Roof Rafter Calculator (Canada)

Roof rafter calculator — what this returns

This tool sizes stick-built common rafters for Canadian residential builds and gives you the order quantity for the lumber yard:

• Rafter length per piece, including the eave overhang tail
• Count per slope and total (both sides combined)
• Total linear footage of lumber (or metres if you toggle units)
• Recommended size (2x6 / 2x8 / 2x10 / 2x12) auto-selected for your snow load
• Lumber cost in CAD at 2026 SPF #2 KD pricing, with an 8% waste allowance

The math follows NBC 2020 Section 9.23 prescriptive tables (and their provincial equivalents in OBC, RBQ, BCBC, Alberta, Manitoba) with snow-load and spacing de-rating applied so the size recommendation tracks your actual ground snow load (Ss from NBC Tbl C-2) rather than a generic default.

How rafter sizing works under Canadian practice

Canadian residential rafter design lives at the intersection of: NBC 2020 Section 9.23 prescriptive tables (Deemed-to-Satisfy for typical houses), CSA O86 engineered timber design (Eurocode-equivalent), and provincial overlays — Ontario OBC SB-12, Quebec RBQ regulation 4.1.2, BC Building Code Subsection 9.23, Alberta Building Code 9.23 and Manitoba Building Code 9.23.

Loads. Per NBC 2020 4.1.6, design ground snow load (Ss) and rain-on-snow (Sr) come from Tbl C-2 climatic data — over 700 Canadian cities listed. Permanent (dead) loads from typical roof construction: asphalt shingles + plywood + 6-mil poly + R-50 cellulose + drywall = ~14 psf. Concrete tile = +6 psf. Metal standing-seam = -8 psf (lighter).

Span direction. NBC 9.23 spans are horizontal projection — wall plate to ridge measured flat. The calculator divides full building span by 2 and adds the eave overhang for the actual rafter length, then sizes against the horizontal half-span only.

Lumber. Canadian lumber yards stock SPF (Spruce-Pine-Fir) #2 and Better as the default; Douglas-fir #2 (BC interior mills) and Western Red Cedar are common alternatives. Imperial sizing (2x6, 2x8, 2x10, 2x12) dominates retail; metric is rare except for engineered LVL (38×140, 38×184, 38×235 mm equivalents to 2x6, 2x8, 2x10).

Spacing. 16” or 24” on-center per NBC 9.23.4. The base table is 16”; tighter spacing (12”) gives a ~10% bonus to allowable span; 24” drops it by ~15%.

Worked example — 30 ft × 36 ft Toronto bungalow, 5/12 pitch

Inputs: 30 ft span, 36 ft long, 5/12 pitch, 16” OC, 1.5 kPa (31 psf) snow Toronto, SPF #2, 1 ft overhang.

Computation:

slopeFactor      = √(1 + (5/12)²) = 1.083
half-span run    = 15 ft
rafter length    = (15 + 1) × 1.083 = 17.33 ft each
count per side   = ceil(36 / 1.333) + 1 = 28
total rafters    = 56
total lumber     = 56 × 17.33 = 970 ft
2x12 SPF #2 max  = 22'-6" at 30 psf, 16" OC, 5/12 — derated to 21'-9" at 31 psf
                  → 2x12 covers 15 ft horizontal, fine
                  → 2x10 max 18'-5", derated to 17'-9" — still covers 15 ft, cheaper
                  → calculator picks 2x10 SPF #2
lumber cost      = 970 × CA$2.30 = CA$2,231
with 8% waste    = CA$2,409 → order qty
peak height      = 15 × 5/12 = 6.25 ft above plate

Order 56 + 8% = 60 sticks of 18 ft 2x10 SPF #2 to cover the 17.33 ft each. The mill cuts to 18 ft and you trim at the ridge.

Lumber size table — NBC 9.23 at 2.0 kPa snow, 16” OC, 14 psf dead

Source: NBC 2020 Section 9.23 supplementary tables. Values for L/240 deflection limit, no rafter-tie reduction. Above 22’-9” SPF, engineered TJI or LVL is required.

For higher snow loads:

• 2.5 kPa (Ottawa): multiply allowable span by 0.89
• 3.0 kPa (St. John’s NL, Quebec City lower): multiply by 0.82
• 4.0 kPa (Quebec City higher, mountain BC): multiply by 0.71
• 5.0 kPa+ (Banff, alpine): engineer mandatory

Pricing — Q1 2026 reference data

Big-box (Home Depot Canada, Rona) and pro-yard (Castle, Home Hardware, Timber Mart) pricing for SPF #2 KD lumber:

• 2x6 SPF #2: CA$1.40–$1.55/lf at Home Depot, CA$1.30–$1.45/lf at Castle/Timber Mart
• 2x8 SPF #2: CA$1.80–$1.95/lf
• 2x10 SPF #2: CA$2.25–$2.45/lf
• 2x12 SPF #2: CA$3.00–$3.20/lf

Specials and engineered:

• TJI 240 mm I-joist (Trus Joist, Boise Cascade): CA$22–$28/m
• LVL 240×45 mm (Microllam): CA$28–$34/m
• Glulam 89×235 mm (Nordic): CA$36–$42/m
• Western Red Cedar #2 5/4×6: CA$5.50–$7.20/lf (cathedral exposed work)

For the 30 × 36 ft Toronto example with 2x10 SPF #2, lumber is roughly CA$2,409 (with waste), plus CA$260–$340 for 1x12 ridge board, hurricane ties (Simpson H1 or Marche Plus), framing brackets and bracing. Crew time for a cut roof on this scale: 2 carpenters × 16–20 hours at CA$48–$58/hr.

Trussed-rafter alternative for the same building (Mitek Canada or local truss yard): roughly CA$3,000–$3,400 for the truss order delivered, plus CA$700–$900 for crane and crew on set day. Trusses are clearly cheaper for standard subdivision houses; cut roofs win on heritage Quebec mansard renos and Newfoundland saltbox additions.

Code and incentive references (Canada)

• NBC 2020 Section 9.23 — Wood-frame construction. Tables 9.23.4.1 to 9.23.4.5 for rafter spans by load case.
• NBC Tbl C-2 — Climatic and seismic design data for 700+ Canadian cities.
• OBC 2024 SB-12 — Ontario supplementary standard for energy efficiency. Drives R-50 attic insulation and 2x10 minimum rafter for vented roof assemblies.
• RBQ Loi sur le bâtiment — Quebec mandatory licensing for any framing contractor on projects over CA$25,000. Owner-builder exemption for own residence.
• BCBC 2024 Subsection 9.23 — British Columbia adds Step Code energy tier requirements; rafter sizing carries higher dead loads for spray-foam ceilings.
• Alberta Building Code 2023 — Mirrors NBC 9.23 with provincial overlays for Class 1 residential.
• CSA O86:24 — Engineered design of wood structures. Required when prescriptive tables don’t cover your case.
• Canada Greener Homes Loan — Up to CA$40,000 interest-free for energy upgrades including R-50+ ceiling insulation that pushes you to deeper rafters. Application via Natural Resources Canada.
• Quebec Programme Renoclimat — Provincial subsidy CA$1.20–$2.50/m² for ceiling insulation upgrades. Often pairs with rafter replacement on older stock.
• BC CleanBC Better Homes — Up to CA$5,500 rebate for ceiling insulation upgrades meeting Step Code 4 minimum.
• Ontario Save on Energy — Enbridge / IESO rebates for ceiling insulation; typically CA$1.00/sqft.

Cut roof vs trussed rafter decision

Build rafters on site when:

• Your span is under 22 ft and within prescriptive NBC 9.23 tables — no engineer needed
• You’re framing a vaulted (cathedral) ceiling with exposed rafter feet
• Quebec heritage mansard or gambrel construction (CCQ approval often required)
• The roof has multiple hips, valleys and dormers that don’t fit standard truss profiles

Order trussed rafters when:

• Span is over 24 ft — outside the prescriptive tables either way
• Building length is over 28 ft — labour savings dominate
• You want a clear-span attic with no internal bearing wall
• Winter framing in Quebec / Northern Ontario — trusses go up in 1 day, cut roofs take 4

For typical Canadian new-build single-storey homes (28–32 ft span, 32–40 ft long, 5/12–8/12 pitch), the lumber package costs about the same and trusses install in a third of the labour — the roof truss calculator gives you the side-by-side cost comparison.

Pair with these calculators

• Roof pitch calculator — convert between X/12, degrees and slope factor before specifying.
• Roof truss calculator — get a trussed-rafter quote to compare against your cut-roof estimate.
• Roof area calculator — total roof surface for sheathing, ice-and-water shield and shingle counts on the same job.

When you change any input above, the output updates immediately. Print the page with your inputs and take it to the lumber yard for a take-off quote — Castle Building Centres and Timber Mart both honour pre-priced take-off lists for 14 days.