Floor-joist depth rule of thumb: span (ft) ÷ 2 + 2 ⇒ joist depth (inches)
HUD RSDG §5.5Description
Field-quick sizing rule for residential floor joists at 16" OC with #2 SPF / Doug-fir-larch: depth (inches) ≈ span (ft) / 2 + 2. So a 12-ft span needs ~8" joists (2x8); a 14-ft span needs ~9" (2x10 routinely used). The rule produces L/360 stiffness under typical 40 psf live + 10 psf dead. Verify against the IRC R502 span table or manufacturer software before specifying.
Why this exists
Floor stiffness is what users feel as bounciness or vibration; L/360 is the residential floor-stiffness target. The rule-of-thumb keeps preliminary architectural sketches realistic so room sizes don't lock in a span that the framing can't actually achieve.
Categories
Source
Solver enforcement
Browsable only — the solver does not currently enforce this directive (no spec-level data to check against). This entry exists so the architect personas can cite it in conversation and the user can read what the rule says.
Related directives
- Continuous load path from roof to foundation · HUD RSDG §2.4
- Residential structural reliability targets 1-in-100 to 1-in-1000 annual probability of failure · HUD RSDG §2.5
- Residential floor live load: 40 psf minimum (30 psf sleeping rooms) · HUD RSDG §3.4
- Wind load design uses ASCE 7 basic wind speed for the locality · HUD RSDG §3.6
- Ground snow load for Virginia: 25 psf eastern, up to 40 psf western mountains · HUD RSDG §3.7
Last reviewed 2026-05-14.