Rafter Span TableComplete Canadian Roof Framing Reference
Comprehensive rafter span tables for Canadian residential roof framing. Find maximum allowable spans for every common rafter size, spacing, wood species, and snow load condition per the National Building Code of Canada.
Quick Answer
A 2x8 SPF #2 rafter at 16" OC can span up to approximately 14'-3" (4.34 m) for a 1.0 kPa (20 psf) live load (light snow). For a 1.5 kPa (30 psf) ground snow load, the span drops to about 12'-10" (3.91 m). A 2x6 rafter at 16" OC spans up to 11'-9" (3.58 m) under light loads.
Understanding Rafter Spans
A rafter span is the horizontal projection distance a rafter can cover between its bearing point at the wall top plate and the ridge board or ridge beam. Rafter spans are measured as the horizontal run, not along the slope. The actual rafter length is longer than the span because it follows the roof pitch.
Rafter design in Canada considers two primary loads: the dead load (roofing material, sheathing, insulation — typically 0.5 to 0.75 kPa depending on covering) and the live load, which includes snow load and the minimum 1.0 kPa construction/maintenance load. Snow load is the dominant design factor in most of Canada.
The NBC provides span tables for different load conditions. Snow load varies dramatically across Canada — from 1.0 kPa in southern coastal BC to over 6.0 kPa in mountain communities. The ground snow load (Ss) for your location is listed in NBC Appendix C, Table C-2. The roof snow load is calculated using shape, exposure, and thermal factors per NBC Part 4.
Rafter Span Tables by Species (1.0 kPa Live Load, 0.5 kPa Dead Load)
The following tables are based on NBC Part 9 for rafters with a 1.0 kPa (20 psf) live load and 0.5 kPa dead load. Spans are the maximum horizontal projection. For higher snow loads, see the reduction factors below.
Spruce-Pine-Fir (SPF)
#2 Grade — standard Canadian framing lumber
| Rafter Size | 12" OC (300 mm) | 16" OC (400 mm) | 24" OC (600 mm) |
|---|---|---|---|
| 2x4 | 9'-0" (2.74 m) | 7'-10" (2.39 m) | 6'-5" (1.96 m) |
| 2x6 | 14'-2" (4.32 m) | 11'-9" (3.58 m) | 9'-8" (2.95 m) |
| 2x8 | 18'-8" (5.69 m) | 14'-3" (4.34 m) | 11'-8" (3.56 m) |
| 2x10 | 20'-2" (6.15 m) | 17'-3" (5.26 m) | 14'-1" (4.29 m) |
| 2x12 | 23'-3" (7.09 m) | 20'-0" (6.10 m) | 16'-4" (4.98 m) |
Douglas Fir-Larch
#2 Grade — common in British Columbia
| Rafter Size | 12" OC (300 mm) | 16" OC (400 mm) | 24" OC (600 mm) |
|---|---|---|---|
| 2x4 | 9'-6" (2.90 m) | 8'-3" (2.51 m) | 6'-9" (2.06 m) |
| 2x6 | 14'-11" (4.55 m) | 12'-10" (3.91 m) | 10'-7" (3.23 m) |
| 2x8 | 19'-8" (5.99 m) | 15'-0" (4.57 m) | 12'-4" (3.76 m) |
| 2x10 | 21'-3" (6.48 m) | 18'-2" (5.54 m) | 14'-11" (4.55 m) |
| 2x12 | 24'-6" (7.47 m) | 21'-1" (6.43 m) | 17'-3" (5.26 m) |
Hem-Fir
#2 Grade — common in BC and Pacific Northwest
| Rafter Size | 12" OC (300 mm) | 16" OC (400 mm) | 24" OC (600 mm) |
|---|---|---|---|
| 2x4 | 8'-9" (2.67 m) | 7'-7" (2.31 m) | 6'-3" (1.91 m) |
| 2x6 | 13'-9" (4.19 m) | 11'-10" (3.61 m) | 9'-9" (2.97 m) |
| 2x8 | 18'-2" (5.54 m) | 13'-10" (4.22 m) | 11'-4" (3.45 m) |
| 2x10 | 19'-8" (5.99 m) | 16'-10" (5.13 m) | 13'-9" (4.19 m) |
| 2x12 | 22'-8" (6.91 m) | 19'-6" (5.94 m) | 15'-11" (4.85 m) |
Snow Load Impact on Rafter Spans in Canada
Snow load is the dominant design factor for Canadian roofs. The ground snow load (Ss) varies dramatically across the country and directly reduces allowable rafter spans.
- Light snow (Ss = 1.0 kPa / 20 psf): Coastal southern BC (Victoria, Vancouver lowlands) — standard span tables apply
- Moderate snow (Ss = 1.5–2.0 kPa / 30–40 psf): Southern Ontario, southern Quebec, Prairies lowlands — span reductions of 10–15%
- Heavy snow (Ss = 2.5–4.0 kPa / 50–80 psf): Northern Ontario, northern Quebec, BC interior, mountain communities — span reductions of 15–25%
- Extreme snow (Ss = 4.0–6.0+ kPa / 80–125+ psf): Revelstoke, Whistler, high-altitude communities — span reductions of 25–40% or engineered solutions required
- Altitude correction: higher elevations receive more snow. Check NBC Appendix C, Table C-2 for the specific Ss value for your municipality.
- Roof slope reduces snow: pitches above 6:12 (26.6°) shed snow more effectively. NBC provides shape coefficients reducing the roof snow load for steep pitches. However, sliding snow creates concentrated loads at lower roofs and valleys.
Always design rafters for the ground snow load specified by your municipality. Undersizing rafters for snow load is one of the most common and dangerous framing errors in Canada.
How Roof Pitch Affects Rafter Sizing
Roof pitch affects rafter design in multiple ways: actual rafter length, load distribution, snow shedding, and the need for collar ties or ridge beams.
- Low pitch (2:12 to 4:12): Rafters carry load almost like a floor joist. High horizontal thrust at the wall plate requires adequate ceiling joists or collar ties. Limited choice of roofing — mainly membrane or metal.
- Medium pitch (5:12 to 8:12): Most Canadian residential roofs fall in this range. Good for asphalt shingles, metal, and some tiles. Ceiling joists or collar ties still required unless a structural ridge beam is used.
- Steep pitch (9:12 to 12:12): Excellent snow shedding — important in heavy snow regions. Actual rafter is much longer than the horizontal span, increasing material cost. Wind uplift becomes a concern.
- Pitch multipliers for rafter length: 4:12 = 1.054, 6:12 = 1.118, 8:12 = 1.202, 10:12 = 1.302, 12:12 = 1.414.
- Structural ridge beam: eliminates the need for ceiling joists. Required for cathedral ceiling (open rafter) designs. Must be engineered per CSA O86 for the specific loads.
- Collar ties vs rafter ties: Collar ties in the upper third prevent ridge separation. Rafter ties (ceiling joists) at the plate level resist outward thrust. Most conventional roofs need rafter ties; collar ties alone are not a substitute.
For most Canadian residential roofs (4:12 to 8:12 pitch), NBC span tables can be used directly. For cathedral ceilings, a structural ridge beam designed per CSA O86 is required.
NBC & CSA Code References for Rafters
The following NBC and CSA sections govern rafter design in Canadian residential construction.
- NBC Part 9 — Rafter span tables: Maximum spans for various species, grades, sizes, spacings, and load conditions including snow load
- NBC Part 4 — Structural design: Loads, load combinations, and structural requirements for non-Part 9 buildings
- NBC Appendix C, Table C-2 — Climatic design data: Ground snow loads (Ss) for every Canadian municipality
- CSA O86 — Engineering Design in Wood: Engineering calculations for rafters, trusses, and ridge beams beyond span table limits
- NBC 9.23.4 — Roof framing: Requirements for ridge boards, collar ties, rafter ties, and connections
- NBC 9.26.2 — Roof sheathing: Minimum sheathing thickness based on rafter spacing
NBC Part 9 span tables are the practical reference for most residential work. For engineered trusses, long spans, or unusual geometries, CSA O86 calculations by a professional engineer are required.
Calculate Your Roof Framing Materials
Once you know your rafter size and spacing, use our calculators to determine how many rafters, how much sheathing, and what fasteners you need.