Hardscape calculator
Deck framing calculator: joists, beams and hangers
The part of the deck nobody sees and every inspector checks. Joist count, joist size against the span, beam posts, hangers and blocking.
Your project
The dimension the deck boards run along — usually the side parallel to the house.
Joists run this way, so this is the number that has to fit the joist span table.
16" on centre is the standard. Composite laid diagonally needs 12".
The calculator checks this against the southern pine span table for your spacing.
Required on spans over 8 ft in most codes, and it is what stops a deck feeling bouncy.
Joists at 16" on centre
13
2x10 × 12 ft · span OK
- Maximum span for 2x10 at 16"
- 14.7 ft
- Your joist span
- 12 ft
- Joist lumber total
- 156 lin ft
- Beam lines
- 1 × 2-ply 2x10
- Beam lumber
- 32 lin ft
- Posts and footings
- 3 posts
- Ledger / rim board
- 40 lin ft
- Joist hangers
- 26
- Mid-span blocking
- 17 lin ft
- Structural screws (approx)
- 278
A 2x10 carries 14.7 ft at 16 inch centres, so a 12 ft span is within the table. Local code and species still govern.
The shareable link reopens this calculator with your exact inputs.
Joist count and joist span are two different questions
Count is arithmetic: deck length in inches, divided by the spacing, plus one for the last joist. A 16 foot deck at 16 inch centres takes 13 joists.
Span is structural. The joist has to reach from the ledger to the beam without exceeding the span table for its size, species and spacing. Southern pine 2x8 reaches about 11 feet 11 inches at 16 inch centres, 2x10 about 14 feet 7 inches, 2x12 about 17 feet. Hem-fir and spruce come up roughly a foot shorter in each case.
When the span is too long there are three honest fixes: a deeper joist, tighter spacing, or a beam under the middle of the span. Sistering two undersized joists is not one of them.
Beams, posts and the ledger
A 2-ply beam of the same depth as the joists typically carries 8 feet between posts. More posts means more footings but smaller lumber, and on most residential decks the extra footing is cheaper than jumping to a 3-ply beam.
The ledger is the single most failure-prone part of a deck. It bolts through the rim joist of the house with structural lag screws or through-bolts on a staggered pattern, never with nails or deck screws, and it needs flashing above it. Most collapsed decks come apart at the ledger, not the framing.
Freestanding decks skip the ledger entirely and take a second beam line along the house side. That costs one more row of footings and removes every water-intrusion detail from the wall — which is why it is the better choice on stucco, brick veneer and any house you did not build.
Hangers, blocking and hardware
Every joist end that lands on a ledger or a flush beam gets a hanger, filled with the structural screws or hanger nails the manufacturer lists. Every hole in the hanger is there for a reason and a half-filled hanger carries roughly half its rated load.
Blocking at mid-span keeps joists from twisting and takes most of the bounce out of the finished deck. Codes generally require it over 8 feet, and it is worth fitting even when it is not required.
Use hardware rated for the treatment chemicals in modern pressure-treated lumber — hot-dip galvanised or stainless. Plain zinc hardware in ACQ lumber corrodes visibly within a couple of seasons.
Maximum joist span by size and spacing
Southern pine No. 2, 40 psf live load. Other species run shorter — check local code.
| Joist | 12" on centre | 16" on centre | 24" on centre |
|---|---|---|---|
| 2x6 | 9.9 ft | 9 ft | 7.6 ft |
| 2x8 | 13.1 ft | 11.9 ft | 9.7 ft |
| 2x10 | 16.2 ft | 14.7 ft | 11.9 ft |
| 2x12 | 18 ft | 17 ft | 13.9 ft |
Joist count by deck length
Includes the closing joist at each end.
| Deck length | 12" on centre | 16" on centre | 24" on centre |
|---|---|---|---|
| 8 ft | 9 | 7 | 5 |
| 10 ft | 11 | 8 | 6 |
| 12 ft | 13 | 10 | 7 |
| 16 ft | 17 | 13 | 9 |
| 20 ft | 21 | 16 | 11 |
| 24 ft | 25 | 19 | 13 |
Frequently asked questions
+How many joists do I need for a 12x16 deck?
Thirteen at 16 inch centres if the boards run the 16 foot direction, each spanning 12 feet. A 2x10 handles a 12 foot span at that spacing; a 2x8 does not.
+How far can a 2x8 deck joist span?
About 13 ft 1 in at 12 inch centres, 11 ft 11 in at 16 inch, and 9 ft 8 in at 24 inch, in southern pine. Softer species lose roughly a foot.
+How far can a 2x10 deck joist span?
Roughly 16 ft 2 in at 12 inch centres, 14 ft 7 in at 16 inch, and 11 ft 11 in at 24 inch in southern pine. It is the default joist for most residential decks.
+How far apart should deck beam posts be?
No more than 8 feet apart on a standard 2-ply beam, and closer if the beam is carrying a long joist span. Each post lands on its own footing.
+Do deck joists need blocking?
Over 8 feet of span, most codes require one row at mid-span. Even where it is optional, blocking is the cheapest way to stop a deck feeling springy underfoot.
+Should my deck be freestanding or attached to the house?
Attached is cheaper by one row of footings. Freestanding is safer on brick veneer, stucco or EIFS walls, where a properly flashed ledger is difficult and a badly flashed one rots the wall out.