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Roof Truss Calculator

Compute rafter length, peak rise, and chord length from span and pitch.

What is roof truss calculator?

A roof truss is a prefabricated triangular frame that spans between exterior walls and supports the roof. Its geometry is set by three numbers: the span (wall to wall), the pitch (rise-in-12), and the overhang (how far the rafter extends past the wall).

This calculator returns the three lengths you need to either order a pre-made truss or stick-frame the same geometry by hand: total rise at the peak, rafter / top chord length (the slanted member), and bottom chord length (the horizontal tie between walls).

The math is geometry only — no engineering load tables or web-member layouts. For real structural design you’ll still need an engineered truss spec, but this tool answers the most common question on the framer’s clipboard: how long does the rafter need to be?

When to use a roof truss calculator

  • Ordering pre-fab trusses — Confirm the truss vendor's quoted lengths against your own span + pitch math before committing to an order.
  • Estimating rafter cuts — Get top chord length to sketch a cut list for stick-framing rafters with the right overhang.
  • Lumber takeoff for rough framing — Multiply rafter length by truss count, then look up board feet to price the rough lumber order.

How to use the Roof Truss Calculator

  1. Measure the building span — Span is the outside-of-wall to outside-of-wall distance — the full width the truss must clear.
  2. Pick the pitch — Pitch is entered as 'rise in 12' (e.g., 6 for a 6:12 pitch). Steeper pitches use more lumber but add attic space.
  3. Add the overhang — Overhang is the horizontal distance the rafter extends past the wall. 1–2 ft is typical.

Worked examples

Standard 24-ft span, 6:12 pitch, 1-ft overhang

Input:  Span=24 ft, Pitch=6, Overhang=1 ft
Output: Rise 6 ft — Top chord ≈ 14.53 ft — Bottom chord 24 ft

Garage 32-ft span, 8:12 pitch, 2-ft overhang

Input:  Span=32, Pitch=8, Overhang=2
Output: Rise ≈ 10.67 ft — Top chord ≈ 21.63 ft — Bottom chord 32 ft

Long agricultural building 40 ft, 4:12, 1.5 ft

Input:  Span=40, Pitch=4, Overhang=1.5
Output: Rise ≈ 6.67 ft — Top chord ≈ 22.66 ft — Bottom chord 40 ft

Frequently asked questions

Are these numbers structural?
No — this is geometry only. A real truss design also accounts for snow load, wind load, web member layout (king / queen / fink), and connector plates. Always have an engineered truss spec'd for your structural use.
Why doesn't the calculator show web members?
Web members (king post, queen post, fink, howe, scissor) depend on truss type and span. Adding those is a future enhancement; for now, the calculator covers the geometry that's identical across all truss types.
Does the bottom chord include the overhang?
No — the bottom chord typically runs only between the wall plates. Overhangs are formed by extending the top chord past the wall, with a fascia / soffit framed below.
What's a typical residential span?
Most stick-built homes use 24-ft or 30-ft trusses. Larger spans (above 40 ft) often need engineered alternatives or interior bearing walls.
How do I figure rafter length without a truss?
It's the same math — rafter length = (half span + overhang) × slope factor. The slope factor comes from the pitch (√(rise² + 144) / 12 for rise-in-12 pitches).
Why is the rafter length larger than half the span?
The rafter follows the slope, not the horizontal. A 12-ft horizontal run at 6:12 pitch becomes √(144 + 36) ≈ 13.4 ft of actual rafter — that's the slope factor at work.
Can I use this for a hip roof?
Hip roofs need a different calculation for the hip rafters (they run diagonally). This calculator covers gable-style truss geometry; hip rafters are a future enhancement.

Roof Pitch Calculator

Convert rise and run into the four standard pitch formats.

Roof Slope Calculator

Convert roof slope between degrees, percent, and rise-in-12.

Roof Shingle Calculator

Bundles and squares of asphalt shingles for any roof.