Lumber species and grades, and what they do to your span

A species group is a group, not a tree

None of the four names in the span tables on this site is a single species of tree. Spruce-Pine-Fir is a Canadian grouping of spruces, pines and firs that grade and perform similarly; Douglas Fir-Larch pairs Douglas fir with western larch; Hem-Fir covers western hemlock and several true firs; and Southern Pine takes in loblolly, longleaf, shortleaf and slash pine from the southeastern United States. Lumber within a group is graded and marketed interchangeably, and the group shares one set of assigned design values.

That is why the tables can print one row per group: the row describes the design values assigned to the group, not the particular tree your board came from. It is also why substituting within a group is routine while substituting between groups changes the row you are allowed to read. The floor tables here carry all four groups; note that the ceiling joist table reproduced on this site covers Douglas Fir-Larch and Hem-Fir only, so a four-species comparison is only possible in the tables that publish four species.

One more nuance recorded in the source data: the Southern Pine rows reflect the Southern Pine Inspection Bureau design values referenced by the IRC. Southern Pine's values were re-evaluated in the 2010s, so older printings of the same tables show different Southern Pine spans than current ones - one more reason to read a cited, current table rather than a remembered one.

A grade is a visual inspection, not a species

Within a species group, lumber is sorted by grade: Select Structural, No. 1, No. 2 and No. 3 in the tables here. A grade describes the defects a piece is permitted to have - the size and placement of knots, splits and checks, wane along the edges, and the slope of the grain relative to the length of the piece. Fewer and smaller defects mean higher assigned design values, so each grade gets its own column in the span table.

The grade of the stick in your hands is whatever the grade stamp says it is, and the stamp is the only evidence that counts. A clear-looking board with a No. 2 stamp is No. 2 for design purposes; an unstamped board has no grade and no row in any table. When a table row and a lumber pile disagree, the stamp wins, which is why the practical first step of any table lookup is reading the stamp, not judging the wood.

The ranking is not stable across grades

Here is the comparison folklore cannot survive: all four species groups at all four grades, for one member and one layout - a 2x10 floor joist at 16 in on center in the 40 psf living-area table.

Read it column by column. At Select Structural, Southern Pine's 17 ft 0 in beats Hem-Fir's 16 ft 5 in and Spruce-Pine-Fir's 16 ft 0 in, trailing only Douglas Fir-Larch. Drop to No. 2 - the grade most lumber yards actually stock - and the order flips: Southern Pine's 14 ft 0 in now trails every other group, a foot and seven inches behind Douglas Fir-Larch and behind SPF and Hem-Fir as well. At No. 3 the gap widens further.

So "which species is strongest" has no table-free answer. The assigned design values move with grade at different rates in different groups, and the only ranking that exists is the one in the column you are actually buying from. If a span comparison is driving a purchasing decision, compare the two exact cells - the lookup tool and the per-combination pages linked below put any two side by side.

When a grade change buys nothing

Look at the Spruce-Pine-Fir row again: No. 1 and No. 2 both read 15 ft 5 in. That is not a typo, and it is not unique to this cell - SPF publishes identical No. 1 and No. 2 spans throughout the 40 psf floor table reproduced here. Paying for the higher grade buys exactly zero inches of floor span in that group.

The reason connects to the deflection guide linked below. A floor span at L/360 is often limited by stiffness rather than strength, and stiffness barely changes between adjacent grades of the same group - the knots that separate a No. 1 from a No. 2 cost strength, not much stiffness. Where the deflection check governs the cell, a grade upgrade moves nothing. Where strength governs - lower grades, and looser limits like a rafter's L/180 - grade moves the number substantially, which is why the same SPF group shows a real gap between No. 2 and No. 3.

The practical rule that falls out: never assume a grade upgrade fixes a span problem, and never assume it does not. Read the two cells. If they match, spend the money on depth or spacing instead.

What is actually on the rack

Availability is regional, and it is an observation about commerce rather than a rule of code. Spruce-Pine-Fir dominates much of the northern and central lumber market; Douglas Fir-Larch is the staple of the western United States; Southern Pine fills the same role in the Southeast; Hem-Fir circulates widely in the West alongside Douglas Fir-Larch. Most yards stock No. 2 as the default framing grade, sell Select Structural at a premium if at all, and rarely rack No. 3 for joist work.

None of that decides anything structurally. It just means the realistic comparison for most projects is between the No. 2 columns of whichever groups your local yards carry - which, as the table above shows, is exactly the column where the folklore ranking is least reliable.

Reading your stamp into the table

The workflow is short. Find the grade stamp on the lumber you have or intend to buy; it names the species group and the grade. Those two facts select your row and column. Add your size, your spacing and your load case, and the span lookup tool returns the published cell with its source table cited underneath; the full floor, ceiling and rafter tables are reproduced on their own pages if you would rather see the whole grid.

Then treat the number as what it is: a published reference value for preliminary sizing under one set of assumptions. Allowable spans depend on species, grade, spacing, live and dead load, and the deflection limit, and the code edition adopted where you build is decided locally - confirm the member with your local building official, and bring in a licensed design professional wherever the situation steps outside the table's assumptions.

Frequently asked questions