Span Tables
Span tables and framing calculators for joists, rafters, studs and lumber, explained by a structural engineer. A span table tells you how far a piece of lumber can reach between supports before it bends too much or breaks. The tools here show the code table they use and the assumptions behind it, so you can see why a span works, not just whether it does.
Spans
- Deck Joist Span Calculator – IRC deck joist spans for 2x6 to 2x12
- Deck Joist Cantilever – overhang limits and the extra load on the beam
- Rafter Length Calculator – rafter length, overhang and birdsmouth
- Live Load vs Dead Load – the loads every span table is based on
Lumber and walls
- Lumber Dimensions – nominal vs actual sizes in inches and mm
- Stud Calculator – studs, plates, king and jack studs
- Board Foot Calculator – board feet and lumber cost
- Is This Wall Load Bearing – how to tell, and what to do next
- When Do You Need a Structural Engineer – when tables are enough and when they are not
What sets a span
| Factor | Effect |
|---|---|
| Depth of the board | Stiffness grows with depth cubed: a 2x10 is about twice as stiff as a 2x8 |
| Spacing | Closer spacing means each joist carries less, so it can span farther |
| Species and grade | Southern pine and Douglas fir span farther than hem-fir or SPF of the same grade |
| Load | Floors (40 psf), decks (40 psf), sleeping rooms (30 psf) and attics differ |
| Deflection limit | Floors and decks are usually limited to span ÷ 360 |
| Wet or dry use | Outdoor (wet) lumber is weaker, so deck spans are shorter than indoor floor spans |
Spans are measured as the clear distance between supports, not the total board length.
Engineer's note: a span table is a shortcut through two checks: strength (will it break) and stiffness (will it bounce). For long floor spans, stiffness almost always governs, which is why a floor can be strong enough and still feel springy. When a span is close to the table limit, going one size deeper or adding blocking costs little and makes a noticeable difference.
Everything in this section
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Maintained by: Engr. Abdul Latif, civil and structural engineer. Every calculator here shows its formula and the code table it follows. For planning only; confirm with your local building department.