Which rafter size do you need? The span determines everything. Use the wrong size and your roof sags or worse. This guide compares 2x6 and 2x8 rafters with complete span tables.
Quick Answer
2x6 rafters: Maximum span 8-14 feet (depending on grade and spacing)
2x8 rafters: Maximum span 11-19 feet (depending on grade and spacing)
If your span exceeds these limits, go up a size or reduce spacing.
Understanding Span
Span = horizontal distance from support to support (wall to ridge or wall to wall)
Actual rafter length is longer than span due to roof pitch.
| Pitch | Rafter Length per Foot of Span |
|---|---|
| 4:12 | 1.054 feet |
| 6:12 | 1.118 feet |
| 8:12 | 1.202 feet |
| 10:12 | 1.302 feet |
| 12:12 | 1.414 feet |
2x6 Rafter Span Table
Based on #2 grade lumber, 20 psf live load, 10 psf dead load
| Spacing | Select Structural | No. 1 | No. 2 |
|---|---|---|---|
| 12” OC | 14’ 2” | 13’ 8” | 13’ 4” |
| 16” OC | 12’ 11” | 12’ 5” | 11’ 6” |
| 24” OC | 10’ 8” | 10’ 0” | 9’ 0” |
2x6 with Snow Load (30 psf)
| Spacing | Select Structural | No. 1 | No. 2 |
|---|---|---|---|
| 12” OC | 12’ 4” | 11’ 11” | 11’ 0” |
| 16” OC | 11’ 2” | 10’ 5” | 9’ 6” |
| 24” OC | 9’ 2” | 8’ 5” | 7’ 9” |
2x8 Rafter Span Table
Based on #2 grade lumber, 20 psf live load, 10 psf dead load
| Spacing | Select Structural | No. 1 | No. 2 |
|---|---|---|---|
| 12” OC | 18’ 9” | 18’ 0” | 17’ 8” |
| 16” OC | 17’ 0” | 16’ 4” | 15’ 2” |
| 24” OC | 14’ 0” | 13’ 2” | 11’ 10” |
2x8 with Snow Load (30 psf)
| Spacing | Select Structural | No. 1 | No. 2 |
|---|---|---|---|
| 12” OC | 16’ 3” | 15’ 9” | 14’ 6” |
| 16” OC | 14’ 9” | 13’ 9” | 12’ 7” |
| 24” OC | 12’ 1” | 11’ 1” | 10’ 0” |
When to Use Each Size
Use 2x6 When:
- Span under 12 feet
- Low snow load area
- 16” spacing is acceptable
- Budget is tight
- Shed or garage roof
Use 2x8 When:
- Span 12-18 feet
- Moderate to heavy snow load
- Want 24” spacing for cathedral ceiling
- Main house roof
- Long-term durability needed
Spacing Options Explained
| Spacing | Pros | Cons |
|---|---|---|
| 12” OC | Longest spans, stiffest roof | More lumber, more cost |
| 16” OC | Standard, matches insulation | Medium spans |
| 24” OC | Less lumber, easier insulation | Shortest spans, more bounce |
OC = on center (measured from center of one rafter to center of next)
Lumber Grade Definitions
| Grade | Description | Use |
|---|---|---|
| Select Structural | Highest quality, few knots | Critical spans |
| No. 1 | Good quality, minor defects | Most residential |
| No. 2 | Standard quality, some knots | Budget builds |
| No. 3 | Economy, large knots | Not for rafters |
Note: No. 2 is most common at lumber yards.
Load Calculations
Dead Load
- Roofing material: 2-5 psf
- Sheathing: 2-3 psf
- Rafters: 2-3 psf
- Ceiling (if attached): 5-10 psf
- Typical total: 10-15 psf
Live Load
- No snow: 20 psf (minimum code)
- Light snow: 20-30 psf
- Moderate snow: 30-50 psf
- Heavy snow: 50+ psf
Check local codes for your snow load requirement.
Cost Comparison
| Item | 2x6x12 | 2x8x12 |
|---|---|---|
| Price per board | $8-12 | $12-18 |
| Board feet | 12 | 16 |
| Cost per foot | $0.75-1.00 | $1.00-1.50 |
Full Project Cost Example
20-foot wide building, 12-foot span:
| Size | Boards Needed | Material Cost |
|---|---|---|
| 2x6 @ 16” OC | 16 boards | $130-190 |
| 2x8 @ 24” OC | 11 boards | $130-200 |
Often similar cost due to spacing differences.
Real-World Examples
Garden Shed (10x12)
- Span: 6 feet
- Use: 2x6 @ 24” OC
- Boards needed: 6
Garage (24x24)
- Span: 12 feet
- Use: 2x8 @ 16” OC
- Boards needed: 19
House Addition (14-foot span)
- Use: 2x10 @ 16” OC
- (Exceeds 2x8 limits at this span)
When 2x6 and 2x8 Aren’t Enough
| Span | Recommended Size |
|---|---|
| 0-12 ft | 2x6 |
| 12-16 ft | 2x8 |
| 16-20 ft | 2x10 |
| 20-24 ft | 2x12 |
| 24+ ft | Engineered lumber |
Code Requirements
Always check local building codes. They specify:
- Maximum spans by lumber size
- Required snow loads
- Minimum lumber grades
- Connection requirements
Get a permit for any structural roof work.
Common Mistakes
- Using span tables wrong - Span is horizontal, not rafter length
- Ignoring snow load - Can reduce span by 30%
- Wrong lumber grade - No. 3 doesn’t meet span tables
- Skipping collar ties - Required for most rafters
- Overloading with heavy shingles - Concrete tile adds 10+ psf
Pro Tips
- When in doubt, go bigger - The cost difference is small
- Match existing - Match size when extending or repairing
- Check both ends - Ridge support affects span calculations
- Consider alternatives - Engineered trusses may be cheaper
Use Our Free Calculator
Our Rafter Calculator handles the math:
- Enter span and pitch
- Get rafter length
- See cut angles
- Get material list
For material estimates, try our Roofing Calculator.
Key Takeaway: For most residential projects with spans under 12 feet, 2x6 @ 16” OC works fine. For spans 12-16 feet, use 2x8. Always check local codes for snow load requirements—they often reduce span tables by 20-30%.