump to what you need

Wrong Angle

If the ridge joint gaps at the top or crushes at the bottom, the plumb cut is not the pitch. Classic causes: saw set to the seat angle (90° minus pitch) instead of the plumb; Speed Square read as a 6° mark instead of 6/12 (26.6°); or a hip plumb used on a common.

• Diagnose. Put a Speed Square on the cut. The plumb should match the calculator's pitch angle. A 6/12 common is 26.6°, not 6° and not 63.4°.
• Fix a single pair. Recut the ridge plumb on the waste side if you have extra tail. If the cut is already into the measuring line, that pair is a shorter rafter or scrap. Do not shim a gap at the ridge and call it framing.
• Fix the pattern. Reset the square to 12 and the pitch number. Recut the pattern pair. Clone from the proven pair, not from the bad one.

Hip and valley plumb cuts are not the common pitch angle. If the board is a hip and you used 26.6° on a 6/12 roof, the angle is wrong even if the length were accidentally close. Stop and use the hip/valley calculator, then hip/valley troubleshooting. Decision: common vs hip/valley.

Rafter Too Short or Too Long

Measure what you actually cut against the calculator's three lengths: line (plate to ridge), tail, and total (ridge to fascia). They fail for different reasons.

Too long

• Span was entered as run (24 ft of run instead of 12). The triangle is doubled.
• Ridge thickness was not subtracted. Each common should lose half the ridge (0.75" run on a 2x ridge).
• Overhang was added as level inches and as tail inches.
• Measuring along the bottom edge after the birdsmouth was cut — that path is shorter than the top-edge measuring line, so people add mystery length and overshoot.

Too short

• Overhang entered as 12" of board instead of 12" horizontal. On 6/12 you lose about 1.4" of tail.
• Run measured to the center of the ridge instead of the face, then the ridge board still took its half.
• Birdsmouth dropped the measuring line and nobody added the drop back when ticking the fascia.
• Plate is not where the plan said (sheathing, extra foam, a moved wall). The triangle changed; the board did not.

A short rafter that is only short in the tail can sometimes take a longer fascia plumb if stock remains. A short line length cannot be stretched at the ridge. Cut a new common. Sistering a scab onto a ridge cut is not a roof.

Ridge Board Misalignment

A ridge that walks off layout is usually the first pair, not the ridge board. If the two commons are different lengths or different plumbs, the ridge is pushed toward the short side and rotated toward the open plumb.

1. Pull layout on both plates. The first pair should land on matching layout marks, not “close enough.”
2. Hang the first pair, then check: ridge on the building centerline, top of ridge level along its length, plumb in the short direction.
3. If the ridge is high, the commons are long or the birdsmouth is too shallow (rafter not sitting down). If the ridge is low, the commons are short or the birdsmouth is too deep.
4. If the ridge is off-center, one run is wrong or one wall moved. Measure plate-to-ridge on both sides with a tape held level. Those two runs must match the calculator run (minus half ridge).

Temporary ridge support (a post or a kicker) keeps the first pair from spreading while you fill in. Spreading looks like “rafters too short” by afternoon. The layout guide is the mark-and-cut sequence; this page is what to measure when the ridge is already wrong.

Birdsmouth That Will Not Sit

If the seat teeters on the outside corner of the plate, the seat cut is not level (wrong complementary angle) or the heel is not plumb. If the rafter sits but the remaining depth looks like a 2x4, the seat is too deep for the stock.

• Re-mark heel at the building line and seat at plate width. Recut only if you have wood to give.
• Do not pack the seat with shims as a plan. A shim under a birdsmouth is a slip plane.
• Hurricane ties and hangers do not fix a birdsmouth that does not bear. They keep a bearing rafter from lifting.

FAQ