Framing and Rough Carpentry (C-5) · Lesson 2 of 5

Floor Joist Deflection: The L/360 Limit and the Span Table

"Floor Joist Deflection: The L/360 Limit and the Span Table" is lesson 2 of 5 in the Framing and Rough Carpentry (C-5) Learning Hub. It opens with "Why Deflection Is a Framing Problem" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Why Deflection Is a Framing Problem
  2. Which Limit Applies (2025 CRC Table R301.7)
  3. The Span Tables Already Build In L/360 (R502.3)
  4. Reading Table R502.3.1(2): Depth, Spacing, Grade, and Dead Load
  5. How to Check a Joist Against the Table

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Find the allowable deflection" through "Add a heavier floor".

What you'll be able to do

  • Explain the main idea of "Floor Joist Deflection: The L/360 Limit and the Span Table" in plain language you can use on the exam.
  • Apply this step with confidence: "The Span Tables Already Build In L/360 (R502.3)".
  • Apply this step with confidence: "Reading Table R502.3.1(2): Depth, Spacing, Grade, and Dead Load".
  • Apply this step with confidence: "How to Check a Joist Against the Table".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Floor Joist Deflection Check: L/360Living area: 40 psf live, 10 psf dead Δ L = 14 ft = 168 in. clear span Sag shown exaggerated TABLE R502.3.1(2): DF-L NO. 2, DL 10 PSF Limit, L/360 = 168 / 360 = 0.47 in. 2x8 at 16 in. o.c.: max span 12-9 2x10 at 16 in. o.c.: max span 15-7 2x10 at 19.2 in. o.c.: max span 14-3 2x10 at 24 in. o.c.: max span 12-9 Room span to carry: 14-0 Heavier floor, DL 20 psf: 2x10 at 19.2 in. = 13-0 2x10 at 16 in. = 14-3 2x8 at 16 in.: 12-9 < 14-0 span = FAILS 2x10 at 16 in.: 15-7 ≥ 14-0 span = PASSES 2x10 at 19.2 in.: 14-3 passes at 10 psf DL, but 13-0 fails at 20 psf DL 2x10 at 24 in.: 12-9 < 14-0 span = FAILS Source: 2025 CRC Table R301.7 (floors L/360), R502.3.2 and Table R502.3.1(2)CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Why Deflection Is a Framing Problem

A joist can be strong enough not to break and still be too flexible. A floor that sags or bounces by a visible amount cracks tile and grout, shakes dishes, and cracks the ceiling finish hung below it. The 2025 California Residential Code (CRC) therefore limits how far a member may bend under live load, as a fraction of its span L (Section R301.7). This is a different check from bending strength or shear, and for longer spans it is usually the check that decides the joist size.

Step 2: Which Limit Applies (2025 CRC Table R301.7)

Section R301.7 says the allowable deflection of any structural member under the live loads of R301.5 and R301.6 (or wind) may not exceed Table R301.7. The table prints: floors L/360; ceilings with brittle finishes (plaster and stucco) L/360; ceilings with flexible finishes (gypsum board) L/240; rafters with slopes greater than 3:12 and a finished ceiling not attached to the rafters L/180; all other structural members, excluding guards and handrails, L/240. Note b: for a cantilever, L is taken as twice the length of the cantilever. For a floor joist the allowable sag is the span in inches divided by 360, so a 14 ft (168 in.) joist may deflect no more than 168 / 360 = 0.47 in.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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