How far can a Double (2-ply) 2 × 8 beam span carrying 12 ft joists?
| Maximum beam span, post to post | 5′ 11″ |
|---|---|
| Maximum overhang past the postone quarter of the actual span, not of this maximum | 1′ 6″ |
| Posts needed across a 20 ft side | 5posts |
| Posts needed across a 30 ft side | 7posts |
| Species groupthe strongest of the common deck species and the usual choice in the south-east; it gets its own column in DCA 6 | Southern Pine |
| — Same size, other ply counts — | |
| Double 2x8the beam on this page | 5′ 11″ |
| Triple 2x8 | 7′ 6″ |
| — Other sizes, double — | |
| double 2x6 | 4′ 7″ |
| double 2x8 | 5′ 11″ |
| double 2x10 | 7′ 1″ |
| double 2x12 | 8′ 4″ |
| — Same beam, other joist spans — | |
| Joists spanning 6 ft | 8′ 6″ |
| Joists spanning 8 ft | 7′ 4″ |
| Joists spanning 10 ft | 6′ 6″ |
| Joists spanning 12 ft | 5′ 11″ |
| Joists spanning 14 ft | 5′ 6″ |
| Joists spanning 16 ft | 5′ 1″ |
| Joists spanning 18 ft | 4′ 9″ |
Notes
- The column is the joist span, not the load width. This page answers for joists that span 12 feet. A common mistake is to halve the joist span first because the beam only carries half of it — the table has already done that. Read the column that matches the joists you are actually building.
- Adding a ply is usually cheaper than going up a size. A triple 2x8 reaches 7′ 6″ against 5′ 11″ for the double — 1′ 7″ more span for one extra board. Going up to a double 2x10 instead reaches 7′ 1″.
- Species is worth a foot. The same beam is 5′ 11″ in Southern Pine and 4′ 7″ in the Douglas Fir / cedar group. Buying whatever the yard has in stock quietly changes the post spacing you are allowed.
- Joists from both sides double the column. DCA 6 assumes joists bear from one side only. If joists frame in symmetrically from opposite sides of the same beam, look up twice the joist span — 12 ft joists from both sides means reading the beyond the 18 ft column, which is off the table and needs a designer.
- Four assumptions, and all four have to hold. 40 psf live plus 10 psf dead, No. 2 grade, wet service, single level. A hot tub, a roof over the deck, a planter wall, or a high-snow jurisdiction all put you outside this table entirely.
- The building official has the last word. Local amendments to the IRC are common and are frequently stricter. Pull the permit before buying lumber — a beam sized from the wrong column fails with people standing on it.
A Double (2-ply) 2 × 8 carrying 12 ft joists
5′ 11″ between posts. Overhang past the post is capped at 1′ 6″.
The column people read wrong
The table column is the joist span — how far the joists run — not the load width. The beam only carries half that load, and the table has already accounted for it. Halving the joist span before looking it up is the most common way to oversize a beam, and doubling it is the most common way to undersize one.
The one case where you do double: joists framing in symmetrically from both sides of the same beam. DCA 6 assumes joists bear from one side only, so two sides means reading the column for twice the joist span.
Ply or size
Adding a ply and going up a size both buy span, and they cost differently. A third ply is one more board of the same length. Going up a size changes every board, and often changes the hangers and the ledger detail with it.
The table above compares both at this joist span, in this species. Read the two rows against what the lumber yard actually charges — the cheaper answer is not the same in every market.
What this table does not cover
Four assumptions carry the whole thing: 40 psf live plus 10 psf dead, No. 2 grade, wet service conditions, single-level deck. A hot tub, a roof, a planter wall, or a jurisdiction with higher snow load all put the deck outside it.
Beyond a triple, DCA 6 stops. The next steps are a shorter beam span with an extra post, or a glued laminated beam under Table 3B — not a fourth ply.
Nearby sizes
- How far can a Triple (3-ply) 2 × 8 beam span carrying 12 ft joists?
- How far can a Double (2-ply) 2 × 6 beam span carrying 12 ft joists?
- How far can a Double (2-ply) 2 × 10 beam span carrying 12 ft joists?
- How far can a Double (2-ply) 2 × 12 beam span carrying 12 ft joists?
- How far can a Double (2-ply) 2 × 8 beam span carrying 12 ft joists?
- How far can a Double (2-ply) 2 × 8 beam span carrying 10 ft joists?
- How far can a Double (2-ply) 2 × 8 beam span carrying 14 ft joists?
- How far can a Double (2-ply) 2 × 8 beam span carrying 8 ft joists?
- How far can a Double (2-ply) 2 × 8 beam span carrying 16 ft joists?
Read more
- How to Tell if a Deck Is Unsafe: The Seven Checks, in Order of What Kills PeopleDeck collapses are almost never a broken board. They are the ledger letting go, all at once, with everyone standing on it.
- Deck Post Size by Height: When 4×4 Stops Being AllowedThe 8 ft line is where 4×4 stops. Above it the question is not only size but bracing, and the connection at each end matters more than the post between them.
- Deck Beam Span Table: Why the Span Depends on the Joists, Not the DeckA beam span table has two inputs, and the one people forget is the joist span the beam is carrying. Read it with one input and the answer is wrong by feet.
- Deck Joist Cantilever: One Quarter of the Actual Span, Not the MaximumThis is the most misread line in the deck code, and it is misread in the direction that puts the beam too far in.