Roof Framing Fundamentals: A Complete Guide

Understanding Roof Geometry

Roof framing is one of the more challenging aspects of carpentry, but the underlying geometry is straightforward once you understand the basics. Every roof is essentially a series of right triangles. The rise is the vertical distance from the top of the wall to the ridge. The run is the horizontal distance from the wall to the ridge. The rafter is the hypotenuse. Use our Roof Pitch Calculator to work with these dimensions quickly.

The pitch is expressed as the rise per 12 inches of run. A 6/12 pitch rises 6 inches for every 12 inches of horizontal travel. Steeper roofs shed water and snow better but require more material and labor. Shallower roofs are easier to build but more prone to leakage. Most residential roofs fall between 4/12 and 12/12.

Common Rafter Layout

The common rafter runs from the ridge board down to the wall plate. To lay out a common rafter, you need to know the total run (half the building width minus half the ridge board thickness), the pitch, and the rafter length. The rafter length is the total run multiplied by the rafter multiplier, which you can find on a framing square or calculate from the pitch angle.

Start by marking the plumb cut at the ridge end of the rafter. This is the angle cut that sits against the ridge board. Then measure down the rafter the calculated length and mark the birdsmouth cut, which is the notch that sits on the wall plate. The birdsmouth has two cuts: a plumb cut (vertical) and a level cut (horizontal, also called the seat cut). The level cut bears fully on the wall plate.

Ridge Board and Rafter Spacing

The ridge board runs along the peak of the roof and ties the rafters together. It is typically one size larger than the rafters. For 2x8 rafters, use a 2x10 ridge. The ridge does not carry structural load in a standard gable roof because the opposing rafters push against each other. Its main purpose is to provide alignment and nailing surface for the rafters.

Rafter spacing is typically 16 or 24 inches on center, matching the wall framing below. Mark the rafter positions on the ridge board and the wall plates before installation. Use our Stair Calculator for similar right-triangle calculations used in stair framing.

Hip and Valley Rafters

Hip roofs have sloped ends instead of gable ends. The hip rafter runs diagonally from the ridge corner down to the wall corner. It is longer than a common rafter at the same pitch and sits at a shallower angle relative to the horizontal. Calculating hip rafter length requires finding the diagonal of the common rafter run.

Valley rafters are essentially the same geometry in reverse, running from the ridge down to the intersection of two roof planes. Both hip and valley rafters are typically one size larger than common rafters because they carry more load. Jack rafters are shortened rafters that run from the hip or valley to the wall plate, decreasing in length as they approach the corner.

Practical Tips

Always cut your first rafter as a pattern, test-fit it, and make any adjustments before cutting the rest. Small errors multiply across dozens of rafters. Use a framing square with stair gauges for consistent layout. Pre-assemble pairs of rafters on the ground and erect them as trusses for faster installation on smaller buildings. For complex roofs, consider hiring a structural engineer or using pre-fabricated trusses, which eliminate most of the field calculation and cutting.