Rafter Span Calculator

Rafter span tables

The longest span for No.2 rafters of each species under 20 psf roof live load and 10 psf dead load, with no ceiling attached (deflection up to the span over 180), on a 6 in 12 roof. Spans are level, from the wall to the ridge. Worked out by the calculator, not copied from the code tables.

Douglas Fir-Larch No.2

Southern Pine No.2

Hem-Fir No.2

Spruce-Pine-Fir No.1/No.2

Rafter spans for snow

The same rafters under roof snow loads of 30, 50 and 70 psf, with 10 psf dead load. Use the snow load on the roof, not the ground snow load on the map; on a heated building with a sloped roof, it is about 0.7 times the ground snow load, and less on steep, slippery roofs.

Douglas Fir-Larch No.2, 16 in apart

Southern Pine No.2, 16 in apart

How the calculator works out a rafter span

Each rafter is a simple span checked to the 2018 National Design Specification (NDS) for wood, with allowable stress design (ASD), as dry lumber. Rafters 24 in apart or closer share load through the sheathing, so they take the repetitive member factor of 1.15. Like the rafter tables in the International Residential Code (IRC), the span is measured level and the roof load is on the level area under the roof. The dead load is along the roof's surface, so on a 6 in 12 roof it grows by 12% when spread over the level area. The longest span is the longest one that passes every check:

• Bending and shear under the dead and roof load, with the load duration factor for the roof load: 1.15 for snow and 1.25 for roof live load, which lasts only while the roof is worked on.
• Bending under dead load alone, with the load duration factor of 0.9 for a load that never goes away.
• Deflection under the roof load: up to the span over 180, or over 240 with a ceiling attached to the rafters, the limits in IRC Table R301.7.

The IRC's rafter tables (R802.4.1) come from the same kind of check. A building department may ask for the code table; for a roof outside it, a calculation like this one.

Rafter questions

How far can a 2x6 rafter span?

Douglas Fir-Larch No.2 2x6 rafters 16 in apart span 14′ 3″ under 20 psf roof live load and 10 psf dead load; 12 in apart, 16′ 5″.

How far can a 2x8 rafter span?

18′ 1″ at 16 in apart and 14′ 9″ at 24 in apart for Douglas Fir-Larch No.2, or 16′ 8″ at 16 in apart for Southern Pine No.2.

How far can a 2x10 rafter span?

22′ 1″ at 16 in apart and 18′ at 24 in apart for Douglas Fir-Larch No.2. Under 30 psf of snow, 18′ 5″ at 16 in apart.

How far can a 2x12 rafter span?

25′ 7″ at 16 in apart and 20′ 11″ at 24 in apart for Douglas Fir-Larch No.2.

Does a ceiling attached to the rafters change the span?

It can. A ceiling cracks if the rafters sag too much, so the limit tightens from the span over 180 to the span over 240. Douglas Fir-Larch No.2 2x6 rafters 16 in apart drop from 14′ 3″ to 14′; deeper rafters are usually limited by bending first, so they change less.

What about a tile roof?

Enter its dead load: clay or concrete tile adds 10 psf or more over shingles. At 20 psf dead load, Douglas Fir-Larch No.2 2x8 rafters 16 in apart span 15′ 6″ instead of 18′ 1″.

Is the span measured along the rafter?

No, level: from the wall to the ridge, measured on the floor plan. The rafter itself is longer, by the slope; the calculator gives its length along the slope.

What holds the walls from spreading?

Rafters push out on the walls unless ceiling joists or rafter ties tie them together, or a ridge beam carries the top ends. To size a ridge beam or header, use the wood beam span calculator or the beam designer. For the floor below, see the floor joist span calculator.