Rafter Span Calculator
Maximum rafter spans from IRC 2021 Tables R802.4.1(1) through R802.4.1(8), for SPF, Douglas Fir-Larch, Hem-Fir and Southern Pine in SS, No.1, No.2 and No.3, from 2x4 to 2x12 at 12, 16, 19.2 and 24 inches on center, across roof live load and 30, 50 and 70 psf ground snow, at both deflection limits.
Built for reroof framing, additions and cathedral ceiling questions, where the answer decides whether the ridge is a board or an engineered beam.
The table span is the horizontal run, not the rafter you cut
- 2x8 SPF No.2 at 24 in o.c., 30 psf ground snow, 10 psf dead load
- Table answer: 12 ft 4 in, measured along the horizontal projection
- On a 6:12 pitch that same rafter runs about 13 ft 10 in along its own length
- Steeper pitch, longer stick, and the tabulated span does not change
Ordering rafters off the table figure is how a roof package arrives eighteen inches short on every piece.
Four things that decide the rafter
Horizontal projection, not rafter length
IRC R802.4.1 measures rafter span along the horizontal projection, so the table figure is the run from the outside of the wall to the ridge in plan, not the length of the stick you cut. Multiply by the pitch factor to get the ordering length: a 6:12 pitch adds about twelve percent, a 12:12 adds forty-one. The 2x8 rafter span at 24 oc in the reference above is 12 ft 4 in of run and roughly 13 ft 10 in of actual rafter on a 6:12 roof.
Snow load, and the dead load column beside it
Ground snow moves this more than anything else. That same 2x8 SPF No.2 at 24 in o.c. reads 14 ft 10 in under a 20 psf roof live load with no snow governing, 12 ft 4 in at 30 psf ground snow, 10 ft 1 in at 50 psf and 8 ft 9 in at 70 psf. Dead load stacks on top: asphalt shingles over one layer of sheathing usually sit in the 10 psf column, while tile, slate or a second layer move you to 20 psf and drop that 30 psf figure to 11 ft 0 in.
Deflection depends on what is under the rafter
IRC Table R301.7 permits L/180 for rafters with no finished ceiling attached and tightens to L/240 where a ceiling is attached to the rafters, and the IRC prints those as separate tables rather than one table with a note. Most reroof and attic work sits in the L/180 set. A cathedral ceiling puts you in the L/240 set and usually into an engineered ridge at the same time, so check both before quoting the framing.
On center spacing, and what is not tabulated
The tables carry 12, 16, 19.2 and 24 in o.c. and nothing wider. That 2x8 at 30 psf runs 17 ft 5 in at 12 in o.c., 15 ft 1 in at 16 in and 12 ft 4 in at 24 in, so tightening spacing is often cheaper than going up a depth. Framing rafters at 4 ft on center is not a prescriptive option at all; it becomes an engineered roof with purlins and heavier members, and the building official will ask for calculations.
Ridge beam vs ridge board code, and rafter ties
IRC R802.3 allows a ridge board of not less than 1 inch nominal thickness only where ceiling joists or rafter ties provide a continuous tie across the structure. Rafter ties sit in the lower third of the rafter height and resist the outward thrust the rafters put on the bearing walls. Collar ties are a different member for a different job: R802.4.6 places them in the upper third of the attic space, not less than 1x4 nominal, spaced not more than 4 feet on center and fastened per Table R602.3(1). Remove the ties for a cathedral ceiling and the ridge becomes a structural ridge beam carrying half the roof load, which is an engineered member sitting on a continuous post-and-footing load path down to the foundation.
Eight IRC tables, checked against the printed text
Most rafter span calculators on the web will not tell you which edition they run, which of the eight tables the number came from, or whether the span is a run or a rafter length. Here is ours in full.
4,991 cells across eight tables
The dataset carries IRC 2021 Tables R802.4.1(1) through R802.4.1(8), covering roof live load and three ground snow loads, two dead load columns and both deflection limits, for four species groups and four grades from 2x4 to 2x12.
Verified against the code text
Values were taken from the 2021 Seattle Residential Code Chapter 8 PDF, which reproduces the IRC rafter span tables verbatim with no local amendments in R802.4.1, and were checked against that printed text rather than retyped from a summary.
Section numbers checked, not assumed
R802.4.5 is Purlins and R802.4.6 is Collar Ties. Those two are commonly cited the other way round online, so both were confirmed against the printed chapter before anything on this page referenced them.
9’-9″
SPF No.2 at 24 in o.c., 30 psf ground snow, 10 psf dead, L/180.
12’-4″
The workhorse depth on most reroofs and the one people quote from memory.
15’-1″
Two feet nine more than the 2x8 for one step up in depth.
17’-6″
The deepest sawn rafter the tables carry before engineered members.
All four are horizontal runs, not rafter lengths. Take that 2x8 to 70 psf ground snow and it falls from 12 ft 4 in to 8 ft 9 in; keep the snow at 30 psf and tighten to 16 in o.c. and it reaches 15 ft 1 in. Ground snow and spacing are the two columns that decide the roof.
When this stops being a table lookup
A hip rafter and its valley members are not tabulated as a span condition, because they collect concentrated loads from the jack rafters, so those go to an engineer or a manufacturer's beam design. Below a 3:12 slope a low slope roof is treated as a beam rather than a rafter system, the ridge cannot be a simple ridge board, and IRC Chapter 9 also restricts roof coverings by minimum slope. An engineer is also required when the ground snow load falls outside the tabulated range, when the member is engineered lumber, when a cathedral ceiling removes the ties, or when the building official asks for calculations. This tool runs the 2021 IRC tables; the edition your building department has adopted may be older, states and municipalities amend the model text, and IRC R104 and R106 make the building official the final word, so the AHJ decides your permit. A prescriptive span table is a worst-case lookup, not an engineering calculation, and nothing here is a stamped design.
Sources
- ICC Digital Codes — 2021 International Residential Code, Chapter 8, Roof-Ceiling Construction, for Tables R802.4.1(1) to R802.4.1(8), R802.3, R802.4.5 and R802.4.6.
- 2021 Seattle Residential Code, Chapter 8 (PDF), which reproduces the IRC rafter span tables verbatim and was the text the dataset and the section numbers were verified against.
- American Wood Council, Span Tables for Joists and Rafters, PS 20-20 sizes (PDF), the parent standard the IRC tables derive from.
Data retrieved 27 August 2026, section numbers verified 12 September 2026. Deliberately absent: no hip or valley rafter spans, because the IRC does not tabulate them as a span condition; no engineered lumber spans, because those come from the manufacturer; no pitch conversion baked into the tabulated figure, because the code measures the span on the horizontal projection and hiding that would be the error this page exists to prevent.
Rafter span FAQs
Is the rafter span table figure the run or the rafter length?
The run. IRC R802.4.1 measures rafter span along the horizontal projection, so the tabulated figure is the plan distance from wall to ridge, not the length of the member. Multiply by the pitch factor for the ordering length: about twelve percent more on a 6:12 and forty-one percent more on a 12:12. Cutting a list straight off the table leaves every rafter short.
What is the 2x8 rafter span at 24 oc?
It depends on snow load, dead load and deflection limit, so there is no single number. SPF No.2 at 24 in o.c. with a 10 psf dead load and L/180 reads 14 ft 10 in under a 20 psf roof live load, 12 ft 4 in at 30 psf ground snow, 10 ft 1 in at 50 psf and 8 ft 9 in at 70 psf. Run your actual ground snow rather than a remembered figure.
Does the ridge need to be a beam, or is a ridge board enough?
A ridge board of not less than 1 inch nominal is acceptable under IRC R802.3 only where ceiling joists or rafter ties provide a continuous tie across the structure. Remove those ties for a cathedral ceiling and the ridge becomes a structural ridge beam carrying half the rafter load, which is an engineered member on a continuous post-and-footing load path.
Where do rafter ties and collar ties actually go?
They are different members. Rafter ties sit in the lower third of the rafter height and resist the outward thrust on the bearing walls. Collar ties go in the upper third of the attic space per IRC R802.4.6, not less than 1x4 nominal, spaced no more than 4 feet on center and fastened per Table R602.3(1). A collar tie does not substitute for a rafter tie.
Can I frame rafters at 4 ft on center?
Not prescriptively. The IRC 2021 rafter span tables tabulate 12, 16, 19.2 and 24 inches on center and nothing wider. Anything beyond that leaves the table and becomes an engineered roof, typically purlins with heavier rafters or a timber-frame design, and the building official will ask for calculations before issuing the permit.
What changes below a 3:12 roof slope?
Low-slope roofs are treated as beams rather than rafters. Below 3:12 the ridge cannot be a simple ridge board, the thrust and deflection assumptions behind the tables no longer hold, and IRC Chapter 9 restricts roof coverings by minimum slope as well. Size those members as beams and confirm the covering is listed for the slope you are building.
Running the span is free. Keeping the crew busy is not
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