Deck frame anatomy
A deck frame is a load path: decking to joists to beam to posts to footings, with the ledger taking half the load back to the house. Every member has one job, and confusing the jobs is how frames get built wrong.
The members and their jobs
| Member | Carries | Decided by |
|---|---|---|
| Ledger | Half the deck load, into the house band joist | Attachment table + flashing detail |
| Joists | Decking load, spanning to beam and ledger | Span table (size, spacing, species) |
| Rim joist | Aligns joist ends, carries guard posts | Arithmetic + blocking for posts |
| Beam | Joist loads, spanning to posts | Span table (size, span, species) |
| Posts | Beam loads, down to footings | Load + height + bracing |
| Footings | Post loads into soil, below frost | Bearing capacity + frost depth |
| Blocking | Prevents joist rotation and lateral movement | Arithmetic + diaphragm design |
| Hangers | Joist-to-ledger and joist-to-beam connections | Manufacturer table by joist size |
Which decisions are arithmetic
- Joist count from spacing and width — see the joist calculator.
- Blocking rows from span and load direction.
- Footing concrete volume from diameter and depth.
- Fastener count from a spacing you were given.
- Railing post and baluster counts.
Which decisions are not
- Joist and beam size. Depends on species, grade, spacing, span and load. A span table answers it; arithmetic does not.
- Footing size. Depends on the load and the soil's bearing capacity, which is a site-specific number.
- Ledger fastener schedule. Depends on the wall assembly and the fastener, and it is tabulated.
- Lateral bracing. Depends on the geometry and the connections; the code gives a load, not a detail.
The distinction matters because a calculator that produces a joist size is producing a number it cannot justify. That is why the span page here explains tables rather than computing spans.
Where frames actually go wrong
- The ledger is not flashed. Water gets behind it, the band joist rots, and the ledger has nothing to hold. This is the most common serious defect on an existing deck.
- Guard posts are bolted to the rim only. The rim is 1½″ thick and the post has a 36″ lever arm. Without blocking or a through-bolt to a joist, the post moves.
- Blocking is omitted. Joists rotate under load, the decking moves, and the frame loses its diaphragm action — which is what resists racking.
- Beam is bolted to the side of the post rather than bearing on top of it, so the bolts carry the load in shear instead of the post in compression.
- No lateral bracing. Covered above, and it is what most collapses trace back to.
What this leaves to the engineer
- It does not size any member. That is deliberate.
- It does not cover deck-to-wall lateral connectors in detail — those are manufacturer-specific.
- It does not cover cantilevers beyond pointing to the cantilever page.
- It does not replace a framing plan for a permit.
Questions people actually ask
What are the parts of a deck frame called?
Ledger against the house, joists spanning from it to a beam, a beam spanning between posts, posts down to footings, rim joists at the edges, blocking between joists, and hangers at the connections.
Does the beam go on top of the posts or bolted to the side?
On top is better. A beam bearing on a post transfers load in compression; a beam bolted to the side of a post puts the bolts in shear and relies on them for the entire load.
Why does a deck need blocking?
Blocking stops joists rotating under load and ties the frame into a diaphragm that resists racking. Without it the deck feels springy and the frame moves sideways.
Can I build a deck without a ledger?
Yes, as a freestanding deck with posts and beams on all sides. That is the correct approach when the house wall cannot take a ledger, such as brick veneer.