Roof Measurements and Rafter Geometry

Start with the endpoints

A length is only meaningful when its endpoints are clear. RoofFolio uses horizontal run and vertical rise to describe a right triangle. Its sloping edge is a geometric reference line. A physical rafter has depth, bearings and cuts that this line does not describe.

Fig. R1.1The reference triangle. Run and rise are measured between the same two endpoints; the sloping edge is a geometric line, not a member.

The measurement vocabulary

Term	Meaning in these tools
Span	Outside-wall width across a symmetric gable.
Run	Horizontal distance between the chosen endpoints of one roof slope.
Rise	Vertical change over that same run.
Pitch, x:12	Rise for every 12 horizontal units. A 6:12 slope rises 6 in over 12 in of run.
Overhang	Here, the horizontal projection beyond the wall. It is not the sloping eave length.
Ridge thickness	Actual horizontal thickness used to locate the upper reference endpoint at the ridge face.
Common rafter	A rafter following a regular roof slope toward the ridge, rather than a hip or valley line.

Centreline and ridge face are different

For a symmetric gable, half the outside-wall span reaches the ridge centreline. To end at the ridge face, subtract half the actual ridge thickness. RoofFolio combines those steps as R = (S − t) / 2.

A 20 ft span with a 1.5 in ridge gives an effective run of (20 − 0.125) / 2 = 9.9375 ft. At 6:12, the rise above the wall reference is 4.96875 ft. A 1 ft horizontal overhang extends the same slope by approximately 1.1180 ft.

Fig. R1.2Half section of a symmetric gable. Run R stops at the ridge face, so R = ½(S − t). The eave extension E continues the same slope.

This arithmetic assumes a centred ridge and symmetric geometry. It does not decide which ridge construction a building needs.

Choose the right worksheet

Use the Rafter Length Calculator when you already have a run and rise or pitch for one triangle. Use the Rafter Calculator when working from a building span with a separate ridge thickness and overhang. Use the Roof Pitch Calculator to translate between ratios, degrees and percent slope.

Keep all measurements in consistent units. RoofFolio provides separate feet and inches fields; 6.8 entered in Feet means 6.8 decimal feet, not 6 ft 8 in. In an Inches field, 1 1/2 means one and a half inches.

A reference line is not a cut schedule

Birdsmouth, member depth, bearing, fascia, ridge connections and fabrication allowances are outside these tools. No result specifies member size, permissible span, loads or a connection detail.

Geometric planning estimate. This does not size structural members or verify loads, connections, or code compliance. Verify structural and code requirements against local code or with a qualified professional.