How do you frame an L-shaped deck?
Where the joists stop at the inside corner, they need a beam under them. Most mistakes start there.
Think of it as one rectangle with a corner taken out. The joists run front to back as normal, but the ones in the cut-out part are shorter and stop at the inside edge of the cut.
Those short joists need something to land on. That means a beam along the inside edge of the cut, with a post at each end. Leave it out and the corner of the deck is hanging off nothing, which is the bit people notice bouncing a year later.
Boards can run straight across both legs of the L. You don't need a mitred join at the corner unless you want the look, and a straight run wastes less.
Check before you buy or dig
- Count the posts on the inside-corner beam as well as the outer ones.
- The longest joist span sets the joist size, even if most joists are shorter.
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Open an L-shaped deck in 3DA general answer, not a structural design. Local rules, ground and loads decide the final build.