What are Floor Joists?
Floor joists are horizontal structural members that span between walls or beams to support the floor above. They transfer the load of the floor, furnishings, and occupants down to the foundation or supporting walls. Joists are typically made from dimensional lumber (2x6 through 2x12), engineered wood I-joists, or floor trusses.
Proper joist sizing and spacing are critical to preventing floor bounce, squeaking, and structural failure. Building codes specify minimum joist sizes based on the span length, spacing, wood species, grade, and expected load conditions.
Joist Count Formula
The "+1" accounts for the end joist. Additionally, two rim joists (also called band joists) are needed, one at each end of the floor system, running perpendicular to the common joists.
Maximum Span Tables
Maximum allowable spans for Douglas Fir-Larch #2 grade lumber at 40 psf live load:
| Joist Size | 12" O.C. | 16" O.C. | 24" O.C. |
|---|---|---|---|
| 2x6 | 10' 11" | 9' 11" | 8' 8" |
| 2x8 | 14' 5" | 13' 1" | 11' 5" |
| 2x10 | 18' 5" | 16' 9" | 14' 7" |
| 2x12 | 22' 5" | 20' 4" | 17' 9" |
Joist Spacing Explained
- 12" O.C.: Provides the strongest floor with minimal bounce. Used for heavy loads such as stone tile or heavy furniture.
- 16" O.C.: The most common spacing for residential construction. Provides good strength and accommodates standard 4x8 subfloor panels.
- 19.2" O.C.: Economical spacing that still works with standard 8-foot panels (five spaces of 19.2" equals exactly 8 feet).
- 24" O.C.: Used for lighter loads or with larger joist sizes. Common in floor systems using engineered I-joists.
Worked Example
A floor 20 feet long and 12 feet wide with 2x10 joists at 16" O.C.:
Plus 2 rim joists at 20 feet each. Total lumber: 16 joists at 12 ft + 2 rim joists at 20 ft.
Frequently Asked Questions
Can I use a smaller joist size with closer spacing?
Yes, closer spacing with smaller joists can achieve the same load capacity as wider spacing with larger joists. However, this uses more lumber. Always verify with span tables and local building codes.
What is blocking between joists?
Blocking consists of short pieces of lumber installed between joists to prevent twisting and distribute loads. Most codes require blocking at bearing points and at midspan for joists deeper than 2x12. Blocking also stiffens the floor and reduces bounce.
Do I need a beam for long spans?
If the floor width exceeds the maximum span for your chosen joist size and spacing, you will need a support beam at midspan. This effectively halves the joist span, allowing smaller or more widely spaced joists.
What is the difference between a joist and a beam?
Joists are repetitive, closely spaced framing members that support the floor deck. Beams are larger structural members that support the joists themselves. Beams carry concentrated loads from multiple joists down to posts or columns.