Check how far a rafter can span against the actual code tables — IRC 2021 Tables R802.4.1(3) and (4), for four No. 2 species, three spacings, both ceiling conditions and both standard dead loads.
| Southern Pine #2 | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|
| 2×4 | 8 ft 7 in | 7 ft 6 in | 6 ft 1 in |
| 2×6 | 12 ft 11 in | 11 ft 2 in | 9 ft 2 in |
| 2×8 | 16 ft 4 in | 14 ft 2 in | 11 ft 7 in |
| 2×10 | 19 ft 5 in | 16 ft 10 in | 13 ft 9 in |
| 2×12 | 22 ft 10 in | 19 ft 10 in | 16 ft 2 in |
Ground snow load 30 psf, dead load 10 psf, ceiling not attached. Spans are horizontal projection, per the 2021 IRC. For other snow loads use the full tables R802.4.1(1)–(8) or the AWC calculator.
Ask a framer how far a rafter spans and the answer is a horizontal number. The IRC measures rafter span along the horizontal projection of the rafter — bearing point to bearing point, as if the roof were flat. On a 6/12 roof the board itself is about 12% longer than its span; on a 12/12 roof, 41% longer. This checker follows the code and works in horizontal spans throughout, so the number you enter is the horizontal distance between the bearing points of your walls or beams, never a tape laid along the slope.
That also means span is not the same as run, though the two are related. On a simple gable roof with a ridge board, each rafter's span is its run — half the building width. Add a structural ridge beam or a mid-span purlin and the span becomes the longest unsupported stretch between bearings, which is the figure the table cares about.
The numbers behind this checker are transcribed from the 2021 International Residential Code: Table R802.4.1(3) for rafters with no ceiling attached (deflection limit L/180) and Table R802.4.1(4) where a ceiling is attached to the rafters (L/240). Both are written for a 30 psf ground snow load, and each publishes two dead-load halves — 10 psf for a standard shingle roof and 20 psf for heavy coverings. All four species are No. 2 grade: Douglas Fir-Larch, Hem-Fir, Southern Pine and Spruce-Pine-Fir, in sizes 2×4 through 2×12 at 12, 16 and 24 in spacing.
One row shows the shape of the data. A Southern Pine #2 2×8 under a 10 psf dead load with no ceiling attached is permitted to span 16 ft 4 in at 12 in spacing, 14 ft 2 in at 16 in, and 11 ft 7 in at 24 in. Spacing is doing heavy work in those numbers — the same board loses nearly 5 ft of span when the spacing doubles.
| Southern Pine #2, 2×8 — DL 10 psf, ceiling not attached | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| Maximum horizontal span | 16 ft 4 in | 14 ft 2 in | 11 ft 7 in |
The checker returns the tabulated maximum for your exact combination and sets your span against it. A pass means the code table permits that member at that spacing — nothing more and nothing less. Treat a pass with only an inch or two to spare as a prompt to think about what the table cannot see: an unusually heavy roof build-up, a long overhang, or lumber that is graded No. 2 but visibly poor. Tables assume sound, correctly graded material.
A fail is not a verdict on the roof, just on that combination. The same span usually passes at closer spacing or one size up, and the margin shown tells you how far short the combination falls. Once the size is settled, work out its length and cut angles with the common rafter calculator, which turns the horizontal span into the actual board.
Every span table is a bundle of assumptions, and using one outside its assumptions is worse than using none. These two tables assume a 30 psf ground snow load. That covers a large share of the country, but snow-belt regions designed for 50 or 70 psf live in Tables R802.4.1(5) through (8), and warm regions where snow never governs use the roof-live-load Tables (1) and (2). Nothing on this page should be stretched to cover them.
The tables also assume ceiling joists or rafter ties right at the wall plate, resisting the outward push of the rafters. Raise the ties into the attic — for a vaulted look or storage — and Table R802.4.1(9) applies reduction factors to every span here. And a practical warning from the code's own footnote territory: spans past 20 ft call for lumber lengths many yards do not stock, so a passing result can still be a board you cannot buy.
Two roofs with identical rafters can have different legal spans. If a gypsum ceiling is fixed directly to the rafters, the code tightens the deflection limit from L/180 to L/240, because a sagging rafter telegraphs straight into cracked plaster. That is the whole difference between Table (4) and Table (3) — the lumber is the same, the tolerance for bend is not.
Dead load is the other lever. It is the weight of the roof itself — decking, underlayment and covering — before snow or anyone sets foot on it. Tile and slate roughly double the standard allowance, and the 20 psf half of each table exists for them. Choose the half that matches your covering, not the one that gives the answer you want.
Every result on this page comes from the method described above, using standard framing geometry and the published data credited below. Found a number that looks wrong? Use Report an Error — every report is read.
No. Rafter spans in the IRC tables are measured along the horizontal projection of the rafter — the run, not the slope. A rafter on a 6/12 roof whose horizontal span is 14 ft is checked against the 14 ft figure even though the board itself is over 15 ft long.
Not with this checker. Tables R802.4.1(3) and (4) are built on a 30 psf ground snow load. Heavier snow country is covered by Tables R802.4.1(5) through (8), and regions governed by roof live load alone use Tables (1) and (2) — use the full tables or the AWC span calculator for those conditions rather than stretching this data.
Then the tabulated spans no longer apply directly. Footnote (a) to both tables assumes ties at the plate; ties raised into the attic must apply the adjustment factors of Table R802.4.1(9), which shorten the permitted span — the higher the tie, the bigger the reduction.
Yes, twice over. The four species differ by up to a foot on the same size, and these tables are for No. 2 grade only. No. 1 or Select Structural would span further, No. 3 less — the grade stamp on the actual lumber is what counts.
Three usual fixes, in order of cost: move to closer spacing (24 in to 16 in buys back a lot of span), go one size deeper, or switch to a stronger species. The size calculator on this site runs the same tables backwards and finds the smallest option that passes.
No. The table span is the horizontal distance between the bearing points of the supports. The eaves overhang cantilevers past the wall and is not part of the tabulated span, though very long overhangs have their own limits in the code.
The full common rafter: length, angles, birdsmouth and materials.
Line and cut lengths and angles for the diagonal rafter on a hip roof.
Where two roof planes meet — valley length and its cut angles.
First jack length and the common difference for your spacing.
Single-slope rafters between two different wall heights.
Seat cut, heel height and HAP, checked against the IRC one-quarter limit.
Plumb, seat and cheek angles — plus the saw settings to cut them.
Convert pitch, rise/run, degrees and percent, both ways.
The lumber size a span actually needs. The inverse of the span tool.
Two pitches, one roof — upper and lower rafter lengths.
The full pitch × run table, printable for the workshop wall.
Figure any rafter by hand — formula and framing-square methods.