The span, the conditions, and the table it came from
Pick the assembly, size, species, and spacing — get the allowable span from the 2021 International Residential Code, cited to the exact table. No rules of thumb, no rounded guesses: the number your inspector can check.
What the lookup covers
Floor joists (30 and 40 psf live, 10 psf dead, L/360), ceiling joists (attics without storage and with limited storage, L/240), rafters (20 psf roof live and 30 psf ground snow, ceiling not attached, L/180), deck joists and deck beams per the R507 deck tables, and built-up headers for exterior and interior bearing walls. Each per-size page carries the full species-by-spacing table for that size, the load assumptions up top, and the citation on every table.
Point loads, cantilevers beyond the table footnotes, unusual snow loads, or anything else outside the table's stated conditions — that's an engineer's question.
Common questions
Where do these span numbers come from?
Every span is quoted from a 2021 International Residential Code table — R502.3.1 for floor joists, R802.5.1 for ceiling joists, R802.4.1 for rafters, R507.6 for deck joists, R507.5 for deck beams, and R602.7 for headers. The exact table is cited next to every number, and each value was hand-verified against the published table before it shipped.
Why does the same 2x8 span differently in different tables?
Loads and deflection limits differ by assembly. A floor carries 30 or 40 psf live load at L/360; a rafter with no ceiling attached is checked at L/180; deck tables build in wet-service factors. Pick the table that matches what the lumber is doing.
Can I use these numbers for my permit drawings?
These are the code tables your plan reviewer uses, cited so they can check them. Local amendments still apply and your inspector has the final word — confirm with your building department.