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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.

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The wood's tabulated (reference) allowable design value, in whatever unit your design tables use (e.g. psi or MPa).

This is the base design value from the NDS Supplement design value tables, before any adjustment factors are applied.

The shortest-duration load governing this design check, which sets the applicable CD factor.

Wood carries a brief load better than a sustained one, so a short duration raises the allowable stress — but the governing case is whichever combination gives the worst ratio of demand to ADJUSTED capacity, and that is not always the largest load. A permanent dead load with a modest live load frequently governs over a much larger wind case, because the wind case gets the generous factor and the permanent one does not.

Reference value with CD applied

1,150 (CD factor)

Medium confidence

This applies the load-duration factor CD alone. NDS requires the full adjustment chain — wet service CM, temperature Ct, size CF/CV, beam stability CL, flat use, incising and repetitive member as applicable — before a number is a design value. Do not use this figure as a final allowable stress.

CD factor applied
1.15
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • 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)

Inputs used

Reference (Tabulated) Design Value
1000
Governing Load Type
Snow (CD=1.15)

Intermediate steps

CD factor applied
1.15
Final result1,150 (CD factor)

Confidence note: This applies the load-duration factor CD alone. NDS requires the full adjustment chain — wet service CM, temperature Ct, size CF/CV, beam stability CL, flat use, incising and repetitive member as applicable — before a number is a design value. Do not use this figure as a final allowable stress.

What this calculation does not cover

  • CD belongs to allowable stress design and to nothing else. The same member checked by LRFD uses the time effect factor lambda in its place, with different values applied against factored rather than service loads. Carrying a CD into an LRFD check, or lambda into an ASD one, mis-scales the capacity by a factor no later step in the design will flag.
  • It multiplies whatever is typed in, including values CD is not permitted to touch. Alongside modulus of elasticity, NDS also excludes compression perpendicular to grain from load duration adjustment — so a bearing check on a sill, a plate or a beam seat run through this page comes back with a capacity the code does not actually grant, and bearing is where a wood frame most often runs short.
  • The largest CD is not the governing case, and checking one combination hides that. Each combination is checked with the CD of its own shortest-duration load, so a smaller load with a lower factor can control: dead alone at 0.9 can govern a member that looks comfortable under dead plus snow at 1.15. The design check is the worst result across every combination, not the one from the biggest load.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1
  1. 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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Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

  • Checking an Exposed Timber Beamuses this calculator

    One glulam justified twice: full section with snow's duration factor, then the residual section an hour of fire leaves behind.

  • Checking Truss Shop Drawingsuses this calculator

    Forty-one sheets and one seal: what to read on a truss design drawing, from the load block to the plate callout, before the pack reaches the saw.

  • Checking Whether a Floor Will Take a Heavy Loaduses this calculator

    The load side is arithmetic anybody can do. The capacity side is a judgement about a floor nobody drew, and the two halves live in different documents.

Called something else where you work? Timber / lumber — the term in each market, how close the equivalence really is, and the standard that governs it.

How to calculate wood load duration factor (CD) adjustment in 3 steps

  1. Reference (Tabulated) Design ValueThe wood's tabulated (reference) allowable design value, in whatever unit your design tables use (e.g. psi or MPa).
  2. Governing Load TypeThe shortest-duration load governing this design check, which sets the applicable CD factor.
  3. Reference value with CD appliedThe tool computes the reference value with CD applied from those figures and shows the formula, its sources, and a confidence rating alongside it.

Reference value with CD applied by reference (Tabulated) Design Value

Page defaults, not your figures above.

Reference (Tabulated) Design ValueReference value with CD applied ((CD factor))
100115
200230
500575
1,0001,150
2,0002,300
5,0005,750
10,00011,500

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.
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 — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.