Rafter Length Calculator

Understanding Rafter Layout

The rafter length calculation starts with the horizontal run — half the building width for a simple gable — plus the overhang. Multiply the run by the pitch multiplier (which comes from the Pythagorean theorem: √(1 + (rise/12)²)) to get the sloped rafter length. Before cutting, subtract half the ridge board thickness from the top end so the rafter seats correctly against the ridge.

Common Roof Pitches in Grayson County

Residential homes in Grayson County and surrounding areas typically have pitches between 4/12 and 8/12. 5/12 and 6/12 are the most common — steep enough to shed Kentucky's rain and occasional snow load, but not so steep as to make roofing difficult. Low-slope structures like pole barns and equipment sheds often use 3/12 or 4/12 for minimal cost. Metal roofing can be installed on pitches as low as 1/12 with the right panel type and sealant, while asphalt shingles require a minimum 2/12 pitch and are more commonly used at 4/12 and steeper.

From Calculation to Cut

Use this calculator to determine what length lumber to purchase — always round up to the next standard length (8, 10, 12, 14, 16, 18, 20 ft). On site, use a framing square or rafter square (Speed Square) to mark the plumb cut at the ridge and the seat cut (bird's mouth) at the wall plate. For complex roofs with hips and valleys, use the hip rafter calculator. For residential remodeling that involves roof modifications, UIR's framing crews calculate and verify all rafter dimensions before cutting. Contact UIR for framing work in Grayson County.