Skip to main content
Calcimator

Timber Truss Design Calculator

King-post truss member forces, rafter sizing, and bottom chord tension from span, rise, and loading.

Inputs

ft
ft
psf
ft
in
in

Results

Rafter axial force

6,923 lbs

≈ 7 grand pianos

Bottom chord tension

2,880 lbs

≈ 16 adults

King post tension3,840 lbs
Horizontal thrust2,880 lbs
Rafter compression capacity48,000 lbs
Rafter utilization ratio0.14
Min bottom chord size2.1 in
Min king post size2.4 in
Pitch angle33.7°
Pitch (x:12)8:12
Rafter length14.42 ft
Total load on truss7,680 lbs
How to Use This Calculator
  1. Enter Truss span, Truss rise, and Total roof load.
  2. Set Truss spacing, Rafter width, and Rafter depth.
  3. Adjust Wood species as needed.
  4. Review Rafter axial force (lbs) and Bottom chord tension (lbs).
  5. Use King post tension (lbs) and Horizontal thrust (lbs) to inform your decision.

How the result changes with Truss span

Truss spanRafter axial forceBottom chord tension
132,680 lbs845 lbs
267,937 lbs3,380 lbs
4218,877 lbs8,820 lbs
5531,504 lbs15,125 lbs

What each input means

Truss span
Clear span of the truss from support to support.
Truss rise
Vertical height from the bottom chord to the peak.
Total roof load
Combined dead + live + snow load on the roof (psf of horizontal projection).
Truss spacing
On-center spacing between trusses.
Rafter width
Width of the rafter timber cross-section.
Rafter depth
Depth of the rafter timber cross-section.
Wood species
0 = Douglas Fir-Larch, 1 = Southern Pine.

What each result means

Rafter axial force
Compression force in each rafter.
Bottom chord tension
Tension force in the bottom chord (tie beam).
King post tension
Tension force in the vertical king post.
Horizontal thrust
Outward thrust at each support.
Rafter compression capacity
Axial compression capacity of the chosen rafter section.
Rafter utilization ratio
Rafter force / capacity. Must be ≤ 1.0.
Min bottom chord size
Minimum square member size for the bottom chord (tension).
Min king post size
Minimum square member size for the king post (tension).
Pitch angle
Roof pitch angle from horizontal.
Pitch (x:12)
Roof pitch expressed as rise per 12 inches of run.
Rafter length
Length of each rafter from plate to ridge.
Total load on truss
Total gravity load carried by this truss.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Truss span = 24, Truss rise = 8, Total roof load = 40, Truss spacing = 8 = 7 input(s) provided
  2. Calculate Rafter axial force
    6923 = 6923
  3. Calculate Bottom chord tension
    2880 = 2880
  4. Calculate King post tension
    3840 = 3840
  5. Calculate Horizontal thrust
    Horizontal thrust = (wLbPerFt * spanFt * spanFt) / (8 * riseFt)
    2880 = 2880

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

Related Calculators

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Timber Framing

Timber Frame Design Calculator

Beam sizing from span, load, and wood species using NDS bending and deflection checks.

Timber Framing

Timber Volume Calculator

Board feet, cubic volume, and weight from timber dimensions and species.

Timber Framing

Post and Beam Layout Calculator

Post spacing, post count, beam quantities, and post load capacity for rectangular post-and-beam structures.

More in Construction & Building Trades.