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 | floor joist spacing.

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.

**Floor joist size**

Standard floor size is 2×8 (2 by 8). Others 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.

**Floor joist spacing**

Most standard, floor joist spacing is 16 inches apart on center, but this can vary depending on building codes and the requirements of the structure. So, 12 inches, and 24 inches apart on centre are also used. Floor joists help distribute the load of a structure. The joist which are kept at lower spacing can allow longer distance to span as compared to joist spaced further apart.

**How far apart are floor joists**

Most often, floor joist spacing is 16 inches apart on center used in old houses, on a deck, for a shed and in a mobile home, but this can vary depending on building codes and the requirements of the structure in the blueprint. Floor joists help distribute the load of a structure.

**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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**How do i figure out how many floor joists i need**

To figure out number of floor joists you would need, measure the length of subfloor in feet. Multiply the subfloor length in feet by 12 to convert in inches. Then divide it by spacing i.e 24″ or 16″, 19.2″, or 12″ and add 1 in result to get total number of floor joists you needed.

The simple formula to find number of floor joists are following: floor joists count = ((subfloor length × 12)/16) + 1. This equation will help you to figure how many floor joists you needed. For example, if your subfloor is 10 feet long, you will need atleast 9 floor joists. i.e (10×12)/ 16) +1 = 7.5 +1 = 8.5, round it as 9.

How far apart do floor joists need to be?, typically, floor joists need be spaced 16″ apart in the center. But it can be also spaced 12″ or 24″ or 19.2″ apart. However, floor joists are designed to the individual specification of a particular house. For example, if you have some special application stairs or commercial structure, you can move the floor joists need to be spaced 12″ in the center.

**How many floor joists do I need for a 10×10 deck**

A 10×10 deck measures 10 feet in length and 10 feet in width. A 10 foot long deck with floor joists 16″ on centre will require 9 joists. Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((10 × 12)/16) + 1 = (120/16) + 1 = 7.5 + 1 = 8.5, round it as 9. Thus, you would need atleast 9 floor joists for a 10×10 deck.

**How many floor joists do I need for a 8×8 deck**

A 8×8 deck measures 8 feet in length and 8 feet in width. A 8 foot long deck with floor joists 16″ on centre will require 7 joists. Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((8 × 12)/16) + 1 = (96/16) + 1 = 6 + 1 = 7. Thus, you would need atleast 7 floor joists for a 8×8 deck.

**How many floor joists do I need for a 12×12 deck**

A 12×12 deck measures 12 feet in length and 12 feet in width. A 12 foot long deck with floor joists 16″ on centre will require 10 joists. Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((12 × 12)/16) + 1 = (144/16) + 1 = 9 + 1 = 10. Thus, you would need atleast 10 floor joists for a 12×12 deck.

**How many floor joists do I need for a 16×16 deck**

A 16×16 deck measures 16 feet in length and 16 feet in width. A 16 foot long deck with floor joists 16″ on centre will require 13 joists. Use the formula for floor joists count = ((subfloor length × 12)/16) + 1, and put the span of your deck: joists count = ((16 × 12)/16) + 1 = (192/16) + 1 = 12 + 1 = 13. Thus, you would need atleast 13 floor joists for a 16×16 deck.

**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.