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Wood Load Duration Factor (CD) Adjustment Calculator

Apply the NDS load duration factor (CD) to adjust an allowable wood design value for the governing load type.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Adjusted design value

1150 (CD factor)

High confidence
CD factor applied
1.15

Running these inputs gives 1150 (CD factor) as the adjusted design value. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • NDS (National Design Specification for Wood Construction) Table 2.3.2 load duration factors: CD = 0.9 (permanent/dead), 1.0 (ten-year/normal floor), 1.15 (two-month/snow), 1.25 (seven-day/construction & roof live), 1.6 (ten-minute/wind & seismic), 2.0 (impact)

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Frequently asked questions

What is the load duration factor CD?
CD is an NDS adjustment factor that increases or decreases a wood member's allowable design value based on how long the governing load is expected to act on it — wood can safely sustain higher stress for a shorter duration than for a sustained (permanent) one.
Which CD applies when multiple load types combine, like dead plus snow?
Per NDS, the CD used for a load combination is the value associated with the shortest-duration load in that combination — a dead+snow combination uses the snow CD (1.15), not the dead-load CD (0.9).
Does CD apply to every wood design value?
No — per NDS Table 2.3.2 and its footnotes, CD does not apply to the modulus of elasticity (E) used for deflection calculations, and connection values may be governed by their own separate adjustment factors.