RAFTER AND ROOF PITCH

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This calculator is built to make roof layout faster and more precise for framers, designers, and DIY builders. You enter the roof pitch and either the total building span with ridge thickness or a direct half-run. You can also add an optional overhang. From that, the tool calculates the common rafter run, rise, rafter length to the ridge, tail length, and total rafter length, plus useful slope factors for hips, valleys, and jack rafters. Units can be switched between inches and millimetres, and you can set how aggressively you want the output rounded so that measurements are easy to mark and cut on site.

Beyond simple lengths, the calculator also gives you key roof geometry info: the exact roof angle in degrees, the common rafter factor (length per unit of horizontal run), the hip/valley factor, and the jack shortening per spacing along a hip. Those values are the same ones you’d normally pull from framing tables or memorize as “per-12” factors, but here they’re generated automatically based on the actual pitch you’re using. That makes it easier to check your layout, communicate with crew members, and double-check numbers before cutting material.

Start by selecting the Units (in or mm). The pitch can be entered in one of two ways:

• Rise per unit: for example, in inches, a value of 6 means a 6-in rise per 12-in run (a 6/12 pitch).
   
• Angle (degrees): if you know the roof angle, switch to Angle mode and enter it directly; the calculator will convert that angle into an equivalent “rise per unit” behind the scenes.
   

Next, choose your Run Input method.

• In Total Span + Ridge Thickness mode, enter the span (outside to outside) and the ridge thickness. The calculator will take half the span and subtract half the ridge to get the common run for one side.
   
• In Half-Run mode, just enter the distance from the outer plate to ridge centerline if the span isn’t easily measured or is already known.
   

Optionally, enter a horizontal overhang to get the tail length and total rafter length including tail, and, if you’re working with hips and jacks, add a jack rafter spacing to see the jack shortening per step along the hip. Once your inputs are in, the Results panel will show the pitch, angle, runs, rises, rafter lengths, and slope factors, all formatted in your chosen units and rounding.

Use this tool as a layout helper and cross-check before cutting rafters or ordering stock. It doesn’t replace local building codes or engineering, but it gives you clear, practical dimensions and factors that match how carpenters actually frame roofs in the field.