Rafter Span Charts
Maximum allowable spans for roof rafters
Rafter span charts determine the maximum horizontal distance a rafter can span based on lumber size, species, grade, spacing, roof pitch, and snow load. Rafters are measured horizontally, not along the slope.
Douglas Fir-Larch, No. 2 Grade, 16" on center
| Rafter Size | 3/12 Pitch | 4/12 Pitch | 6/12 Pitch |
|---|---|---|---|
| 2x6 | 11' - 4" | 10' - 5" | 9' - 0" |
| 2x8 | 15' - 0" | 13' - 9" | 11' - 10" |
| 2x10 | 19' - 1" | 17' - 6" | 15' - 1" |
| 2x12 | 23' - 3" | 21' - 4" | 18' - 5" |
Southern Pine, No. 2 Grade, 16" on center
| Rafter Size | 3/12 Pitch | 6/12 Pitch | 12/12 Pitch |
|---|---|---|---|
| 2x6 | 9' - 9" | 7' - 9" | 5' - 5" |
| 2x8 | 12' - 10" | 10' - 2" | 7' - 2" |
| 2x10 | 16' - 4" | 13' - 0" | 9' - 1" |
| 2x12 | 19' - 11" | 15' - 10" | 11' - 1" |
Understanding Roof Pitch
Roof pitch is expressed as rise over run (e.g., 4/12 means 4 inches of rise for every 12 inches of horizontal run).
- Low Slope: 2/12 to 4/12 - Less structural stress
- Medium Slope: 4/12 to 9/12 - Common residential
- Steep Slope: 9/12 to 12/12+ - More structural stress
Snow Load Considerations
Snow load varies by geographic location and elevation:
- Light Snow: 20 PSF (Southern US)
- Moderate Snow: 30-40 PSF (Mid-Atlantic, Lower Midwest)
- Heavy Snow: 50+ PSF (Mountain regions, Northern states)
Check local building codes for required snow load in your area.
Rafter vs. Truss
Stick-Built Rafters
- Built on-site, piece by piece
- Allows for vaulted ceilings
- Provides attic storage space
- More expensive and time-consuming
- Requires skilled labor
Roof Trusses
- Pre-engineered at factory
- Faster installation
- Can span longer distances
- More economical
- Limited attic access
- Ceiling typically follows bottom chord
Important Notes
- These spans assume rafters are supported by a ridge beam or ridge board with ceiling joists
- For cathedral ceilings, rafters must be sized as both rafter and ceiling joist
- Collar ties required on every other rafter pair for spans over code limits
- Ridge beam (vs. ridge board) carries load and allows longer spans
- Wind loads may govern design in coastal and high-wind areas
When to Use Engineered Lumber
Consider engineered lumber (LVL, I-joists) when:
- Spans exceed conventional lumber limits
- You need longer, straight members
- Dimensional stability is critical
- You want to minimize callbacks for warping
Disclaimer: These charts are general guidelines. Always verify with local building codes and consult a licensed engineer or building official for your specific conditions including snow load, wind load, and seismic requirements.