A deck frame is a system, not a parts list. Joist span sets beam load. Beam span sets post count. Post count sets footing count, and the ledger ties the whole thing to the house. Sizing the members one at a time is how decks end up with a stout beam on undersized posts. This calculator frames the deck as one system so the pieces agree with each other.
Open the CalculatorJoists. Size and spacing by species and grade under wet-service conditions, including allowable cantilever past the beam.
Beam. Ply build-up and span from the joist layout above it, not from a generic table row.
Posts. Size and maximum height from the load each post actually collects, which depends on everything upstream.
Ledger. Fastener type and spacing per the attachment tables, plus the lateral load connection the code requires and inspectors increasingly check.
Guard posts. Flagged where they land, because a guard post connection that only works in theory is the most common correction on a deck inspection.
Change joist spacing from 16 to 12 inches and the decking requirement relaxes, the beam load per foot rises slightly, and the label on your joist hangers changes. Stretch the beam span two feet and each post picks up several hundred pounds, which can change the footing size in soil that was already marginal. None of these interactions are hard. All of them are easy to miss when each member is sized on a different web page. The calculator carries every change through the whole frame.
A 12 by 16 deck, attached with a ledger on the 16-foot side, 40 psf live and 10 psf dead.
Enter the footprint, species, and decking direction. The calculator returns a complete frame: joist size and spacing, beam build-up and post positions, post size, ledger fastening schedule, and the load delivered to each footing, ready to check against your soil bearing. Change any single input and the full frame re-sizes.
For common species at 16 inches on center under standard deck loads, a 12-foot clear span typically lands at 2x8 or 2x10 depending on species and grade, before cantilever. Run your actual species and layout rather than borrowing a neighbor’s answer; SPF and Southern Pine do not span the same.
As many as the beam span requires, and post count is usually cheaper to add than beam size. Footings are typically the expensive unit, so the real optimization is beam build-up against footing count. The calculator prices that trade in members, not money, and your lumber quote finishes the job.
Not directly. Veneer is not a structural attachment surface. Those cases need a free-standing deck or an engineered standoff detail, and the calculator will flag the condition rather than pretend a fastener schedule fixes it.
It is the tension tie that keeps the deck from pulling off the house, separate from the ledger bolts that carry gravity load. Current code requires it on ledger-attached decks, and hardware for it is inexpensive relative to what it prevents.
Yes. Removing the ledger adds a beam line and changes the bracing requirement, and the frame re-sizes accordingly.
Results from this calculator are preliminary sizing guidance for comparison and planning. They are not engineering, not sealed drawings, and not a substitute for design by a licensed professional under your local code. Final member sizing, connections, and load paths must be confirmed for your specific project.
Bardowl Design produces structural layouts and material specifications that builders and lumberyards can actually work from. Clear scope, defined assumptions, documented output.
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