Rafter Span Calculator

The smallest #2 rafter size that covers your span, read straight from the published IRC tables (no interpolation, no invented numbers). For cut length and angles, use the rafter length calculator.

Measure in
Roof load
Species and grade
Rafter spacing
Span to cover (horizontal) Horizontal distance the rafter must cover, wall plate to ridge - not the sloped board length, and not counting overhang.
SMALLEST SIZE
2x6
Smallest #2 size that covers 12 ft 0 in at 16 in oc - max 14 ft 4 in per IRC Table R802.4.1(1)
Shopping list
  • 2x4 max spanSPF #2 at 16 in oc 9 ft 5 in - short
  • 2x6 max spanSPF #2 at 16 in oc 14 ft 4 in - PASS
  • 2x8 max spanSPF #2 at 16 in oc 18 ft 2 in - PASS
  • 2x10 max spanSPF #2 at 16 in oc 22 ft 3 in - PASS
  • 2x12 max spanSPF #2 at 16 in oc 25 ft 9 in - PASS
  • SourceDead load 10 psf columns, L/180 deflection, ceiling not attached, rafter ties at the plate IRC Table R802.4.1(1)

Max rafter span - 20 psf roof live load, 10 psf dead load, ceiling not attached, L/180 (2021 IRC Table R802.4.1(1), #2 grade)

Spans are horizontal projection, published values only. Over 26 ft means the code prints note b (the allowable span exceeds 26 ft, the ceiling of the prescriptive tables). Tile or slate roofs need the 20 psf dead load columns of the same table, not shown here.

SizeSpacingDouglas Fir-Larch #2Hem-Fir #2Southern Pine #2Spruce-Pine-Fir #2
2x412 in oc10 ft 10 in10 ft 1 in10 ft 4 in10 ft 4 in
2x416 in oc9 ft 10 in9 ft 2 in9 ft 0 in9 ft 5 in
2x419.2 in oc9 ft 1 in8 ft 8 in8 ft 2 in8 ft 10 in
2x424 in oc8 ft 2 in7 ft 11 in7 ft 4 in8 ft 0 in
2x612 in oc16 ft 10 in15 ft 11 in15 ft 7 in16 ft 3 in
2x616 in oc14 ft 7 in14 ft 2 in13 ft 6 in14 ft 4 in
2x619.2 in oc13 ft 3 in12 ft 11 in12 ft 3 in13 ft 1 in
2x624 in oc11 ft 11 in11 ft 7 in11 ft 0 in11 ft 9 in
2x812 in oc21 ft 4 in20 ft 8 in19 ft 8 in21 ft 0 in
2x816 in oc18 ft 5 in17 ft 11 in17 ft 1 in18 ft 2 in
2x819.2 in oc16 ft 10 in16 ft 4 in15 ft 7 in16 ft 7 in
2x824 in oc15 ft 1 in14 ft 8 in13 ft 11 in14 ft 10 in
2x1012 in oc26 ft 0 in25 ft 3 in23 ft 5 in25 ft 8 in
2x1016 in oc22 ft 6 in21 ft 11 in20 ft 3 in22 ft 3 in
2x1019.2 in oc20 ft 7 in20 ft 0 in18 ft 6 in20 ft 3 in
2x1024 in oc18 ft 5 in17 ft 10 in16 ft 6 in18 ft 2 in
2x1212 in ocover 26 ftover 26 ftover 26 ftover 26 ft
2x1216 in oc26 ft 0 in25 ft 5 in23 ft 10 in25 ft 9 in
2x1219.2 in oc23 ft 10 in23 ft 2 in21 ft 9 in23 ft 6 in
2x1224 in oc21 ft 4 in20 ft 9 in19 ft 6 in21 ft 0 in

Max rafter span - 30 psf ground snow load, 10 psf dead load, ceiling not attached, L/180 (2021 IRC Table R802.4.1(3), #2 grade)

Ground snow load is a mapped site value set by your jurisdiction, not the flat roof snow load. Above 30 psf, use IRC Tables R802.4.1(5)-(8) or a design professional.

SizeSpacingDouglas Fir-Larch #2Hem-Fir #2Southern Pine #2Spruce-Pine-Fir #2
2x412 in oc9 ft 6 in8 ft 10 in8 ft 7 in9 ft 1 in
2x416 in oc8 ft 3 in8 ft 0 in7 ft 6 in8 ft 2 in
2x419.2 in oc7 ft 7 in7 ft 4 in6 ft 10 in7 ft 5 in
2x424 in oc6 ft 9 in6 ft 7 in6 ft 1 in6 ft 8 in
2x612 in oc14 ft 0 in13 ft 7 in12 ft 11 in13 ft 9 in
2x616 in oc12 ft 1 in11 ft 9 in11 ft 2 in11 ft 11 in
2x619.2 in oc11 ft 0 in10 ft 9 in10 ft 2 in10 ft 11 in
2x624 in oc9 ft 10 in9 ft 7 in9 ft 2 in9 ft 9 in
2x812 in oc17 ft 8 in17 ft 2 in16 ft 4 in17 ft 5 in
2x816 in oc15 ft 4 in14 ft 11 in14 ft 2 in15 ft 1 in
2x819.2 in oc14 ft 0 in13 ft 7 in12 ft 11 in13 ft 9 in
2x824 in oc12 ft 6 in12 ft 2 in11 ft 7 in12 ft 4 in
2x1012 in oc21 ft 7 in21 ft 0 in19 ft 5 in21 ft 4 in
2x1016 in oc18 ft 9 in18 ft 2 in16 ft 10 in18 ft 5 in
2x1019.2 in oc17 ft 1 in16 ft 7 in15 ft 4 in16 ft 10 in
2x1024 in oc15 ft 3 in14 ft 10 in13 ft 9 in15 ft 1 in
2x1212 in oc25 ft 1 in24 ft 4 in22 ft 10 in24 ft 8 in
2x1216 in oc21 ft 8 in21 ft 1 in19 ft 10 in21 ft 5 in
2x1219.2 in oc19 ft 10 in19 ft 3 in18 ft 1 in19 ft 6 in
2x1224 in oc17 ft 9 in17 ft 3 in16 ft 2 in17 ft 6 in

The eight IRC rafter span tables and what this tool covers

This tool carries the dead load 10 psf columns of tables (1) and (3) only. The 20 psf dead load halves of every table (tile, slate, heavy roofs) and all attached-ceiling and high-snow tables are deliberately out of scope. Read them in the code text, or hand the job to a design professional.

IRC tableConditionsIn this tool
R802.4.1(1)Roof live load 20 psf, ceiling not attached, L/180Yes - 10 psf dead load columns
R802.4.1(2)Roof live load 20 psf, ceiling attached, L/240No
R802.4.1(3)Ground snow 30 psf, ceiling not attached, L/180Yes - 10 psf dead load columns
R802.4.1(4)Ground snow 30 psf, ceiling attached, L/240No
R802.4.1(5)Ground snow 50 psf, ceiling not attached, L/180No
R802.4.1(6)Ground snow 50 psf, ceiling attached, L/240No
R802.4.1(7)Ground snow 70 psf, ceiling not attached, L/180No
R802.4.1(8)Ground snow 70 psf, ceiling attached, L/240No

What this answers

A 2x6 SPF #2 rafter makes 14 ft 4 in at 16 in on center. That’s under a 20 psf roof live load. Every other version of that question (other sizes, other species, wider spacing, snow) lives in two IRC tables, and this tool reads them for you.

Pick the load, species, spacing and the span you need to cover. It returns the smallest #2 size that passes, plus a pass or short verdict for every size from 2x4 through 2x12.

Every figure is the published value from the 2021 IRC, Table R802.4.1(1) or Table R802.4.1(3), dead load 10 psf columns, #2 grade. Nothing is interpolated, averaged or extended. If your case falls outside these cells the tool says so instead of guessing.

Table (1) is the roof live load 20 psf table, ceiling not attached to rafters, L/180 deflection. Table (3) is the ground snow load 30 psf table under the same conditions.

How to use it

Choose 20 psf roof live load where design snow is light or zero (most of the southern and coastal US). Choose 30 psf ground snow where your building department lists a ground snow load up to 30 psf, which takes in much of the lower snow belt. Your local jurisdiction publishes the ground snow load for your parcel. When in doubt, check with them, not a map on a forum.

Species comes off the grade stamp. SPF #2 dominates big-box racks nationally, Southern Pine is typical in the Southeast, Douglas Fir-Larch in the West, Hem-Fir on the West Coast. Can’t read the stamp? Pick the species with the lowest value for your size and spacing and you’ll land on the safe side.

Then set spacing (16 and 24 in on center are the common layouts, and 19.2 in fits five bays in 8 ft) and enter the horizontal distance from the wall plate to the ridge. The head result is the smallest size that makes it. The rows show the max span for every size, so you can also read it the other way around. Pick a size, see how far it goes.

Where the numbers come from

The IRC publishes eight prescriptive rafter span tables, R802.4.1(1) through R802.4.1(8). This tool carries the dead load 10 psf columns of tables (1) and (3) for the #2 grade of four species.

Those tables are built on the design values behind the AWC Span Tables for Joists and Rafters. Dead load 10 psf covers ordinary shingle roofs, including two layers of asphalt shingles. Tile and slate push into the 20 psf dead load columns, which aren’t in this tool.

The tables assume ceiling joists or rafter ties at the top plate resisting the outward thrust of the rafters. Ties raised up into the attic shrink the allowable spans by the factors in Table R802.4.1(9), which this tool doesn’t apply. Grade matters too. Select Structural and #1 span farther than the #2 values here, while #3 and stud grade span less.

Canadian builds fall under the NBC 9.23 span tables, which differ from these IRC values, so these numbers shouldn’t be carried over the border.

Rafter span is not rafter length

Span is measured along the horizontal projection, not along the sloped board (IRC R802.4.1). It’s the flat distance the rafter covers. A 12 ft span at a 6/12 pitch means roughly 13 ft 5 in of lumber before the overhang. Sizing uses span. Buying and cutting use length. This page answers the sizing question only, so for the cut length from span and pitch, use the rafter length calculator.

When the table is not enough

Plenty of roofs fall outside this tool. Dead loads over 10 psf (tile, slate, heavy build-ups) need the heavier columns. Ground snow above 30 psf needs the 50 and 70 psf tables, R802.4.1(5)-(8).

Ceilings attached to the rafters use the L/240 tables, which aren’t here either. Raised rafter ties, engineered lumber and any span past 26 ft are out as well. This is a prescriptive-table lookup for typical light-frame roofs, not engineering. Point loads from solar racks or equipment, heavy snow country, and anything beyond these cells belong with a design professional or your local building department.

Common questions

How far can a 2x6 rafter span?

For Spruce-Pine-Fir #2 at 16 in on center, 14 ft 4 in under a 20 psf roof live load, 10 psf dead load, no ceiling attached (2021 IRC Table R802.4.1(1)). Douglas Fir-Larch #2 reaches 14 ft 7 in, Hem-Fir #2 14 ft 2 in and Southern Pine #2 13 ft 6 in. Under 30 psf ground snow, SPF #2 drops to 11 ft 11 in at the same spacing.

How far can a 2x8 rafter span?

SPF #2 makes 18 ft 2 in at 16 in on center under a 20 psf roof live load. Douglas Fir-Larch #2 makes 18 ft 5 in, Southern Pine #2 17 ft 1 in. Drop to 24 in on center and SPF #2 makes 14 ft 10 in. Under 30 psf ground snow it’s 15 ft 1 in at 16 in on center.

How far can a 2x10 rafter span?

22 ft 3 in at 16 in on center for SPF #2 at 20 psf roof live load. Douglas Fir-Larch #2 makes 22 ft 6 in. At 24 in on center SPF #2 makes 18 ft 2 in. Under 30 psf ground snow, SPF #2 makes 18 ft 5 in at 16 in on center.

How far can a 2x4 rafter span?

9 ft 5 in at 16 in on center for SPF #2 at 20 psf roof live load, and 8 ft 0 in at 24 in on center. That makes 2x4 rafters workable only for small sheds and lean-tos, really. Under 30 psf ground snow the 16 in figure drops to 8 ft 2 in.

What is the maximum span for a 2x12 rafter?

For SPF #2 at 16 in on center and 20 psf roof live load, 25 ft 9 in. At 12 in on center the table prints note b, meaning the allowable span exceeds 26 ft. Under 30 psf ground snow, SPF #2 2x12 makes 21 ft 5 in at 16 in on center. Past 26 ft the prescriptive tables end and the roof needs engineered design.

Is rafter span measured horizontally or along the slope?

Horizontally. The IRC measures rafter span along the horizontal projection of the rafter, so a 12 ft span at an 8/12 pitch actually takes about 14 ft 5 in of lumber before overhang. Enter the flat plate-to-ridge distance here, and use the rafter length calculator for the sloped cut length.

What if my ground snow load is higher than 30 psf?

Use the 50 and 70 psf tables, IRC R802.4.1(5) through (8). They’re not in this tool because misreading them is expensive. Your building department publishes the ground snow load for your parcel. Above 70 psf ground snow the IRC tables end entirely and the roof must be engineered.

Do these spans require rafter ties or ceiling joists?

Yes. The tabulated spans assume ceiling joists or rafter ties at the top plate resisting the outward push of the rafters. Where ties sit higher in the attic, the spans must be multiplied by the reduction factors in Table R802.4.1(9). A structural ridge beam changes the framing model entirely. Neither adjustment is applied by this tool.

Which species should I pick if the stamp is unreadable?

Use the lowest value across the four species for your size and spacing. At 2x8, 16 in on center, 20 psf that’s Southern Pine #2 at 17 ft 1 in. The stamp on dimensional lumber names species group and grade. Big-box framing stock in most of the US is SPF #2, the Southeast usually stocks Southern Pine, and the West Douglas Fir-Larch.

How this page is checked: written and maintained by Salman Khalid. Every formula on this site is pinned by an automated test suite: 140+ named checks against manufacturer data sheets and code sections, plus a property sweep of every option combination and edge input. The build refuses to publish if a single check fails. Full methodology on the how we calculate page. Spot an error? Tell us. Fixes are logged, dated, on the corrections page. Sources for this page: IRC Table R802.4.1(1) rafter spans (UpCodes) · AWC 2024 Span Tables for Joists and Rafters (PDF) · AWC span options calculator
Cite or link this page: Salman Khalid, “Rafter Span Calculator,” LevelCalcs, levelcalcs.com/rafter-span-calculator/, updated August 23, 2026. The reference tables and worked figures are free to quote with a link back.