When constructing a new home or addition, properly sizing and spacing the floor joists will create a durable, sturdy foundation. Determining the correct joist dimensions and maximum spans between supports prevents problems like floor bounce, sagging, and instability. This guide covers the key factors that go into calculating proper floor joist spans so you can frame a solid, long-lasting floor.
There are several factors to consider when framing a floor for a home building project, including:
- Wood species
- Lumber grade
- Lumber size
- Load on the floor
- Joist span
Wood Species
Different types of wood vary in stiffness and resistance to bending, with some species much stronger than others. Even within the same species, strength variation depends on the tree’s growing conditions.
The lumber from slow-growing trees, which have more end-grain growth rings per inch, is much stronger and denser than faster-growing trees of the same species.
Some common wood species for flooring and framing include:
- High Bending Strength: Southern yellow pine and Douglas fir
- Medium Bending Strength: Hemlock, spruce, and redwood
- Low Bending Strength: Western red cedar, Eastern white pine, and ponderosa pine
Lumber Grade
The grade assigned to the lumber, which indicates quality and strength, is a key factor in determining appropriate joist spans.
Because knots and other defects weaken wood, higher grades of lumber, designated as clear, select, or #1, are stronger than lower grades. However, higher grades are also more expensive. Lumber graded as #2 is the most common choice for floor joists. It has some knots and defects, but usually not enough to significantly reduce bending strength.
Lower grades, designated as #3, utility, and #4, often have too many large knots or defects to be suitable for joists.
Regardless of grade, choose boards with few knots/defects that are straight with minimal crook along the edges.
Position joists with any crook facing up so weight will straighten it. Knots near edges should also face up so they compress rather than stress.
Size of Lumber
A board’s width plays a much larger role than thickness in determining joist span distances.
Doubling joist thickness or number increases span by about 25%. Doubling width increases span 80–100%, even with the same linear footage of lumber.
Load on Floor
The floor’s load also affects the joist size and span length you need.
Dead load is the weight of building materials (joists, subfloor, etc.) and is commonly 10 pounds per square foot. Calculate it by adding material weights and dividing by the square footage covered.
The live load includes furnishings like bookshelves and beds, people, and other items that you may place on the floor. It ranges from 30+ pounds per square foot, with 40 pounds typical for residential rooms and decks.
Increase joist size or reduce spans for heavy loads like pianos or crowded decks.
Joist Span
Considering all the factors, we can give the minimum joist sizes needed for various wood types, grades, sizes, spacing, and floor loads.
Check local building codes before construction. Consult an engineer for unusual situations or nonstandard floor plans.
Below is a table of minimum joist sizes for 16′′ and 24′′ on-center spacing using #2 lumber with 10 psf dead load and 40 psf live load.
| Yellow Pine, Douglas Fir | Redwood emlock, Spruce | Western red cedar, Eastern white pine | ||||
| Joist Size | 16′′ o.c. | 24′′ o.c. | 16′′ o.c. | 24′′ o.c. | 16′′ o.c. | 24′′ o.c. |
| 2×6 | 9′ 9′′ | 8′ 3′′ | 8′ 8′′ | 7′ 6′′ | 7′ 6′′ | 6′ 3′′ |
| 2×8 | 12′ 8′′ | 10′ 8′′ | 11′ 0′′ | 10′ 2′′ | 10′ 5′′ | 8′ 6′′ |
| 2×10 | 16′ 0′′ | 13′ 0′′ | 14′ 6′′ | 12′ 4′′ | 12′ 9′′ | 10′ 5′′ |
| 2×12 | 18′ 6′′ | 15′ 0′′ | 17′ 6′′ | 14′ 4′′ | 14′ 9′′ | 13′ 0′′ |
When in doubt, always use bigger lumber, closer spacing, or smaller spans. Don’t use smaller lumber, wider spacing, or longer spans. Err on the side of caution rather than trying to push the envelope.
So, How Do You Determine Proper Floor Joist Spans?
Calculate maximum spans between supports to prevent sagging, bouncing, and related issues. Consider:
- Wood species/grade — Denser woods like Southern pine allow longer spans than lighter types. Higher grades have fewer defects.
- Joist dimensions — Wider joists can span further than narrower ones.
- Load – Heavier loads need shorter spans than lighter loads.
- Joist spacing — Closer spacing shortens spans compared to wider spacing.
Follow local building codes for minimum joist sizes and spans. Use the span table for safe spans under normal conditions. Consult an engineer if you need to.
Proper joist sizing and spacing prevent problems and guarantee your floor will be strong, stable, and durable.
FAQs About Floor Joists
What is the best wood for joists?
Southern pine and Douglas fir provide high durability for the longest spans. Where unavailable, hemlock and spruce work if your spans are reduced.
Should joists rest on the foundation or sill plate?
Your joists should rest on the sill plate, not directly on the foundation. The sill minimizes moisture transfer.
How deep should single-story home joists be?
For one floor, 2x10s or 2x12s spaced 16 inches apart usually suffice. This is strong enough for spans under 18 feet.
Can trusses substitute for sawn joists?
Yes, engineered floor trusses from smaller lumber can efficiently span long distances between supports. These are often used in production homes.
Should the joist crown be face up or down?
The crown should face up so gravity helps straighten your joists. Install bracing to resist bowing before adding sheathing.
