Deck framing calculator
Joists, blocking, rim, ledger and hangers — counted from the frame geometry rather than a rule of thumb.
- Ledger attachment and flashing is the most common point of deck failure. The fastener schedule and flashing detail must come from AWC DCA6 or your local building department, not from a calculator.
- Span limits are not yet verified against AWC DCA6 and are withheld. Size your joists and beams with your local building department.
Framing
| Item | Qty | Unit |
|---|---|---|
| 2x8 joists, 12 ft Joists run the 12 ft width, spaced 16 in on centre along the 16 ft length: floor(192 in / 16 in) + 1 = 13. The +1 is the joist that closes the far end. | 13 | each |
| 2x8 rim joist, 16 ft One rim joist closes the outboard edge, running the full length of the deck. | 1 | each |
| 2x8 ledger board, 16 ft The ledger carries the house-side end of every joist and runs the full length. | 1 | each |
| 2x8 blocking, cut to fit between joists A 12 ft joist span gets 1 row(s) of blocking (one per 8 ft of span). Each row needs a piece in each of the 12 bays: 1 x 12 = 12. | 12 | each |
Hardware
| Item | Qty | Unit |
|---|---|---|
| Joist hangers One hanger per joist where it meets the ledger: 13 joists = 13 hangers. | 13 | each |
How this calculator works
Why joists are floor(span / spacing) + 1
Joists are spaced on centre along the length of the deck. Dividing the span by the spacing counts the gaps between joists, not the joists themselves. The +1 is the final joist that closes the far end — miss it and you are one joist short on every deck you build.
On centre means centre to centre
Sixteen inches on centre is measured from the middle of one joist to the middle of the next, not the clear gap between them. The clear space between 1.5 in joists at 16 in OC is 14.5 in. Decking span ratings are quoted on-centre, so this is the number that matters.
When blocking gets added
Long joists can twist under load. This calculator adds a row of blocking for each 8 ft of joist span, fitted between every pair of joists. A 12 ft deep deck gets one row; a 16 ft deck gets two.
The ledger is the dangerous part
Where a deck is attached to a house, the ledger carries one end of every joist. Ledger attachment and flashing is the most common point of deck failure, and the fastener schedule depends on your joist span, ledger size and wall construction. This tool counts hangers; it does not tell you how to bolt the ledger on. Get that from AWC DCA6 or your building department.
What we will not guess
Joist and beam span limits are structural-safety data. Until our span tables are verified line by line against the published AWC DCA6 tables, this site withholds them and says so, rather than printing a number that looks authoritative. Footing depth must reach below your local frost line, which varies from nothing in the far south to over eight feet in the far north. Confirm both with your local building department — see our methodology.
Other calculators
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