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Truss Bottom Chord Deflection Calculator

Check a truss bottom chord's calculated deflection (acting as a ceiling joist) against the standard L/240 ceiling limit.

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
19.69 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Bottom chord deflection

0.38 in

High confidence

PASSES the standard L/240 ceiling deflection limit.

L/240 ceiling limit
0.98 in

For the dimensions entered, expect a bottom chord deflection of 9.59 mm. Of the working steps, l/240 ceiling limit dominates at 25 mm. Note that moment of inertia i (×10⁶ mm⁴) sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Add the equipment this sizes

This result is a specification — 0.377 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

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

  • Uniformly-loaded simple beam deflection D = 5wL^4/(384EI), applied to the truss bottom chord acting as a ceiling joist, checked against the standard L/240 ceiling deflection limit

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Frequently asked questions

Why check the bottom chord against L/240 rather than L/360?
L/240 is the standard code deflection limit for ceilings (non-plaster), which is what a truss bottom chord typically supports — L/360 is the tighter limit generally reserved for floors or plaster ceilings.
What loads should be included in the uniform load input?
Include the ceiling dead load (drywall, insulation, and the chord's own weight) plus any applicable attic storage live load, expressed as a load per linear meter of chord.
Does passing this check mean the truss is fully engineered?
No — this checks bottom chord deflection alone. The overall truss (top chord, webs, plates, and bearing) must be designed and verified by the truss manufacturer's engineering per ANSI/TPI 1.