You’re standing there with a circular saw, a framing square, and a $150 piece of LVL or a standard 2x10, and your heart is thumping because if you mess up this one cut, the ridge board won't sit right. It's stressful. Most people think they need to be a math genius to figure out how to cut a rafter, but honestly, it’s more about understanding geometry than memorizing long formulas. If you can use a speed square and a pencil, you can frame a roof.
Framing is messy. It's loud. It involves sawdust in your eyes and the constant calculation of "line-of-sight." But there is a specific rhythm to it that makes sense once you stop overcomplicating the "rise over run" aspect.
I’ve seen guys on job sites try to eyeball a birdsmouth cut and end up with a structural nightmare that looks like a sagging shelf. Don't be that guy. We are going to break down the actual physics of the cut, the tools you absolutely cannot skip, and why the "plumb cut" is the hill most DIY projects die on.
The Secret Language of the Framing Square
Before you even touch a saw, you have to talk the talk. A rafter isn't just a piece of wood; it’s a series of specific angles working in harmony to fight gravity. The plumb cut is the vertical cut at the top where the rafter meets the ridge board. Then you have the birdsmouth, which is the notch that lets the rafter sit flat on the wall plate. Without that notch, your roof is basically just sliding off your house.
Most pros use a Swanson Speed Square. It’s that silver or blue triangle you see in every carpenter's back pocket. On that square, you’ll see a "Pivot" point and a "Common" rafter scale. This is your bible. If your roof pitch is a 6/12—which means the roof rises 6 inches for every 12 inches it moves horizontally—you find the 6 on that Common scale.
Why Your Measurements Are Probably Wrong
Here is where it gets tricky. People measure from the very tip of the wood. Big mistake.
When you are figuring out how to cut a rafter, you have to account for the thickness of the ridge board. If you’re using a standard 2x ridge, that board is actually 1.5 inches thick. That means your rafter length needs to be shortened by half that thickness—0.75 inches—measured square from the plumb cut. If you forget this, your rafters will be too long, and your ridge will be pushed up, creating a peak that looks like a broken tent.
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Measure twice. Actually, measure three times. Use a tape measure that doesn't flop around.
Step 1: The Plumb Cut
Lay your rafter board flat on the sawhorses. Take your speed square and pivot it until the "6" (or whatever your pitch is) aligns with the edge of the board. Draw that line. This is your plumb cut. It’s called "plumb" because when the rafter is installed, this line will be perfectly vertical, pointing straight to the center of the earth.
You’re going to cut this first. Use a sharp blade. A dull blade will wander in the wood, especially if you’re cutting through pressure-treated material or dense Douglas Fir. Keep the base of your saw flat. If you tilt it even a degree, the rafter won’t sit flush against the ridge board, and you’ll lose structural integrity.
The Birdsmouth: Where the Magic Happens
This is the hardest part of learning how to cut a rafter. The birdsmouth consists of two cuts: the seat cut (horizontal) and the heel cut (vertical).
- The Seat Cut: This must be at least 3.5 inches long if you’re sitting on a 2x4 wall.
- The Heel Cut: This should never exceed one-third of the depth of the rafter.
If you cut too deep into the rafter to make it fit, you weaken the wood. It’ll crack right at the notch under a heavy snow load. Larry Haun, the legendary framing master, always emphasized that the rafter is essentially a beam, and you can't just hack away at a beam.
To mark it, you need to calculate the "Line Length." This is the diagonal distance from the ridge to the outside of the wall. Once you mark that point on your board, use your square to draw another plumb line. Then, flip the square to draw the level seat cut. It should look like a little triangle is being removed from the bottom of the board.
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Don't Overcut the Corner
When you’re cutting the birdsmouth with a circular saw, the round blade won't reach the corner of the notch without overshooting on the top side. Stop early. Finish the cut with a hand saw. It takes an extra 30 seconds, but it prevents you from creating a "stress riser" where the wood will eventually split.
Handling the Overhang and Tails
The "tail" is the part of the rafter that hangs past the wall to create your eaves. You can cut these to a fancy pattern, or just a simple plumb cut to attach a fascia board.
Some people like to leave the tails long and cut them all at once after the rafters are installed. This is actually a pro tip. By snapping a chalk line across all the installed rafters, you ensure the edge of your roof is perfectly straight, even if your walls have a slight bow in them. It’s much easier than trying to get every single individual rafter cut perfectly to the sixteenth of an inch on the ground.
Real World Math (The Easy Way)
Forget trigonometry. Use a rafter table. Most framing squares have these tables stamped right onto the metal. If you look under the "6" on the table, it will give you the "Length of Common Rafter per Foot of Run."
Let's say your building is 20 feet wide. Your "run" is half that: 10 feet.
If the table says 13.42 inches for a 6/12 pitch, you multiply 13.42 by 10.
That gives you 134.2 inches.
Convert that to feet and inches (11 feet, 2 and 3/16 inches).
Wait. You still have to subtract that 3/4 inch for the ridge board.
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Every single time I see a beginner try to do this, they forget the ridge deduction. Then they get up on the ladder, try to nail it in, and realize they’re fighting the wood. Wood doesn't compress. If it's too long, it's too long.
The Tools You Actually Need
You don't need a $500 miter saw for this. In fact, a miter saw is often worse because it’s hard to support a 16-foot rafter on a small saw table.
- 7-1/4 inch Circular Saw: The gold standard.
- Speed Square: Essential for marking the angles.
- Chalk Line: For the tails.
- Sharp Pencil: A fat carpenter pencil is okay for rough framing, but a mechanical pencil or a sharpened #2 is better for the birdsmouth.
- Tape Measure: Get one with a wide blade that stays stiff when extended.
Common Blunders to Avoid
Don't use a "step-off" method with your square if you can avoid it. This is where you move the square 12 times for a 12-foot run. If you are off by even 1/16th of an inch on the first "step," that error multiplies. By the time you get to the end, you could be nearly an inch off. Use the math from the rafter table or a construction calculator app like BuildCalc.
Another big one: ignoring the "crown" of the wood. Every piece of lumber has a slight curve. Always layout your rafter so the crown (the hump) is facing up. Gravity and the weight of the roofing material will eventually flatten it out. If you put the crown down, your roof will have a permanent sag from day one.
Final Installation Check
Once you’ve mastered how to cut a rafter, the installation is all about bracing. Nail the first pair of rafters to the ridge board and the wall plates. Use three 3-inch nails at the ridge and "toenail" the birdsmouth into the top plate of the wall.
Check for "plumb" frequently. If your first rafter is leaning, every single one after it will be leaning too. Use a 4-foot level to make sure the ridge is centered and the rafters are vertical.
Actionable Next Steps
To get started on your rafter project today:
- Determine your pitch: Check your local building codes, as some areas require steeper pitches for snow shedding.
- Pick your lumber: Select straight, high-grade 2x8 or 2x10 boards, ensuring you check the crown on every piece.
- Mark a "Pattern Rafter": Cut one rafter perfectly, test fit it, and use it as a template for all the others. This ensures consistency across the entire roofline.
- Get a helper: Lifting rafters and holding a ridge board is a two-person job; safety should always be the priority when working at height.