How to read a span table
Every deck calculator that offers a "maximum span" is either copying one line out of a table and hiding its conditions, or making it up. This page explains the table instead, so you can read the real one — which is short, and in the code book.
What a span table is
A span table lists the maximum distance a member can span for one specific combination of species, grade, size, spacing and load. Change any one of those and the number changes. That is not conservatism; it is how the values are derived — each cell is the length at which the member either reaches its allowable bending stress or deflects more than the limit, whichever comes first.
The five variables, and what each one does
| Variable | Effect on span | Where you find it |
|---|---|---|
| Species and grade | Largest single effect — a Select Structural member can span far more than a No. 2 of the same size | Stamped on the board |
| Size | Depth matters roughly as the square; width barely at all | Nominal size, then actual for the table |
| Spacing | Halving spacing roughly doubles allowable span | Your layout, 12/16/19.2/24″ |
| Live load | 40 psf for residential decks; higher for commercial or hot tubs | Code, plus what you put on it |
| Deflection limit | L/360 for floors, L/360 or L/240 depending on the member and finish | Code table footnote |
Read the third column. Four of the five are conditions you supply, and the table's answer is only valid inside them. A calculator that returns "14 ft" without asking for species, grade and spacing has answered a different question.
Reading it, in order
- Find the right table. Joists and beams are different tables; so are floor and roof; so are solid-sawn and engineered.
- Read the header. Species group, grade, live load and deflection limit are stated once at the top and apply to every cell.
- Pick the row for your size. 2×8, 2×10, 2×12.
- Pick the column for your spacing. If your spacing is not a column — say 19.2″ — use the next column up, not an interpolation.
- Read the span. Then check the footnotes, which usually add limits on cantilever, notching, and bearing length.
Beams are a different problem
A beam carries the reactions of many joists, so its load is concentrated rather than distributed, and its span is set by post spacing. The code's deck beam table gives span by number of plies, species, size and the joist span it is carrying — three dimensions of input, which is why a two-input "beam calculator" cannot be right. A built-up beam of two 2×10s is not twice as strong as one 2×10 in the way most people assume, either: the plies act together only if they are fastened together to the nail schedule.
Why we do not publish a span
Because publishing one number invites exactly one mistake: using it outside its range. A visitor reads "2×8 at 16″ spans 12′-4″", remembers the number, and applies it to a different species, a different grade, and a deck with a hot tub. The failure mode of a wrong span is not a board that squeaks — it is a deck that sags permanently or, at the connection, lets go.
What we do instead is count and lay out — the joist calculator tells you how many joists at your spacing and what the real spacing becomes after rounding. The span number you take from the table, for the member you actually bought.
The numbers people get wrong even with the table
- Spacing is not what you drew. A 14 ft run at 16″ spacing needs 11 joists, and the real spacing becomes 16.8″, which may push you into the next column. The calculator shows this; a drawing usually does not.
- Notches and holes. A 2″ notch in the top edge of a joist at the bearing point can cut its capacity by a third. The table assumes an unnotched member.
- Bearing length. A joist needs a minimum bearing at each end — usually 1½″ on wood, more on masonry. Hangers provide it; a joist resting on a beam's edge does not.
- Grade substitution. Upgrading to a better grade raises the span; downgrading to a cheaper one lowers it. The stamp on the board is the input, not the species name on the tag.
Why you still need the table itself
- It does not reproduce the tables. They are in the code book, they are edition-specific, and reproducing them out of context is how the mistake above happens.
- It does not cover engineered lumber. LVL, glulam and I-joists have manufacturer tables with their own assumptions.
- It does not replace an engineer. For cantilevers, hot tubs, roofs over decks or anything unusual, the table is the starting point, not the answer.
Questions people actually ask
Can a 2x8 span 12 feet?
It can, under some combinations of species, grade and spacing, and not under others. That is the point of the table — the size alone does not determine the span, and any calculator that says otherwise is guessing.
What is the deflection limit for a deck?
L/360 is the usual floor limit, meaning the member may deflect no more than its span divided by 360. Some members and finishes allow L/240. The table states which limit it was built on.
Does joist spacing really change the span that much?
Yes — going from 12″ to 24″ spacing doubles the load each joist carries, which reduces the allowable span substantially. It is the variable most often ignored when people copy a span from a table.
Why does my 2x10 need to be a specific grade?
Because grade describes the size and frequency of defects, and defects govern bending strength. Two boards of the same species and size, one No. 2 and one Select Structural, have different allowable spans and different prices.
Can I use the table for a hot tub deck?
No. A hot tub is a concentrated load far above the 40 psf residential live load the table assumes. That is an engineering calculation, not a table lookup.