Standard and code basis
Residential deck design in Canada is governed by the National Building Code of Canada 2020, Part 9 (Housing and Small Buildings). Key requirements:
- Minimum live load: 1.9 kPa (40 psf) for exterior decks and balconies — this is the code floor for any residential outdoor platform in Canada.
- Dead load: Typically 0.5 kPa (10 psf) for decking, joists, beam, and hardware. The calculator uses a combined design load of 50 psf (40 live + 10 dead) for sizing.
- Joist span tables: NBCC Part 9 Table A-1 provides maximum joist spans for common Canadian lumber species and spacings. The calculator uses these tables directly.
- Permit requirement: All decks over 0.6m (2 ft) above grade require a building permit in every Canadian province.
Your inputs — what each field means
| Input | What it represents | How it is used |
|---|---|---|
| Deck length (ft) | The longest dimension of the deck | Combined with width to calculate total deck area and total load. |
| Deck width (ft) | The shorter dimension, measured from the house to the outer beam | Combined with length for area. Also determines the joist span if joists run parallel to the house. |
| Joist span (ft) | The clear span of each deck joist between supports (beam to ledger, or beam to beam) | Used to select joist size from the NBCC span table. Longer spans require larger joists. |
| Beam span (ft) | The distance between posts supporting the outer beam | Used to select beam size based on the tributary load (beam span × joist span × 50 psf). |
| Hot tub | Whether the deck will support a hot tub | A filled hot tub represents a concentrated load of 3,500–8,000+ lbs. This is treated separately as a point load, not a distributed load. |
| Hot tub weight (lbs) | The filled weight of the hot tub (water + shell + occupants) | Converted to PSF over the assumed 16 sq ft footprint. Hot tubs exceeding 100 PSF require engineer review. |
The calculation — step by step
- Calculate total deck load:
total load (lbs) = deck area (sq ft) × 50 psf
Example: 20 ft × 12 ft × 50 psf = 12,000 lbs total load on the structure. - Size the joists from the NBCC span table. The calculator uses NBCC Part 9 joist span table values for SPF lumber at 16″ on centre:
- Joist span ≤ 8 ft → 2×8 @ 16″ o/c
- Joist span ≤ 10 ft → 2×10 @ 16″ o/c
- Joist span ≤ 12 ft → 2×10 @ 12″ o/c
- Joist span ≤ 14 ft → 2×12 @ 16″ o/c
- Joist span > 14 ft → 2×12 @ 12″ o/c (engineer review recommended)
- Size the beam based on tributary load:
beam tributary load (lbs) = 50 psf × joist span × beam span
Beam size is selected from a threshold table:- Load < 3,000 lbs → 3×8 SPF
- Load < 5,000 lbs → 3×10 SPF
- Load < 8,000 lbs → 3×12 SPF
- Load < 12,000 lbs → 4×12 SPF or 3.5″×9.5″ LVL
- Load ≥ 12,000 lbs → Engineer review required
- Hot tub check (if applicable):
PSF = hot tub weight (lbs) ÷ 16 sq ft footprint
A 3,500 lb hot tub = 219 PSF — far exceeding the standard 50 psf deck design load. The calculator flags this and requires engineer review: direct posts under the hot tub footprint are mandatory.
Reading your result
- Total load (lbs): The total weight the deck structure must carry under full live load.
- Joist size: The minimum SPF joist size and spacing to carry the joist span at 50 psf.
- Beam size: The minimum SPF beam or LVL to carry the tributary load over the beam span.
- Hot tub warning: If a hot tub is entered, the calculator shows the PSF and flags whether engineer review is required.
Assumptions and limitations
- Assumes SPF Select Structural lumber. Douglas Fir-Larch has higher Fb and can span farther for the same section; if DF-L is your local species, the results are conservative.
- Joist sizing uses 16″ on-centre spacing. Changing to 12″ o/c increases capacity and is sometimes required for long spans or heavy loads.
- Does not size the posts, footings, or ledger connection. Post size for standard residential decks is typically 4×4 or 6×6 SPF depending on height and load. Footings must extend below frost depth — see the footing table on the calculator page.
- Does not check lateral load resistance (wind, seismic). Decks over 1.5m tall require diagonal bracing or knee-wall design in most provinces.
- The hot tub load check is indicative only. A filled hot tub always requires engineer review for permit submission.
Ready to calculate?
Enter your deck dimensions and get instant joist and beam sizing per NBCC 2020.
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