Free Rafter Calculator with AI - Cut Angles & Blueprint PDFs
Use our Free Rafter Calculator to easily design and customize rafter blueprints - or simply describe your rafter to the built-in AI chat and it designs and builds it for you. Featuring an interactive viewer, our tool allows you to generate, download, print, and save detailed rafter designs as PDFs. Perfect for builders, architects, and DIY enthusiasts looking to create precise rafter plans quickly and efficiently. Try it today to streamline your building projects!
How to Use This Tool
Enter the span and pitch - or ask the AI
Type your building span (or the run to the ridge centerline) and the roof pitch, such as 8/12 - or open the AI chat and describe the rafter in plain language. Either way the calculator solves the rafter triangle instantly: rise, line length, and every cut angle.
Set lumber size, ridge, and overhang
Pick the rafter stock (2x6 through 2x12), the ridge board thickness, and the level overhang for the tail. The tool applies the ridge deduction and adds the tail length along the slope automatically.
Adjust the birdsmouth
Set the seat cut so the rafter gets full bearing on the plate while the notch stays within code limits. The viewer shows the plumb and level cuts and the resulting height above plate (HAP) so every rafter lands at the same ridge height.
Export the blueprint
Download or print the dimensioned rafter blueprint as a PDF - plumb cut, birdsmouth, tail cut, and overall length all labeled - and save the design to your account to re-cut the same pattern later.
The math behind the rafter calculator
A common rafter is the hypotenuse of a right triangle: the horizontal leg is the run (half the span for a central ridge), the vertical leg is the rise, and the pitch sets their ratio. For a pitch of X/12, every foot of run produces sqrt(1 + (X/12)^2) feet of rafter - the pitch multiplier. That one number turns a tape-measure dimension in plan into real lumber length, and the plumb cut angle at the ridge is simply arctan(pitch/12).
Two adjustments turn the theoretical line length into the board you actually cut. The rafter is shortened by half the ridge thickness, measured horizontally and taken along the slope, so it lands on the ridge face instead of the centerline. Then the tail is added past the birdsmouth for the overhang, using the same multiplier. The birdsmouth itself is a level seat cut plus a plumb heel cut; what remains above the seat - the height above plate, or HAP - must be identical on every rafter or the ridge will wander.
run = span / 2 (central ridge)
rise = run x pitch / 12
line length = run x sqrt(1 + (pitch/12)^2)
plumb cut angle = arctan(pitch / 12)
ridge deduction = (ridge thickness / 2) x sqrt(1 + (pitch/12)^2)
tail length = level overhang x sqrt(1 + (pitch/12)^2)
HAP = plumb rafter depth - plumb depth of the birdsmouth
Worked example: 24 ft span at 8/12 with a 2x8
- Building span 24 ft with a central 2x ridge, pitch 8/12, 2x8 rafters, 12 in level overhang. Run = 24 / 2 = 12 ft.
- Pitch multiplier for 8/12: sqrt(1 + (8/12)^2) = sqrt(1.444) = 1.202. Line length = 12 x 1.202 = 14.42 ft, about 14 ft 5 in from plate to ridge centerline.
- Ridge deduction: half of the 1-1/2 in ridge is 3/4 in of run, times 1.202 = about 7/8 in off the top of the rafter along the slope.
- Plumb cuts: arctan(8/12) = 33.7 degrees at the ridge, and the same angle repeats at the heel cut and the tail plumb cut.
- Tail: 12 in of level overhang x 1.202 = about 14-7/16 in of rafter past the birdsmouth heel.
- Rise check: 12 ft x 8/12 = 8 ft to the theoretical ridge point above the plates. The calculator draws the full blueprint from these inputs and labels every cut.
Frequently Asked Questions
What is a common rafter?
A common rafter runs square from the wall plate to the ridge - perpendicular to both in plan view - and sets the height of the whole roof. Hips, valleys, and jacks are all measured relative to the commons. On a simple gable roof, every rafter is a common; on hips and irregular roofs the commons are the full-length rafters in the middle of each plane. If you can lay out one common, the framing square handles everything else.
How do I calculate rafter length by hand?
Multiply the run by the pitch multiplier, sqrt(1 + (pitch/12)^2). A 12 ft run at 8/12 is 12 x 1.202 = 14.42 ft of line length. Then subtract half the ridge thickness along the slope (about 7/8 in for a 2x ridge at 8/12) and add the tail for the overhang (level overhang times the same multiplier). This tool doubles as a rafter length calculator - it runs exactly this arithmetic and prints it on the blueprint, so you can check it against a framing square.
What angle do I cut the top of the rafter?
The plumb cut angle is arctan(pitch/12): 18.4 degrees for 4/12, 26.6 for 6/12, 33.7 for 8/12, 45 for 12/12. Set a circular saw or miter saw to that angle, or use the pitch numbers directly on a framing square - 8 on the tongue, 12 on the blade - and mark along the tongue. The same angle repeats at the heel plumb cut and the tail cut, so one saw setting cuts the whole rafter.
How deep can the birdsmouth be?
Two limits govern it. The seat needs at least 1-1/2 in of bearing on the plate (IRC R802.6.1), and the notch at the end of the rafter should not exceed one quarter of the rafter depth (R802.7.1) - about 1-13/16 in of plumb depth on a 2x8. Cut deeper and you have effectively turned your 2x8 into something smaller right where the load concentrates. The calculator keeps the seat inside both limits as you adjust it.
What is HAP and why does it matter?
HAP - height above plate - is the rafter material left standing above the birdsmouth seat, measured plumb. It matters because the ridge height is set by the HAP, not by the rafter length: if one rafter's birdsmouth is cut 1/4 in deeper than the others, that rafter pushes the ridge down 1/4 in at its station. Cut one pattern rafter, verify its HAP, and gang-cut the rest from it so every rafter matches exactly.
What size rafter do I need for my span?
Size comes from the span tables, not the geometry: IRC Table R802.4.1 and its variants list allowable horizontal spans by species, grade, spacing, and snow load. As a rough anchor, a 2x8 SPF No. 2 at 16 in on center handles about 14 to 18 ft depending on load; heavier snow country needs deeper stock or tighter spacing. Use this calculator for lengths and cuts, then confirm the member size against our span tables and your local ground snow load.
Do I need collar ties or rafter ties?
Usually one or the other, sometimes both. Rafter ties (or ceiling joists acting as ties) belong in the lower third of the roof and resist the outward thrust that tries to push the walls apart - without them you need a structural ridge beam. Collar ties sit in the upper third, at most 4 ft apart, and resist wind uplift at the ridge (IRC R802.4.6). They are not interchangeable, and a collar tie alone does not stop wall spread.
Can the AI design my rafter for me?
Yes. Open the AI chat inside the calculator and describe what you are building in plain language - span, pitch, overhang, lumber size - and the assistant designs and builds the common rafter in the tool. You can then adjust any cut manually in the viewer and export the same blueprint PDF. It is the fastest path from a text description to a cut-ready drawing, and every number it produces is the same trigonometry shown in the formulas above.
Rafters or trusses - which should I use?
Trusses win on speed and span for production work, but stick-framed rafters win on flexibility: full attic space, cathedral ceilings, dormers, and the ability to frame on site from lumber-yard stock with no crane or lead time. For sheds, garages, additions, and any roof you want open inside, cut rafters remain the practical choice - and a printed blueprint from this calculator plus one careful pattern rafter is the whole layout process.