What size lumber do you need for 20′, 16′, 18′, 10′, 12′ & 22 feet span depends on many factors, dead load, live load, snow load, wood species, grade of lumber, construction requirements and local building code. Different dimension of structural lumber used for different span.

There are different depth of lumber as a 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/ lumber 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.

Not all the type of species of wood/ lumber 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.

Spacing between floor joist for different sizes of lumber 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.

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**What size lumber for 20′, 16′, 18′, 12′, 22′, 24′, 25′, 26′ & 30 feet span**

In general, a 2×6 lumber may span from 10 to 13 feet, while a 2×8 lumber may span from 13 to 16 feet. Moreover, a 2×10 lumber may span from 16 to 20 feet, while a 2×12 lumber may span from 20 to 27 feet. In addition, a 2×4 lumber may span from 7 to 9 feet.

**What size lumber for a 20 foot span**

The size of lumber required for a 20-foot span need to be 2×12 at 16″ oc, which includes a 12 inch depth and a 2 inch width. Thus, a 2×12 size of lumber can allow to span up to 20 feet.

**What size lumber for a 10 foot span**

The size of lumber required for a 10-foot span need to be 2×6 at 16″ oc, which includes a 6 inch depth and a 2 inch width. Thus, a 2×6 size of lumber can allow to span up to 10 feet.

**What size lumber for a 12 foot span**

The size of lumber required for a 12-foot span need to be 2×8 at 16″ oc, which includes an 8 inch depth and a 2 inch width. Thus, an 2×8 size of lumber can allow to span up to 12 feet.

**What size lumber for a 14 foot span**

For a 14 foot span, you will need atleast 2×8 size of lumber/ floor joist when spaced at 16″ apart. Thus, a 2×8 size of lumber can allow to span 14 feet. For a 14-foot spans, the lumber has to be at least 8 inches in depth used as floor joist.

**What size lumber for a 15 foot span**

For a 15 foot span, you will need atleast 2×8 size of lumber/ floor joist when spaced at 12″ apart. Thus, a 2×8 size of lumber can allow to span 14 feet. For a 14-foot spans, the lumber has to be at least 8 inches in depth used as floor joist.

**What size lumber for a 16 foot span**

For a 16 foot span, you will need atleast 2×10 size of lumber/ floor joist when spaced at 16″ apart. Thus, a 2×10 size of lumber can allow to span 16 feet. For a 16-foot spans, the lumber has to be at least 10 inches in depth used as floor joist.

**What size lumber for a 18 foot span**

For a 18 foot span, you will need atleast 2×10 size of lumber/ floor joist when spaced at 12″ apart. Thus, a 2×10 size of lumber can allow to span 18 feet. For a 18-foot spans, the lumber has to be at least 10 inches in depth used as floor joist.

**What size lumber for a 22 foot span**

For a 22 foot span, you will need atleast 2×12 size of lumber/ floor joist when spaced at 12″ apart. Thus, a 2×12 size of lumber can allow to span 22 feet. For a 22-foot spans, the lumber has to be at least 12 inches in depth used as floor joist.

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**Conclusion**:-

In general, for a 10 to 13-foot span, you may need 2×6 size of lumber, whereas for a 13 to 16-foot span, you may need 2×8 size of lumber. Moreover, for a 16 to 20-foot span, you may need 2×10 size of lumber, whereas for a 20 to 27-foot span, you may need 2×12 size of lumber. In addition, for a 7 to 9-foot span, you may need 2×4 size of lumber.