A plumb cut rafter is the vertical cut at the top of a rafter where it meets the ridge — the cut that matches the angle of the rafter against the vertical face of the ridge board or ridge beam. The plumb cut is one of three required cuts on every common rafter (the other two being the seat cut, which sits on the wall plate, and the tail cut, which forms the eave overhang). Cutting all three correctly is the foundational skill of stick-frame roof carpentry, and the plumb cut is the most visible — a poorly-cut plumb cut at the ridge creates gaps that are visible from below the ridge once the roof is up.
Several search variants resolve to the same cut question. "Plumb cut rafter" wants the angle and technique for the top cut. "Cut rafter tails" wants the tail (eave) cut detail. "How to cut rafters with speed square" wants the layout method using a Swanson speed square (the dominant carpenter's tool for rafter layout). This guide covers all three: the plumb cut at the top, the rafter tail cuts (plumb, square, decorative), and the speed-square method for laying out and cutting any rafter cut.
The three rafter cuts on a common rafter: (1) plumb cut at the top — vertical, matches the ridge face. (2) seat cut (bird's mouth) at the wall plate — combination of a horizontal cut (seat) and a vertical cut (heel) that lets the rafter sit cleanly on the wall plate. (3) tail cut at the eave — shapes the section of rafter that extends past the wall plate to form the overhang. Each cut has standard layout methods using a framing square, a speed square, or a calculator. This guide walks through each.
Plumb cut at the rafter top
The plumb cut is the cut at the top of the rafter where it meets the ridge. The cut is "plumb" because it's vertical — perpendicular to the level wall plates below, parallel to the gravity vector. The plumb cut angle (measured from horizontal) equals the roof pitch angle: for a 4/12 pitch (18.43° from horizontal), the plumb cut angle is 18.43°.
How to mark a plumb cut with a framing square: place the framing square against the rafter with the long blade representing the run (12 inches) and the tongue representing the rise (the rise per 12 of run). For a 6/12 pitch: align the 6-inch mark on the tongue and the 12-inch mark on the blade with the rafter edge. Mark along the tongue — that is the plumb cut line.
How to mark a plumb cut with a speed square: speed squares have a degree scale and a "common" or "common rafter" scale. Set the speed square to the rise number (e.g., 6 for a 6/12 pitch) on the "common" scale. Align the square against the rafter edge, mark along the angled side. The plumb cut angle reads automatically. Many users search "how to cut rafters with speed square" looking for exactly this technique — the speed square method is the dominant carpenter's approach in U.S. residential construction.
Cutting the plumb cut: place a circular saw against the marked line, with the blade tilted to match the cut. For a typical plumb cut, the saw blade is set at 0° tilt (square to the rafter face), but the rafter is angled so the cut is plumb when the rafter is in position. Cut along the marked line — the resulting plumb cut face is vertical when the rafter is set on the roof.
Plumb cut accuracy is critical because it determines the seat at the ridge. A plumb cut that's off by 1° creates a 3/16-inch gap at the ridge over a typical rafter — visible from below and creating a leak path at the ridge. For best accuracy: lay out the cut carefully, verify the angle with a digital angle finder, and test-cut on scrap before committing to the full set of rafters.
A common variant: the "double bevel" plumb cut for hip rafters. Hip rafters meet at the ridge at an angle (typically 45° in plan view), so the plumb cut is bevel-cut to match the meeting angle. Layout requires both the plumb angle (matching the rafter pitch) and the bevel angle (matching the meeting plane). More complex than common rafter plumb cuts; speed-square layout helps significantly.
Cut rafter tails — three styles
Rafter tails are the section of each rafter that extends past the wall plate to form the eave overhang. Three main styles: plumb cut tails (vertical, the modern standard), square cut tails (perpendicular to the rafter, for exposed Craftsman style), and decorative tails (custom profiles for traditional architecture).
Plumb cut tails: the cut face is vertical, matching the angle of the ridge plumb cut. Fascia (the horizontal trim board at the eave) attaches directly to the vertical cut face — clean, conventional installation. The most common tail cut in modern residential construction; the default for almost all asphalt-shingle homes built since the 1980s.
How to mark a plumb cut tail: same method as the ridge plumb cut. The cut is positioned at the desired distance from the wall plate (typically 12-24 inches for residential overhangs). The rafter tail extends past the wall plate, and the plumb cut at the end of the tail is parallel to the plumb cut at the ridge.
Square cut tails: the cut face is perpendicular to the rafter's long axis (the cut "follows" the rafter rather than being plumb). The cut tilts back at the pitch angle when the rafter is in position — fascia (if used) angles to match. Common on Craftsman bungalows, modern minimalist designs, and any eave where the rafter ends are visible from below as a structural detail.
How to mark a square cut tail: align a framing square or speed square perpendicular to the rafter edge and mark across. The cut is straight perpendicular to the rafter, ignoring the pitch. The result tilts at the pitch angle when the rafter is installed.
Decorative tails: for Craftsman, Tudor, traditional Asian, and other architectural styles where rafter tails are exposed and visible. Common shapes include curved tails, scalloped tails, tapered tails, and double-step "Stickley" profiles. Decorative tails are typically cut with a band saw or jigsaw to a paper template — cut one tail carefully, use it as a template for the rest.
Choosing the tail style: plumb cut for any home with conventional fascia and soffit (most modern construction). Square cut for exposed-tail architectural styles (Craftsman, modern). Decorative for traditional and architectural styles where the tail is a visual feature. The tail style is one of the design decisions made before any rafter is cut — changing tail styles after framing is messy and limits the design.
How to cut rafters with a speed square
The Swanson speed square is the dominant carpenter's tool for rafter layout in U.S. residential construction. Compact (typically 7-inch base), durable (cast aluminum), and engineered specifically for rafter angles. The speed square has a "common rafter" scale that converts pitch directly to cut angles without doing trigonometry.
Speed square anatomy: the square has three sides. The base (long horizontal edge) is the reference for placing against the rafter edge. The pivot corner (90° corner) is the alignment point. The tongue (the angled side opposite the pivot) is the angled edge that traces the cut line. The two scales: "degree" (regular angle scale 0-90°) and "common" (rafter pitch scale, marked 1, 2, 3, ... 12).
How to mark a plumb cut with a speed square: (1) place the speed square against the rafter edge with the pivot corner at one rafter edge. (2) rotate the square until the appropriate "common" mark on the angled scale aligns with the rafter edge. For a 6/12 pitch: align the "6" mark on the common scale. For 4/12: align the "4" mark. (3) trace along the tongue (the angled side) — that is the plumb cut line. Cut along the line for a clean plumb cut.
How to mark a seat cut (bird's mouth) with a speed square: the seat cut combines two cuts — the seat (horizontal, sits on the wall plate) and the heel (vertical, against the outside of the wall plate). To lay out: (1) measure from the plumb cut along the rafter edge to the start of the seat cut location. (2) at that point, place the speed square with the pivot corner at the rafter edge, "common" mark at the rafter edge — trace along the tongue for the heel cut (vertical when rafter is in position). (3) place the square again at the bottom of the heel cut, with the base perpendicular to the heel cut, and trace along the base for the seat cut (horizontal when rafter is in position).
How to mark a tail cut with a speed square: depending on the style, use either the plumb-cut method (for plumb cut tails) or the square-cut method (rotate the speed square so the base is perpendicular to the rafter for a square cut). Decorative tails require a paper template.
Common errors with speed-square layout: (1) confusing the "common" and "hip/valley" scales. The common scale is for common rafters (perpendicular to the wall plate); the hip/valley scale is for hip and valley rafters (which run diagonally). Use the wrong scale and the cut angle is wrong. (2) marking on the wrong side of the speed square — the angled tongue gives the cut line; the base is for laying out perpendicular references. (3) misidentifying which mark on the common scale to use — for a 6/12 pitch, the "6" mark, not the "12" or "0" marks.
Pattern rafter — cut one, copy the rest
For any roof with multiple identical rafters (most residential roofs), the standard practice is to cut one "pattern rafter" carefully, verify it fits perfectly, then use it as a template to cut all the others. Pattern-rafter workflow significantly reduces layout time and ensures all rafters are identical.
Selecting the pattern rafter: choose a straight, knot-free, dry piece of lumber for the pattern. Crown the board (place crown up — the natural curve of the board oriented up) so that any sag goes against gravity in service. The pattern rafter should be slightly longer than the calculated length to allow for adjustment.
Laying out the pattern rafter: use the speed square or framing square to mark all three cuts (plumb cut at top, seat cut at the wall plate, tail cut at the eave). Mark with a sharp pencil — any layout error transfers to every other rafter. Verify all three cuts are accurate before cutting.
Test-fitting the pattern rafter: before cutting more rafters, test-fit the pattern on the actual structure. Place the rafter in position, verify it sits cleanly at the ridge, the bird's mouth fits the wall plate, and the tail extends the correct amount. If the fit is wrong, adjust the pattern and test again before cutting any more rafters.
Using the pattern as a template: lay the pattern rafter on top of each new rafter blank (lined up on one edge). Trace all three cut lines from the pattern onto the new rafter. Cut along the traced lines — every new rafter is identical to the pattern. This method is faster and more accurate than re-laying out each rafter individually.
Common mistakes: (1) failing to crown the pattern (and all subsequent rafters consistently) — uncrowned rafters can sag in service. (2) marking on the wrong face of the rafter — always trace on the same face for all rafters. (3) using a worn or damaged pattern after several uses — a pencil-traced template wears down; remake the template if it loses precision.
Overhang length and fascia attachment
The rafter tail length (the section past the wall plate forming the eave overhang) is a design decision affecting drainage, sun shading, and aesthetics. Typical residential overhangs: 12-24 inches for asphalt shingle homes, 24-48 inches for traditional Craftsman or farmhouse styles with deep eaves.
Drainage benefits of a deeper overhang: water sheds further away from the foundation. A 12-inch overhang directs rainfall about 4-6 inches from the foundation; a 24-inch overhang directs water 12-18 inches away (depending on drip edge geometry). Deeper overhangs reduce foundation moisture and basement seepage in regions with heavy rainfall.
Sun shading benefits: deeper overhangs reduce direct sun exposure on south-facing windows in summer (when the sun is high) while still allowing low-angle winter sun (lower in the sky) to reach the windows. Passive solar design typically targets 24-36 inch overhangs in hot climates. The overhang depth interacts with the window height — deeper for taller windows.
Fascia attachment to plumb cut tails: the fascia (typically 1×6 or 1×8 nominal pine or composite material) attaches directly to the vertical cut face of the plumb-cut rafter tails. Continuous along the eave, with butt joints or scarf joints between fascia boards. The drip edge installs above the fascia, with the fascia hidden below.
Fascia attachment to square cut tails: the fascia (if used) angles to match the rafter pitch. Some Craftsman designs skip the fascia entirely, leaving the rafter ends fully exposed. Others install a "cap" piece across the rafter ends but leave the rafter tail visible from the side. The detail varies by architectural style.
Soffit installation: soffit (the horizontal panel under the eave, between the wall and the fascia) attaches to nailer strips installed on the underside of the rafter tails and at the wall. For plumb-cut tails: soffit is horizontal, with vents installed to allow attic ventilation. For exposed-tail designs (square cut or decorative): no soffit; rafter tails are visible from below.
Common rafter cutting mistakes and how to avoid them
Several common errors plague first-time rafter cutters. Avoiding these saves time and ensures the roof goes up cleanly.
Wrong pitch in the layout: the most common error is laying out the cuts for a different pitch than the actual roof. For a 6/12 pitch, set the speed square to "6" on the common scale; for 4/12, set it to "4". A 6/12 layout used on a 4/12 roof produces rafters that don't fit. Double-check the pitch before cutting.
Wrong direction of the cut: the plumb cut at the top should angle one way (toward the ridge); the seat cut's heel should angle the same way. Reversing the cut direction creates rafters that fit upside-down. Lay out and cut on the same face of the rafter, with the crown up, to maintain consistent direction.
Mismeasuring the rafter length: the rafter length is calculated from the run (half the span, in inches) and the rise (rise per 12). The actual rafter length runs from the ridge plumb cut to the seat cut at the wall plate, plus the tail length past the seat cut. Mismeasuring by 1 inch produces rafters that are too short or too long; use the calculator on this site to verify rafter length before cutting.
Skipping the test-fit: cutting all the rafters before test-fitting the first one means errors compound across the entire set. Always test-fit the pattern rafter before cutting more. The 30 minutes spent on test-fitting saves hours of recutting if the pattern is wrong.
Using a worn pencil or chalk line for layout: layout marks need to be precise to 1/16 inch or better. A worn pencil produces fat marks that introduce 1/8 inch or more of cut error. Use a sharp pencil (not a regular #2 — use a hard-lead pencil or carpenter's pencil sharpened to a point) for layout.
Forgetting to account for the kerf: the saw blade has a kerf (the width of the cut, typically 1/8 inch). When cutting along a layout line, the saw must be on the "waste" side of the line — otherwise the kerf removes material from the rafter, making it too short. Always cut on the waste side, not on the line itself.
Tools needed for cutting rafters
Cutting rafters requires standard carpentry tools plus a few rafter-specific items. The list below covers a complete residential rafter-cutting setup.
Speed square (Swanson 7-inch or equivalent): the primary layout tool for rafter cuts. Cost: $10-25. Essential for any rafter work; also useful for general framing.
Framing square (24×16-inch L-shape): the traditional rafter layout tool, still useful as a backup or for long rafters where the speed square is too small. Cost: $20-40.
Pencil (sharp): for marking layout lines. A worn pencil produces inaccurate marks. Carpenter's pencils are wider and more durable; #2 pencils are sharper. Use whichever the carpenter prefers, but keep it sharp.
Tape measure (25-foot+): for measuring rafter length and overhang distances. A 25-foot tape covers most residential rafter lengths; longer tapes are useful for longer rafters.
Circular saw (7-1/4 inch): for cutting the rafters. The 7-1/4 inch blade depth handles 2x lumber up to 2x10 in a single pass; deeper rafters (2x12, double-2x) require flipping the rafter and cutting the second face.
Saw horses: for supporting the rafter while cutting. A pair of standard saw horses or a portable rafter-cutting jig (used by some framers).
Safety equipment: safety glasses, hearing protection, dust mask. Cutting wood produces flying debris and dust; protect eyes, ears, and lungs.
Optional tools: a digital angle finder (for verifying cut angles after marking), a chalk line (for long straight lines), a band saw or jigsaw (for decorative tail cuts), and a hand plane (for fine-tuning tight-fit cuts).
How we sourced these techniques
Rafter cutting techniques follow standard U.S. residential carpentry practice as documented in the IRC (International Residential Code), the American Wood Council Wood Frame Construction Manual, and standard carpentry references (Larry Haun, Greg Higgins, Mike Guertin). The speed-square method is the dominant residential approach in U.S. practice; framing-square methods are more common in older or specialized work.
Specific tools and pricing reflect 2026 U.S. residential carpentry markets. Available tools (Swanson speed square, Empire framing square, DeWalt circular saws, etc.) are common contractor and DIY choices at the time of writing; specific products change as manufacturers release new versions, but the techniques and tool categories are stable. Recommendations are reviewed annually and updated when industry-standard tools or practices change materially.
Need to run the numbers?Use the free roof pitch calculator on the home page to convert pitch to angle, calculate rafter length, or estimate roof area in any unit.