Guides
Span table guides
The span tables answer “what is the span for this exact combination” across 1,002 published cells. These guides answer the questions around them - how to read a table at all, what the deflection limit in the heading is doing, what spacing costs you, and why one nominal size has a dozen different published spans. Every figure quoted is taken from the tables on this site and linked back to the page it lives on.
How far can a 2x8 joist span?There is no single answer. See the published 2x8 spans by species, grade, spacing and load case, and what the span table leaves out.Joist spacing: 16 in vs 24 in on centerWhat on-center spacing means, what a 2x8 or 2x10 actually loses going from 16 in to 24 in, and when the wider layout is worth it.What does L/360 deflection mean?L/360 is a deflection limit: the span divided by 360. Here is the arithmetic, why floors, ceilings and rafters use different limits, and what it changes.How to read a span tableA span table has five keys before it has a number. Learn what each column means, how to read a cell, and the misreadings that produce undersized framing.Rafter vs joist: what's the difference?Rafters, ceiling joists and floor joists are sized from different tables with different loads and limits. See the same 2x8 read out of all of them.How far can a deck joist span?Deck joists are sized from the AWC DCA6 wet-service table, not the indoor floor table. See the published spans and the overhang column most sites skip.Lumber species and grades, and what they do to your spanWhat SPF, DF-L, Hem-Fir and Southern Pine actually are, what a grade stamp means, and why the species ranking changes as the grade drops.Attic storage vs an attic floor: which span table applies?Ceiling joists with limited attic storage and attic floors are covered by different span tables. See both published numbers and what neither one decides.
Go straight to the numbers
Every guide links into the published data it talks about.