Online Bay Roof Section Calculator - Precise Rafter Calculations for Specialized Bay Roofs
Tailor your bay roof projects with our Online Bay Roof Section Calculator, a specialized tool designed to provide accurate rafter calculations for unique bay roof configurations. This calculator is perfect for complex roof designs, offering precise measurement and detailed rafter lengths to ensure flawless construction. Ideal for advanced builders and architects who need exact specifications for non-standard bay roofs. View, print, and plan your roofing projects with ease, ensuring each section of the roof integrates seamlessly with the overall structure.
How to Use This Tool
Define the section geometry
Enter the wall length, projection, and corner angles for the individual bay section you are solving - useful when the bay is non-standard and each face needs its own treatment.
Set pitch and connection heights
Enter the pitch and where the section ties in - the plate height at the bay wall and the ledger height at the house - so the section planes in with its neighbors.
Read the rafter output
The tool draws the section and returns each rafter length with plumb, seat, and cheek angles, ready to print alongside the other sections of the bay.
Solving one section at a time
Non-standard bays - unequal side angles, a bow made of several short panels, or a bay squeezed against another element - do not yield to a single symmetric solution. The section approach solves each roof plane as its own triangle: the run is the horizontal distance from the section wall to its high line, the pitch multiplier converts run to rafter length, and the hips at each edge of the section bisect whatever corner angle that edge actually has, taking their plan run from the corner angle rather than an assumed 45 or 22.5 degrees.
The discipline that makes sections assemble correctly is a shared rise: every section must reach the same height at the common tie-in line. Fix the rise first, and each section's pitch or run follows from it.
section rafter length = section run x sqrt(1 + (pitch/12)^2)
edge hip plan angle = corner angle / 2
edge hip plan run = section run / cos(corner angle / 2)
shared rise constraint: rise = run_1 x pitch_1 / 12 = run_2 x pitch_2 / 12
Frequently Asked Questions
When do I need the section builder instead of the regular bay roof builder?
Reach for sections when the bay breaks symmetry: side walls at different angles, a bow window with four or five short panels, a bay meeting a wall return on one side only, or a projection that varies per face. The standard builder assumes a symmetric bay and solves it in one shot; the section builder lets you dial in each face independently and still keep the planes meeting cleanly at shared hips.
How do I keep separate sections planing in together?
Lock the rise. Every section must reach the same elevation at the tie-in line against the house, so set that height first and let each section's pitch follow from its own run: pitch = 12 x rise / run. Unequal runs at a common rise mean unequal pitches, and the hip between two such sections becomes an irregular hip that does not bisect the corner in plan. Enter both sections and the tool solves that shared edge for you.
What is different about hips between unequal-pitch sections?
On an irregular hip, the plan angle shifts toward the shallower roof plane instead of splitting the corner evenly, and the two cheek cuts on the hip are no longer mirror images - each side gets its own side angle, and the jacks on each plane have different common differences. This is the hardest layout to do by hand with a framing square, and it is exactly the case the section builder computes and prints per edge.
Can I use this for a bow window roof?
Yes - a bow is the natural section case. Treat each of the four or five panels as one section with its actual corner angles (often 157.5 degrees between panels on a five-panel bow), solve them individually, and the hips at each panel joint come out with their shallow plan angles and short jacks. Print each section drawing and cut list, assemble around the curve, and the segments meet at the shared rise line against the house.
Do these little roof sections need engineering?
Rarely for structure - spans of a foot or two put trivial bending loads on even 2x4 rafters, and IRC prescriptive framing comfortably covers them. What deserves attention is the attachment and waterproofing: the ledger connection to the house, uplift fastening on any cantilevered framing, and the step and head flashing where each section meets the wall. If the bay supports a heavy roof covering or a person could walk on it, ask your building official. Verify locally.