Free tool Decks Span tables

Deck Joist and Beam Span Calculator

Q: What is the smallest joist for a deck with a railing?

A guarded deck needs at least 2x8 joists.

Enter the span you need, your joist spacing and your beam. The calculator checks them against common Ontario deck span limits for S-P-F lumber and tells you the smallest joist that works.

Short version: At 16 in on centre, S-P-F No.1/No.2 joists at a 40 psf deck load span about 11′-9″ (2×8), 13′-8″ (2×10) and 15′-5″ (2×12). A guarded deck needs at least 2×8 joists. Always use the shorter of these and your township’s deck span table.

Check your joists and beam

Joist span you need (feet, e.g. 12.5)Joist spacingJoist size you plan to useBeam (built-up)Beam pliesPost spacing you plan (feet)

The span limits this calculator uses

Joists: S-P-F No.1/No.2 at a 40 psf residential deck load, with bridging.

Joist	12 in o.c.	16 in o.c.	24 in o.c.	Max cantilever
2×8	12′-6″	11′-9″	10′-8″	16″
2×10	14′-6″	13′-8″	12′-10″	24″
2×12	16′-5″	15′-5″	14′-6″	24″

Beams: built-up S-P-F supporting joists up to 3.6 m (11′-10″), per OBC Table 9.23.4.2.-H.

Beam	2-ply	3-ply	4-ply
2×8	5′-10″	8′-7″	9′-8″
2×10	7′-2″	10′-6″	12′-2″
2×12	8′-4″	12′-2″	14′-2″

Full explanation: Deck framing, joists and beams in Ontario.

Before you build

Your municipality may publish its own deck span tables; use the shorter span. Check footings, ledger attachment and guards too: deck footings, ledger attachment and the deck code checker.

Questions people ask

How far can a 2×8 deck joist span?

About 11′-9″ at 16 in on centre for S-P-F No.1/No.2 at a 40 psf deck load, 12′-6″ at 12 in and 10′-8″ at 24 in.

How far can a 2×10 deck joist span?

About 13′-8″ at 16 in on centre, 14′-6″ at 12 in and 12′-10″ at 24 in for S-P-F No.1/No.2.

What is the smallest joist for a deck with a railing?

A guarded deck needs at least 2×8 joists.

Can I use these spans for my permit?

Use them for planning. Your township may have its own deck span tables, and the shorter span governs.

Related guides on BuildersOntario

Deck framing, joists and beamsThe full span guide Deck code checkerRailing, gaps and stairs Deck building calculatorMaterials and cost Deck permit OntarioWhat to submit

Building in Simcoe County or on Georgian Bay?

We are ICFhome.ca, a family builder in Tiny Township with 45 years of custom homes behind us. Pick the path that matches where you are.

Send plans for review Ballpark estimate Book a call Visit the showhome

Updated November 2026. Planning values for S-P-F No.1/No.2 lumber; confirm against your municipality’s deck tables or an engineer.