Roof Truss Calculator

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Walk-through

How to Use This Calculator

4 steps

1

Enter span and pitch

Type the full building span — the wall-to-wall width the truss must clear — in feet, and choose the roof pitch (rise per 12 inches of run) from the dropdown. The calculator updates instantly.

2

Choose a truss type

Pick King Post for short spans (up to about 16 ft), Queen Post for medium spans (up to about 24 ft) where you want open space between the two vertical webs, or Fink (W) for longer spans — the most common shape in residential framing today.

3

Add overhang if needed

If the rafter tails extend past the wall plate, enter that overhang distance. It lengthens the top chord only — it doesn't change the rise, bottom chord, or web member lengths.

4

Read the member lengths

The Member Lengths tab lists every chord and web with its exact length. Check the Rise & Pitch tab for the angle and slope, and the Truss Diagram tab for a schematic view of the truss shape.

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Reference

Formula & Methodology

6 formulas

Rise

rise = (span / 2) × (pitch / 12)

The ridge climbs one unit of rise for every 12 units of horizontal run at the given pitch. Half the span is the horizontal run from wall to ridge.

Top chord (rafter length)

top chord = √((span / 2)² + rise²) [+ overhang / cos(pitch angle)]

The rafter is the hypotenuse of a right triangle formed by the half-span run and the rise (Pythagorean theorem). If there's a rafter-tail overhang past the wall, add its length along the same slope: overhang ÷ cos(pitch angle).

Bottom chord

bottom chord = span

The bottom chord runs the full width of the truss and equals the building span exactly.

King Post web

web = rise

A single vertical member from the center of the bottom chord straight up to the apex. Its length equals the rise.

Queen Post webs

web = rise × 2/3 (each of 2 webs)

Two vertical webs sit at the 1/3 and 2/3 span points. By similar triangles, the rafter line's height at a point span/6 from center is rise × (1 − (span/6)/(span/2)) = rise × 2/3.

Fink (W) struts and diagonals

strut = rise / 2, diagonal = √((span/4)² + strut²)

Each half of the truss has one vertical strut from the bottom-chord quarter point up to the midpoint of that side's rafter (height rise/2, since it's the midpoint of the rise), plus one diagonal from that rafter midpoint down to the truss center — forming the classic W zigzag.

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YMYL

Trust, Methodology & Sources

Written by Quinn Sumner · Updated 2026-07-23 · 3 sources

Editorial accountability

Author: Quinn Sumner - Editor

Owner: Quinn Sumner - Editorial owner

Last reviewed: 2026-07-23

Last verified: 2026-07-23

Data effective: 2026-07-23

Methodology

Applies right-triangle (Pythagorean) rafter geometry and similar-triangle web-member positioning to user-entered span, pitch, and truss-type inputs, consistent with IRC Chapter 8 roof-ceiling framing conventions. Produces planning-level member lengths to support material takeoffs and truss-design research, not to replace engineered truss shop drawings.

Assumption: The truss is a symmetrical gable shape with the ridge centered over the span.

Assumption: King Post, Queen Post, and Fink (W) web layouts follow their standard textbook geometry; real fabricated trusses may vary by manufacturer.

Assumption: Overhang extends only the top chord (rafter tail) along the existing roof slope and does not change the load-bearing span.

Limitations & guidance

Does not size lumber grade, dimension, or connector-plate capacity, and does not check span tables against snow, wind, or seismic load.

Not a substitute for an ANSI/TPI 1 engineered truss design or stamped shop drawings from a licensed truss manufacturer.

Professional guidance: This calculator is for planning and estimating purposes only and is not a substitute for design by a licensed engineer or truss manufacturer. Verify all values against local code requirements and have any load-bearing truss design reviewed by a qualified professional before construction.

Primary sources

International Residential Code (IRC) — Chapter 8, Roof-Ceiling Construction - International Code Council

ANSI/TPI 1: National Design Standard for Metal Plate Connected Wood Truss Construction - Truss Plate Institute

NRCA Roofing Manual - National Roofing Contractors Association

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Glossary

Key Terms Explained

8 terms

Span The horizontal distance between the outside faces of the two supporting walls that the truss must clear, measured in feet.

Rise The vertical distance the roof climbs from the wall plate to the ridge (apex), measured over half the span.

Run The horizontal distance the roof travels for a given rise — for a symmetrical gable truss, the run equals half the span.

Top chord The sloped rafter member running from the wall plate up to the ridge. A gable truss has two top chords, one per side, of equal length.

Bottom chord The horizontal member spanning the full width of the truss at the wall-plate level, tying the two top chords together and resisting outward thrust.

Web member An internal diagonal or vertical member connecting the top and bottom chords, transferring load between them and stiffening the truss.

King post The single vertical web member in the simplest truss style, running from the bottom-chord center to the ridge apex.

Fink truss A truss style using two vertical struts and two diagonal webs arranged in a W pattern — the most common configuration in residential roof framing for spans of roughly 24 ft and up.

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Scenarios

Real-World Examples

3 worked examples

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DIY garage builder

24 ft span, 6/12 pitch, King Post

Span 24 ft Pitch 6/12 Truss type King Post

Half-span run is 12 ft, so rise = 12 × (6/12) = 6 ft. Top chord = √(12² + 6²) = √180 ≈ 13.42 ft per rafter. Bottom chord = 24 ft exactly. The single king-post web = 6 ft (equal to the rise). A 24 ft span is on the long side for a king-post design in practice — a Queen Post or Fink layout distributes the load better at this span, but the geometry math is the same.

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Shed builder with overhang

16 ft span, 4/12 pitch, King Post, 1 ft overhang

Span 16 ft Pitch 4/12 Overhang 1 ft

Half-span run is 8 ft, rise = 8 × (4/12) ≈ 2.67 ft. Top chord without overhang = √(8² + 2.67²) ≈ 8.43 ft. Pitch angle ≈ 18.4°, so each foot of overhang adds 1/cos(18.4°) ≈ 1.05 ft along the slope — total top chord ≈ 9.48 ft. Overhang never changes the 16 ft bottom chord or the 2.67 ft king-post web, since both are measured to the load-bearing span, not the rafter tail.

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Framer estimating a Fink truss

28 ft span, 5/12 pitch, Fink (W)

Span 28 ft Pitch 5/12 Truss type Fink (W)

Half-span run is 14 ft, rise = 14 × (5/12) ≈ 5.83 ft. Top chord = √(14² + 5.83²) ≈ 15.17 ft. Bottom chord = 28 ft. Each vertical strut = rise/2 ≈ 2.92 ft. Each diagonal = √((28/4)² + 2.92²) = √(7² + 2.92²) ≈ 7.59 ft. That's 4 web members total (2 struts + 2 diagonals) plus the 2 top chords and 1 bottom chord — 7 cut pieces per truss.

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Deep Dive

Understanding Roof Truss Member Lengths

3 chapters · 4 min read

A roof truss is a triangulated framework of chords and webs designed to span the full width of a building without interior support. This calculator computes the exact cut length of every member — top chord, bottom chord, and webs — for three common truss styles from just the span and pitch.

How the geometry works

Every gable truss is built from two right triangles mirrored at the ridge. Half the span is the horizontal run; the rise is the vertical climb over that run at the chosen pitch. The top chord (rafter) is the hypotenuse of that triangle — found with the Pythagorean theorem. The bottom chord ties the two wall plates together and always equals the full span.

Web members fill the triangle's interior. Where they land depends on the truss style: a king post drops straight down from the apex to the bottom-chord center; a queen post uses two verticals at the 1/3 and 2/3 span points; a Fink truss zigzags a vertical strut and a diagonal on each side, which is why it's favored for longer spans — it breaks the rafter into shorter unsupported segments and shares load across more members.

Choosing a truss type for your span

King post trusses are simplest and cheapest to build but are only efficient up to roughly a 16 ft span before the unsupported rafter length and web forces get impractical for typical lumber sizes. Queen post trusses extend that range to about 24 ft and have the advantage of leaving usable open space between the two verticals — useful for a small storage loft. Beyond about 24 ft, Fink (W) trusses are the standard choice in residential construction because the extra web members shorten each unsupported segment and spread the load more evenly, which is why they dominate pre-fabricated roof trusses today.

What this calculator doesn't do

This tool computes planning-level geometry only — it does not size lumber grade or dimension, check span tables against snow or wind load, or verify connector-plate capacity. Real trusses that carry structural load must be engineered to ANSI/TPI 1 (the National Design Standard for Metal Plate Connected Wood Truss Construction) and fabricated from stamped shop drawings by a licensed truss manufacturer. Use these numbers for early planning, material takeoffs, and understanding truss geometry — not as a substitute for engineered truss drawings.

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Questions

Frequently Asked Questions

6 questions

What's the difference between king post, queen post, and Fink trusses?+

A king post truss uses a single vertical web from the bottom-chord center to the apex — simplest, and best for spans up to about 16 ft. A queen post truss uses two vertical webs at the 1/3 and 2/3 span points, leaving open space in the middle — suited to spans up to about 24 ft. A Fink (W) truss uses a zigzag of two vertical struts and two diagonal webs, distributing load more efficiently over longer spans (24 ft and up), which is why it's the most common truss shape in residential framing today.

How do I find the top chord (rafter) length?+

The top chord is the hypotenuse of a right triangle formed by half the span (the run) and the rise: top chord = √(half-span² + rise²). If the rafter tails overhang the wall, add overhang ÷ cos(pitch angle) to account for the extension along the same slope.

What is the bottom chord and how long is it?+

The bottom chord is the horizontal member spanning the full width of the truss at the wall-plate level. Its length always equals the building span — it doesn't change with pitch or truss type.

How is rise calculated from pitch?+

Rise equals half the span multiplied by the pitch (expressed as rise-per-12-inches-of-run) divided by 12. For a 24 ft span at a 6/12 pitch: half-span = 12 ft, rise = 12 × (6/12) = 6 ft.

Do overhangs change the web member lengths?+

No. Overhang only extends the top chord (rafter tail) past the wall plate — it does not change the rise, the bottom-chord span, or any web member length, since all of those are measured relative to the load-bearing span between the walls, not the rafter tail.

Can I use this calculator to build a real load-bearing truss?+

This calculator gives planning-level member lengths for estimating lumber and understanding truss geometry — it does not size lumber grade, check span tables, or verify connector capacity. Real trusses that carry roof and snow loads must be engineered per ANSI/TPI 1 and typically fabricated by a licensed truss manufacturer with stamped shop drawings. Always have a truss design reviewed by a qualified engineer before construction.

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Truss Inputs

Rise, Run & Pitch Angle

Truss Elevation

How to Use This Calculator

Enter span and pitch

Choose a truss type

Add overhang if needed

Read the member lengths

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

DIY garage builder

Shed builder with overhang

Framer estimating a Fink truss

Understanding Roof Truss Member Lengths

How the geometry works

Choosing a truss type for your span

What this calculator doesn't do

Frequently Asked Questions

Get a branded PDF of your results

Roof Truss Calculator

Truss Inputs

Rise, Run & Pitch Angle

Truss Elevation

How to Use This Calculator

Enter span and pitch

Choose a truss type

Add overhang if needed

Read the member lengths

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Understanding Roof Truss Member Lengths

How the geometry works

Choosing a truss type for your span

What this calculator doesn't do

Frequently Asked Questions

Get a branded PDF of your results

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