What is a Fink (W-truss)?

A Fink truss is the most common residential roof truss. It has two top chords (rafters) meeting at a peak, a bottom chord (tie beam), and a W-shaped web of three interior members — two diagonals and one vertical king post. This calculator models a symmetric Fink truss.

What is building span?

Span is the total outside-to-outside width of the building measured at the wall plates. For a symmetric Fink truss the run (half-span) is span ÷ 2, which is the horizontal distance from wall to ridge centerline.

How is peak height calculated?

Peak height = Run × (Pitch ÷ 12). For example, a 24 ft span (12 ft run) with a 6/12 pitch gives a peak height of 12 × (6 ÷ 12) = 6 ft above the bottom chord.

What lumber size should I use?

Residential Fink trusses commonly use 2×4 or 2×6 lumber for chords and webs. Enter nominal dimensions; the board-foot formula uses nominal thickness and width with actual measured length.

Does this include the ridge board?

No. A truss roof typically has no ridge board — the two top chords meet at the peak and are held by the truss geometry. This calculator covers only the truss members themselves.

What is the web member layout in a Fink truss?

A Fink truss has five main members: two top chords (rafters), one bottom chord (tie), two diagonal webs running from the bottom chord to points on the top chord (roughly the third points), and one vertical king post at the center. This gives the characteristic W shape.

What is the formula for calculating roof trusses?

The core formulas are: Run = Span ÷ 2; Peak Height = Run × (Pitch ÷ 12); Top Chord (rafter) = √(Run² + Height²) using the Pythagorean theorem. Board feet per member = (Thickness × Width × Length_ft) ÷ 12. Total lumber = BF per truss × truss count × (1 + waste%).

Roof Truss Calculator

Name: Roof Truss Calculator
Availability: InStock
Author: Nham Vu

Calculate Fink (W-truss) rafter lengths, peak height, and lumber quantities from span and roof pitch.

Truss Dimensions

Building Span (ft) — outside-to-outside width

Roof Pitch (in/12)

Number of Trusses

Overhang per Side (ft)

Lumber

Thickness (in)

Width (in)

Price / BF (optional, $)

Waste Factor (%)

Enter your span, pitch, and lumber size on the left to see the truss breakdown.

Truss Results

Peak Height

—

feet above bottom chord

Top Chord Length

—

ft (each side, incl. overhang)

BF per Truss

—

board feet

Total Board Feet

—

all trusses + waste

Member Breakdown (per truss)

Member	Qty	Length (ft)	BF each	BF subtotal
Total BF per truss (before waste)				—

Number of trusses	—
Waste factor	—
Total board feet (with waste)	—
Estimated lumber cost	—

Fink Truss Layout

How the Calculation Works

1
Run (half-span): Run = Span ÷ 2 — the horizontal distance from wall plate to ridge centerline.
2
Peak height: Height = Run × (Pitch ÷ 12) — e.g. 12 ft run × 6/12 pitch = 6 ft peak.
3
Top chord (rafter): TC = √(Run² + Height²) using the Pythagorean theorem, then add overhang slope length.
4
Bottom chord: Equals the full span plus twice the overhang horizontal distance.
5
Fink web members: King post = peak height. Each diagonal web connects a bottom-chord third-point to the top-chord midpoint: computed via Pythagorean theorem from those node coordinates.
6
Board feet: BF = (Thickness × Width × Length_ft) ÷ 12 per member, summed across all members × quantity × truss count × (1 + waste%).

Summary

Calculate Fink (W-truss) rafter lengths, peak height, and lumber quantities from span and roof pitch.

How it works

Enter the total building span — the outside-to-outside width of the structure.
Select or type the roof pitch in X/12 format (rise per 12 inches of run).
Enter the lumber size (nominal dimensions) and an optional waste factor.
The calculator derives half-span (run), ridge height, top chord (rafter) length, and all Fink web members.
Board feet are computed per member type and summed for the full truss, then multiplied by truss count.
Review the per-member table and total material estimate before ordering lumber.

Use cases

Estimating lumber for a residential roof truss package.
Verifying truss geometry before placing a prefab order.
Teaching framing geometry and the Pythagorean theorem.
Preparing a material takeoff for a building permit.
Comparing lumber cost across different pitch options.
DIY shed or garage roof framing planning.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu