What size floor joist to span 10′, 12′, 15′, 16′, 18′, 20′, 24′ & 25 feet | what size floor joist to span 10 feet | what size floor joist to span 12 feet | what size floor joist to span 15 feet | What size floor joist to span 20 feet | what size floor joist to span 24 feet | what size floor joist to span 6 feet, 7 feet & 8 feet.

There are different depth of floor joist are used in floor construction such as 2×6, 2×8, 2×10 and 2×12, made of different species of wood (southern yellow pine & Douglas fir) and different grade such as grade – 1, grade – 2, grade – 3. Southern yellow pine is strongest species of wood that can span more distance compared to other species of wood. Grade 1 lumber have higher strength than other grade Lumber due to fewer knots.

Floor joists can be define as several small beams. Floor joists are typically a horizontal structural member that runs across an open space of building on supporting wall or studs. Their function is similar as beams, except floor joists are supported by wall, foundations, or beams framing.

Floor joists usually run between walls or rafters to support a ceiling or floor. They transfer the dead load of the subfloor and floor finishes and live load of people, furniture and furnishings to beams, walls, headers, or foundation walls. It is like the skeleton/ framing of a building that supports the load which the floor is built to bear. Joists are technically beams that typically spans relatively short distances compared to beams and are made of wood, steel, or engineered wood or lumber.

What size floor joist to span 10′, 12′, 15′, 16′, 18′, 20′, 24′ & 25 feet depending on different size of Lumber, species of wood, grade of wood, spacing between the joist, expected dead and live load of structure that coming to joist. The loading includes dead load of the weight of the frame structure & fixed weight and live load of People, furniture, appliances and a snow on the roof.

Not all the type of species of wood created equally strong and type of tree used to make joist affects the span allowed. Southern yellow pine is strongest species of wood that can span more distance compared to other species of wood. Grade 1 lumber have higher strength than other grade Lumber due to fewer knots.

Proper floor joist sizing is specified by the International Residential Code – the IRC. It outlines max span lengths for wood joists of all sizes – from 2×6 to 2×12 and greater. The code also identifies joist sizing and spans for various joist spacing – from 12” up to 24” apart.

Typical floor joist size are 2×6 (2-by-6), 2×8 (2-by-8), 2×10 (2-by-10), 2×12 (2-by-12) and even greater. A 2×6 size of floor joists, can allows joist max span lengths upto 12 feet and 2×12 size of floor joists, can allows joist max span length up to 25 feet. Spacing between floor joist for different sizes are kept at 12 inches, 16 inches and 24 inches apart. The joist which are kept at lower spacing can allow longer distance to span as compared to joist spaced further apart. Standard spacing between floors joist is 16 inches.

**What size floor joist to span 10′, 12′, 15′, 16′, 18′, 20′, 24′ & 25 feet**

What size floor joist to span 10′, 12′, 15′, 16′, 18′, 20′, 24′ & 25 feet depending on different size of Lumber, species of wood, grade of wood, spacing between the joist, expected dead and live load of structure that coming to joist. The loading includes dead load of the weight of the frame structure & fixed weight and live load of People, furniture, appliances and a snow on the roof.

**What size floor joist to span 10 feet**:- as per general rules and guideline, a 2×6 (2-by-6) size of floor joists made of southern yellow pine, graded as #1 can allows joist spans up to 10 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 16 inches apart. Thus you will need a 2×6 (2-by-6) size of floor joists for 10 foot span at standard 16 inches spaced.

**What size floor joist to span 12 feet**:- as per general rules and guideline, a 2×6 (2-by-6) size of floor joists made of southern yellow pine, graded as #1 can allows joist spans up to 12 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 12 inches apart. Thus you will need a 2×6 (2-by-6) size of floor joists for 12 foot span at standard 12 inches spaced.

**What size floor joist to span 15 feet**:- as per general rules and guideline, a 2×8 (2-by-8) size of floor joists made of southern yellow pine, graded as #1 can allows joist spans up to 15 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 16 inches apart. Thus you will need a 2×8 (2-by-8) size of floor joists for 15 foot span at standard 16 inches spaced.

**What size floor joist to span 16 feet**:- as per general rules and guideline, a 2×8 (2-by-8) size of floor joists made of southern yellow pine, graded as #1 can allows joist spans up to 16 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 16 inches apart. Thus you will need a 2×8 (2-by-8) size of floor joists for 16 foot span at standard 12 inches spaced.

**What size floor joist to span 18 feet**:- as per general rules and guideline, a 2×10 (2-by-10) size of floor joists made of southern yellow pine, graded as #1 can allows joist spans up to 18 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 16 inches apart. Thus you will need a 2×10 (2-by-10) size of floor joists for 18 foot span at standard 16 inches spaced.

**What size floor joist to span 20 feet**:- as per general rules and guideline, a 2×10 (2-by-10) size of floor joists made of southern yellow pine, graded as #1 can allows joist spans up to 20 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 12 inches apart. Thus you will need a 2×10 (2-by-10) size of floor joists for 20 foot span at standard 12 inches spaced.

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**What size floor joist to span 24 feet**:- as per general rules and guideline, a 2×12 (2-by-12) size of floor joists made of southern yellow pine, graded as #1 can allows joist spans up to 24 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 16 inches apart. Thus you will need a 2×12 (2-by-12) size of floor joists for 24 foot span at standard 16 inches spaced.

**What size floor joist to span 25 feet**:- as per general rules and guideline, a 2×12 (2-by-12) size of floor joists made of southern yellow pine, graded as #1 can allows joist spans up to 25 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 12 inches apart. Thus you will need a 2×12 (2-by-12) size of floor joists for 25 foot span at standard 12 inches spaced.

**What size floor joist to span 6′, 7′ & 8 feet**:- as per general rules and guideline, a 2×4 (2-by-4) size of floor joists made of southern yellow pine, graded as #1 can allows joist spans up to 6′, 7′ & 8 feet for a live load of 30 pounds per square foot and dead load of 10 pounds per square foot when spaced 16 inches apart. Thus you will need a 2×4 (2-by-4) size of floor joists for 6 foot, 7 foot and 8 foot span at standard 16 inches spaced.

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**Conclusions:-**

A 2×4 (2-by-4) size of floor joists span up to 6′, 7′ & 8 feet, 2×12 (2-by-12) upto 24 foot, 2×10 (2-by-10) upto 20 foot, 2×8 (2-by-8) upto 16 foot & 2×6 (2-by-6) upto 12 foot when spaced 16 inches apart by using southern yellow pine, graded as #1 when live load of 30 lbs/ ft^2.