Online Bay Roof Rafter Calculator - Accurate Measurements for Bay Window Roofs
Simplify your bay window roofing projects with our Online Bay Roof Rafter Calculator. This tool provides precise calculations for all rafter lengths required for constructing a bay roof. Ideal for builders and DIY enthusiasts, it ensures accuracy in measurements, aiding in the optimal fitting and aesthetic alignment of bay window roofs. Easily view and print the rafter lengths to facilitate smooth construction and planning processes.
How to Use This Tool
Enter the bay dimensions
Type the width of the bay along the house wall, the projection out from the wall, and the angle of the side walls - 45 degrees for a standard bay, 30 or 60 for shallower and deeper bays.
Set the pitch
Enter the roof pitch for the bay. Many bay roofs match the main-roof pitch; others run shallower to tuck under a window band or a second-floor overhang.
Read the rafter set
The tool returns the center commons, the two hips over the bay corners, and the jacks on the angled side planes, each with length and cut angles, and draws the framing plan for you.
Print the plan
Print or export the dimensioned rafter plan and cut list and take it to the saw - a full bay roof usually cuts from a single short list of parts.
Bay roof framing math
A standard 45-degree bay roof is a partial hip roof: a small flat or ridged center section over the front face, two hips over the bay corners, and triangular planes over the angled sides that die into the house wall. The commons over the front face use ordinary rafter math on the projection as run. The corner hips are the interesting members - because the bay corner is 135 degrees in plan, each hip bisects it at 22.5 degrees off the front plane, not 45, and its plan run is the projection divided by cos(22.5 deg).
The side planes climb from the angled walls to a level line against the house, so their jacks shorten on a fixed common difference just like any hip roof. Where the bay tucks under an upper wall, the roof also needs a ledger and flashing line at the house - the tool dimensions the plate heights so that line lands where you want it.
common rafter (front plane) = projection x sqrt(1 + (pitch/12)^2)
hip plan run (45-degree bay) = projection / cos(22.5 deg) = 1.082 x projection
hip rafter length = sqrt(hip plan run^2 + rise^2)
rise = projection x pitch / 12
hip cheek angle = 22.5 deg in plan (45-degree bay corner)
Frequently Asked Questions
How is a bay window roof framed?
As a miniature hip roof. Commons run from the bay front wall back toward the house, hips rise over the two angled corners, and jack rafters fill the triangular side planes. The whole assembly ties to a ledger on the house wall with a continuous flashing line above. Spans are tiny - usually 18 to 30 inches of projection - so 2x4 or 2x6 rafters are common, but every piece carries an angle, which is where the calculator earns its keep.
What angle are the hips on a 45-degree bay?
The bay corner is 135 degrees in plan, so the hip bisects it at 22.5 degrees to the front face - the same plan angle as an octagon corner, which makes sense because a 45-degree bay is half an octagon. The hip plan run is 1.082 times the projection, and the cheek cuts are compound: hip plumb angle combined with the 22.5-degree side angle. The tool prints both saw settings for your pitch on the plan.
Should the bay roof match the main roof pitch?
Match it when the bay roof is visually prominent and near the main roof - consistent pitch reads as intentional. Drop to a shallower pitch when the bay must tuck under a window sill, a band board, or a soffit line above; bays commonly run 3/12 to 6/12 for exactly this clearance reason. Just respect roofing minimums: asphalt shingles need 2/12 with doubled underlayment below 4/12 per IRC R905.2.2. Verify your covering requirements locally.
How does the bay roof attach to the house wall?
With a ledger fastened through the sheathing into framing, carrying the upper ends of the commons and the side-plane members, and a properly lapped flashing above - step flashing on the sloped side edges and continuous head flashing where the roof meets the wall. On brick veneer, the flashing detail changes and the ledger anchoring needs specific fasteners. Water management is the failure point on most bay roofs, so do not shortcut this line.
Can I frame a bay roof without a center ridge?
Yes - narrow bays often frame as a pure hip point where the two hips and the center common meet at a single peak, no ridge at all. Wider bays get a short level ridge across the front plane between the hip apexes, exactly like a long hip roof in miniature. The builder picks the correct configuration from your width and projection, and shows the peak or ridge geometry in the drawing so you can see which case you have.
What about a curved or segmented bow window roof?
A bow is a segmented curve - typically four or five wall panels at gentle angles instead of two 45-degree sides. Frame the roof the same way with a hip over each panel joint; each hip bisects its own corner angle, so the plan angles are shallower than a standard bay and there are more of them. Use the bay roof section builder to solve each segment individually when the panels are unequal, then assemble the sections around the curve.