Deck calculator: boards, joists, footings and balusters
One row per deck surface: its length along the house and its projection. Enter your boards, spacing, footings and guard: the calculation counts the rest. The span tables and the rules sit beside it, as references.
Joist spans: IRC Table R507.6 (reference)
Maximum deck joist spans in the US at a 40 psf live load, in feet-inches, by joist spacing. Canada has no deck-specific table: the NBC points to Span Tables 9.23.4.2 (floor joists in residential use), and a deck serving one dwelling carries the roof snow load or 1.9 kPa, whichever is greater (9.4.2.3).
| Species and size | 12 in | 16 in | 24 in |
|---|---|---|---|
| Southern pine 2x8 | 13-1 | 11-10 | 9-8 |
| Southern pine 2x10 | 16-2 | 14-0 | 11-5 |
| Douglas fir-larch, hem-fir or spruce-pine-fir 2x8 | 12-6 | 11-1 | 9-1 |
| Douglas fir-larch, hem-fir or spruce-pine-fir 2x10 | 15-8 | 13-7 | 11-1 |
Sources: IRC 2021, R507; NBC 2020, Part 9
AWC DCA 6 (a separate reference)
The American Wood Council's prescriptive guide, based on the 2015 IRC. It has its own tables, which sometimes differ from the IRC: where they differ, the IRC applies.
| DCA 6 | |
|---|---|
| Posts | 6x6 or larger |
| Footings | At least 12 in below undisturbed ground or below the frost line, whichever is deeper; within 5 ft of the house, at the house footing level |
| Stringers | At least 2x12; at least three cut stringers; at most 18 in on center on stairs wider than 36 in |
Source: AWC DCA 6
Footing depth (reference)
As read. Also check the frost depth your city recognizes.
| Rule as read | |
|---|---|
| IRC 2021, R507.3 | At least 12 in below undisturbed ground, and below the frost line when the deck is attached to a frost-protected structure; no footings for a freestanding deck of 200 sq ft or less, 20 in or less above grade, on precast pier blocks without beams or posts |
| NBC 2020, Table 9.12.2.2 | By soil and drainage: 1.2 m (4 ft), or below the frost depth |
| NBC 2020, 9.12.2.2.(7) | Not required for a deck of 1 storey or less, 55 m² (592 sq ft) or less, joists 600 mm (24 in) or less above the ground, no roof and not attached |
Sources: IRC 2021, R507; NBC 2020, Part 9
Guards (reference)
As read, in the US and in Canada.
| IRC 2021 (US) | NBC 2020 (Canada) | |
|---|---|---|
| Required | More than 30 in up within 36 in of the edge | A drop of more than 600 mm within 1.2 m of the edge |
| Height | 36 in; 34 in along stairs | 1 070 mm; 900 mm for an exterior guard serving one dwelling where the deck is 1 800 mm or less above the ground |
| Openings | A 4 in sphere does not pass | A 100 mm sphere does not pass |
| Climbing | No such rule read | The rule against climbable members applies only to guards more than 4.2 m up |
Sources: IRC 2021, R312.1 and R311.7; NBC 2020, Part 9; NBC 2020, 9.8
Composite boards: the makers (reference)
What the installation guides say, cited as a guide: your spacing, gap and fasteners stay yours.
| Trex | TimberTech | |
|---|---|---|
| Joist spacing | 16 in for 1x6 boards (US chart); 4 in less at 45 degrees | 16 in at most; 12 in for angled boards |
| Gap between boards | 3/16 in (5 mm) | 1/8 to 3/16 in |
| Fasteners | Two screws per joist | Per the fastening system |
| Lengths | 12, 16 and 20 ft | Per the product |
Sources: Trex 2026 guide; TimberTech composite guide
Wood boards (reference)
For a wood deck, as read.
| Rule as read | |
|---|---|
| Joist spacing | IRC R507.7, 5/4 boards: 12 in on a single span, 16 in on several, 8 or 12 in diagonal |
| Gap between boards | About 1/8 in on a finished deck; damp boards butted, they shrink as they dry (Wood Preservation Canada) |
| Fasteners | At least two nails or screws at each joist (IRC); 2 screws at each joist and 2 at each end (Wood Preservation Canada) |
Sources: IRC 2021, R507; Wood Preservation Canada, building tips
How it is calculated
For each surface: board rows = projection / (width + gap), rounded up; decking length = rows x length x (1 + waste); boards = decking length / length bought, rounded up at the total. Joists = length / spacing, rounded up, + 1; joist length = joists x projection; fasteners = rows x joists x fasteners per crossing. Posts = beam lines x (length / spacing, rounded up, + 1); concrete per footing = PI x diameter² / 4 x depth; bags = posts x concrete / bag yield, rounded up at the total. Balusters = guard length / (width + gap), rounded up. No span or load is checked, and nothing comes filled in: these are your values.
The whole job in MesuPro
Deck survey
Surfaces and decking, joists, hangers and fasteners, beams, posts and concrete, guards, stairs and the old deck.
Feet and inches by default
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