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.
The tables, in full
Each assembly has one page carrying every table it is read from — all sizes, species, grades, and spacings in one place — with links down to the page for each size:
Floor joist span tables — 2021 IRC Tables R502.3.1(1) and (2).
Ceiling joist span tables — 2021 IRC Tables R802.5.1(1) and (2).
Rafter span tables — 2021 IRC Tables R802.4.1(1) and (3).
Deck joist span tables — 2021 IRC Table R507.6, spans and
cantilevers.
Deck beam spans — 2021 IRC Tables R507.5(1)–(4).
Header and girder spans — 2021 IRC Tables R602.7(1) and (2).
Or read the same tables one species at a time — every cell the code lists for that species, floor, ceiling, rafter and deck:
Southern pine spans · Douglas fir-larch spans · Hem-fir spans · Spruce-pine-fir spans · all four compared
Roof pitch and rafter length
A rafter span in the code is measured along the horizontal projection of the rafter — the run, not the sloping stick. The roof pitch pages carry every whole-inch pitch with its angle, the length of rafter each foot of run takes, the rafter spans those 2021 IRC tables list at that pitch, and the minimum slope Chapter 9 gives each roof covering. The same multiplier turns a footprint into a roofing order: figuring metal roofing works the panel count, panel length and roof area off it.
What the lumber weighs
Sizing a member and carrying it up a ladder are different questions, and the code only answers the first. The lumber weight page answers the second — pounds per foot and pounds per piece, by size and by the same four species groups the span tables are read by, with the moisture content every figure assumes stated beside it. It cites the published wood standards it works from, and it does not cite the IRC, which publishes no such number.
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.