Roof Builder & Roof Framing Generator - Auto-Generate Rafters in 3D
Effortlessly design and automatically generate roof framing with our Online Custom Roof Builder. Simply define the perimeter walls of your structure, and this advanced tool calculates all roof rafters instantly. View your custom roof in 3D, and access detailed blueprints for individual rafters, valley rafters, hip rafters, and ridge rafters. Customize roof angles for irregular roofs and easily view the perimeter and area of your roof. Ideal for architects, builders, and DIY enthusiasts seeking precision and efficiency in roof design and construction.
How to Use This Tool
Trace the perimeter walls
Draw the outside wall lines of your structure, or type the footprint dimensions. The builder accepts rectangles, L-shapes, T-shapes, and irregular footprints with inside and outside corners.
Set the pitch for each roof plane
Enter a pitch such as 6/12 for the whole roof, or assign a different pitch to individual planes for irregular roofs. The tool generates ridges, hips, and valleys automatically from the footprint geometry.
Review the framing in 3D
Orbit the 3D model and check every common, hip, valley, and ridge member. Click any rafter to see its length, plumb cut, and seat cut before you commit to lumber.
Export blueprints and cut lists
Download the rafter blueprints and full cut list as a PDF, or print directly. Each hip and valley rafter gets its own drawing with bevels and cheek cuts labeled.
The math behind the roof builder
Every stick-framed roof reduces to one idea: the pitch multiplier. A rafter is the hypotenuse of a right triangle whose horizontal leg is the run and whose vertical leg is the rise. For a pitch of X/12, every foot of run produces sqrt(1 + (X/12)^2) feet of rafter. That single number - 1.118 for 6/12, 1.202 for 8/12, 1.4142 for 12/12 - converts plan dimensions to real lumber lengths, and it is the same factor that converts plan area to sheathing area.
Hips and valleys travel diagonally across the plan, so their unit run is not 12 inches but 16.97 inches (12 times sqrt(2)) for a regular 45-degree hip. The tool applies the correct diagonal run automatically, including for irregular corners where the hip does not bisect the angle at 45 degrees, and it computes the backing bevels and cheek cut angles that follow from that geometry.
pitch multiplier = sqrt(1 + (pitch/12)^2)
common rafter length = run x sqrt(1 + (pitch/12)^2)
hip/valley rafter length = common run x sqrt(2 + (pitch/12)^2)
hip unit run = 16.97 in per 12 in of common run (regular 45-degree hip)
ridge length (simple hip roof) = building length - building width
roof area = plan area x pitch multiplier
Worked example: 24 x 32 ft hip roof at 6/12
- Footprint: 24 ft wide by 32 ft long, full hip roof, 6/12 pitch. The common run is half the width: 24 / 2 = 12 ft.
- Pitch multiplier for 6/12: sqrt(1 + (6/12)^2) = sqrt(1.25) = 1.118. Common rafter line length = 12 x 1.118 = 13.42 ft, about 13 ft 5 inches before the ridge deduction and tail.
- Hip rafters: 12 x sqrt(2 + 0.25) = 12 x 1.5 = 18.00 ft line length. The 6/12 case is a rare clean number - sqrt(2.25) is exactly 1.5.
- Ridge: 32 - 24 = 8 ft between the hip apexes. Rise at the ridge = 12 ft x 6/12 = 6 ft above the plates.
- Sheathing area: plan area 24 x 32 = 768 sq ft, times 1.118 = 859 sq ft. Order 27 sheets of plywood plus waste for the hip cuts.
- The builder draws all of this from the two footprint dimensions and the pitch, then exports each rafter blueprint with the 26.6-degree plumb cut (6/12) marked.
Frequently Asked Questions
How does the roof builder calculate rafters from just the walls?
The footprint fixes the geometry. Once you draw the perimeter, every ridge sits midway between parallel walls, every outside corner generates a hip, and every inside corner generates a valley. Add a pitch and each member length follows from right-triangle math: run times the pitch multiplier for commons, diagonal run times its multiplier for hips and valleys. The tool solves all of it at once and updates the 3D model live as you edit the walls.
What is the pitch multiplier and why does it matter?
The pitch multiplier is sqrt(1 + (pitch/12)^2) - the length of sloped roof per foot of horizontal run. For 4/12 it is 1.054, for 8/12 it is 1.202, for 12/12 it is 1.414. Multiply any plan dimension by it to get true length along the slope, and multiply plan area by it to get sheathing and shingle area. Estimators call the same number the slope factor; it is the fastest way to convert a takeoff from plan view.
Can it frame an L-shaped or irregular roof?
Yes. Inside corners on the footprint produce valleys, and the tool calculates the valley rafter, the shortened commons, and the valley jack rafters on both sides. You can also assign different pitches to different planes - say 8/12 on the main roof and 4/12 on a rear addition - and the builder solves the resulting off-angle valley, which is tedious to do by hand because the valley no longer runs at 45 degrees in plan.
What size ridge board does code require?
IRC R802.3 calls for a ridge board at least 1 inch nominal thickness and at least as deep as the cut end of the rafter - so a 2x10 rafter with a plumb cut needs a ridge board deep enough to catch the full cut face, typically a 2x12. A structural ridge beam is different: it carries load and needs sized supports to the foundation. Always verify with your local building department, since amendments vary by jurisdiction.
How do I know what rafter size to use for my span?
Rafter size depends on span, spacing, species, grade, and snow load. IRC Table R802.4.1 and its variants list allowable spans - for example, a 2x8 SPF No. 2 at 16 inches on center handles roughly 14 to 18 ft of horizontal span depending on load. Use the roof builder for geometry and lengths, then confirm the member size against our span tables and your local ground snow load before ordering lumber.
Does the tool account for the ridge deduction and rafter tails?
Yes. The theoretical line length runs from the plate to the ridge centerline; the actual rafter is shortened by half the ridge thickness measured perpendicular to the plumb cut - 3/4 inch for a 1.5-inch ridge. The tail for the overhang is added past the birds mouth using the same pitch multiplier: a 16-inch level overhang at 6/12 adds about 17-7/8 inches of tail. Both adjustments appear on the exported blueprint for each rafter.
Can I get a cut list for the whole roof?
Every member - commons, hips, valleys, jacks, and ridges - lands on a single exportable cut list with lengths, plumb cut angles, and cheek cuts for jack rafters. You can print it or save the PDF blueprint set for the job site. The list groups identical rafters, so a hip roof with sixteen matching jacks shows one line with a quantity instead of sixteen entries, which makes gang-cutting on sawhorses much faster.
Is this accurate enough to cut lumber from directly?
The math is exact trigonometry, the same numbers a framing square or construction calculator produces, so the lengths and angles are cut-ready. That said, real buildings are rarely perfectly square. Standard practice is to cut one pattern rafter from the blueprint, test it in place at both ends of the building, and then gang-cut the rest from the pattern. Field-verify wall dimensions before cutting hips and valleys, which are less forgiving.