Floor Joists vs Deck Joists

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Why Floor and Deck Tables Are Not the Same Spreadsheet

Both common rows in this cluster use 40 psf live load and 10 psf dead load and L/360. That is where the similarity ends.

• Service condition. Floors are dry. Decks are wet. Wet lumber has lower published design values. Table R507.6 says wet service factor included.
• Incising. Treated DF-L, Hem-Fir, and SPF are often incised. The deck table includes that factor for those species. Southern Pine is tabulated on its own row.
• Cap and cantilever. Some deck 2x12 cells stop at 18′-0″. The floor table can list a longer dry-service 2x12. Cantilevers are a separate deck column; the calculator returns main span only.
• Grade rows. The floor table lists SS, #1, #2, #3. The deck table is No. 2. Running a Select Structural floor cell on a #2 treated deck joist is reading the wrong book.

Snow country is another fork: R507.6 has 50 / 60 / 70 psf ground-snow rows. This cluster uses the 40 psf live-load deck row only. If your ground snow is higher, the 40 psf cell is the wrong cell.

Same Stick, Two Cells (IRC 2021)

No. 2 grade, 16" on center. Floor = Table R502.3.1(2) at 40/10, L/360. Deck = Table R507.6 at 40 psf live, wet service, no cantilever.

Southern Pine happens to match on several 16" o.c. 2x8/2x10/2x12 cells between these two tables. That is a coincidence of those particular rows, not a rule that “deck equals floor.” DF-L 2x10 at 16" o.c. is the example to remember: 15′-7″ inside, 13′-7″ on the deck.

Why DF-L, Hem-Fir, and SPF Match on the Deck Table

Table R507.6 prints one row for “Douglas fir-larch, Hem-fir, Spruce-pine-fir” with a footnote that the incising factor is included. The floor table prints those species separately, so Hem-Fir #2 and DF-L #2 are different interior numbers and the same exterior number. The calculator repeats that on purpose. If a website gives SPF a different deck span than DF-L, it is not reading the 2021 IRC grouping.

Which Table to Use on the Job

Inside, dry, living area, sawn lumber: floor table. Bedrooms can use the 30 psf table (not in this tool). Heavy finishes may need 20 psf dead load (also not in this tool).

Outside, exposed, treated joists under decking: deck table, 40 psf live row unless snow controls. Then size the deck stairs separately. Ledger, laterals, and post footings are their own R507 sections.

Covered porch that is still wet-service lumber: usually still the deck table. Do not upgrade it to the floor table because there is a roof. Wet service and treatment still apply unless the designer says otherwise.

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