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How Do I Choose the Correct Floor Joist Size?

B. Leslie Baird
B. Leslie Baird

Choosing the correct floor joist size depends on your building application, joist spacing, and several other factors. These include the length of the span as well as the type and grade of the lumber being used. Common joist spacings are 12 inches (304.8 mm), 16 inches (406.4 mm), and 24 inches (609.6 mm). The amount of weight that needs to be supported can also be a significant factor. For instance, the stronger the wood, the more weight it will be able to support over a longer span.

When determining the floor joist size, measure the length of the span. The span is the inside dimension from the support on one end to the support on the opposite end. The next factor to consider is the joist spacing that will be used and the amount of weight the flooring will need to support. For example, a standard outdoor deck will not be required to support as much weight as a dining room floor, unless the deck with have a large hot tub.

View of second story floor joists from below.
View of second story floor joists from below.

For a 12 inch (304.8 mm) joist spacing, longer lengths of lesser grade lumber can be used. In this example, there will be a greater number of joists supporting the weight of the deck or flooring. So, for an 8 foot long (2.44 m) deck, a Redwood joist of grade 2 lumber would need to be 2 x 6 (50.8 x 152.4 mm ). A larger joist size would be perfectly acceptable, whereas a smaller size would not. The 2 x 6 joist size is suitable for most types of grade 2 lumber at 8 feet (2.44 m) in length.

Floor joists on new construction.
Floor joists on new construction.

A structure with a 10 foot (3.05 m) length, and joists set 12 inches (304.8 mm), apart would require using a floor joist size of 2 x 8 (50.8 x 203 mm). If the joist spacing will be 16 inches (406.4 mm), a joist size of 2 x 8 should be used, even if the length is only 8 feet (2.44 m). With a joist spacing of 24 inches (609.6 mm), the same 2 x 8 barely meets the minimum requirement for 8 foot (2.44 m) lengths. A better choice would be using a 2 x 10 (50.8 x 254 mm) joist size. Using grade 1 lumber will provide more structural support with a smaller joist size.

Calculations for choosing a floor joist size depend on the grade and type of the lumber, the length of the span, and the joist spacing. In addition, the weight of both live and dead loads must be considered. A dead load refers to the weight of all the materials involved in the building or deck. Live loads are the addition of furniture and people using the area.

Formulas or engineering programs are available that can be referenced when dealing with unusual span lengths or lumber varieties. Many areas are also subject to building code regulations and those requirements must be followed. When in doubt, moving to the next floor joist size up may be more expensive but worth the cost for eliminating the worry.

What Is Standard Floor Joist Size?

When it comes to floor joists, there is no standard size. Since joists form the support structure for a building, their size must be carefully calculated based on the load they will need to bear. Each separate measurement of a joist can vary and each of those variances will impact its stability.

  • Thickness - Typically floor joists are 2 inches thick, although, for some spaces needing greater support, builders will double the thickness in order for the joist to take more weight
  • Width - Floor joists range from 6 inches to 12 inches in width. Generally, the wider the joist, the further it can span without support and the more weight it can bear. However, it is not always possible to use the widest available joist as it can have a significant impact on the finished room dimension.
  • Length - Joists can be of many different lengths depending on the placement of support beams, the width of the joist, the type of wood used and the spacing between one joist and the next.

What Size Floor Joists Are Used in Residential Construction?

In older homes, it is common to find floor joists that are 2 inches thick by 6 inches wide. This may be one of the reasons that sloping or sagging floors are one of the most common issues that owners of vintage homes have to deal with.

In newer construction, it is rare to find these smaller joists. Typically, builders will choose 2-by-8, 2-by-10 or 2-by-12 joists. The wider joists offer a higher level of stability for a greater length of time.

How To Calculate Floor Joist Sizes?

Floor joists are the foundation for any structure. Using boards that are not the right size or spacing them incorrectly can degrade stability. A great many factors go into calculating the correct width and length of floor joists.

What Load Must They Carry?

Builders must know what load a floor needs to support before deciding on what size joist to use. There are two different kinds of load they need to understand:

  • Dead Load - This is the measurement of all the permanent elements that comprise the structure, including floors, walls, ceilings, etc. The common dead load for a residential building is 10 pounds per square foot.
  • Live Load - This is an estimation of the load of transient items within a structure, including people, furniture, appliances, etc. The typical live load in a residence is 40 pounds per square foot.

What Wood is Being Used?

Not all types of wood are optimal for the construction of floor joists. When choosing which wood to use, builders will look at two factors; species and grade:

  • Different species of wood have varying degrees of strength and flexibility. Woods that have a high to medium level of strength and flexibility, such as yellow pine or redwood, are good choices for joists.
  • Wood is also graded for the presence of natural flaws that can weaken it, such as knots, shakes or cracks. When building a support structure it is best to choose grade 1 or 2 lumber to ensure the integrity of the build.

How Much Space Between Joists?

The space between joists is typically 12, 16 or 24 inches. Joists that are closer together provide maximum support, those that are further apart may lead to unwanted bounce or sagging in a floor. Spacing joists at 16 inches off center is the most common measurement used in residential housing.

A builder must know the exact combination of all of these factors before he or she can determine the maximum length of a floor joist. For example: with a standard dead load of 10 pounds per square foot and a live load of 40 pounds per square foot, a 2-by-12 joist made of grade 1 lumber with a joist spacing of 16 inches, has a maximum span of 16 feet without the need for additional supporting structures.

Changing even one element in this equation can change the maximum span significantly. A joist of the same width, thickness and spacing, made of grade 2 lumber, only has a maximum span of 15 feet. A joist made of the same materials and using the same spacing that measures 2 inches by 10 inches will have a maximum span of 13 feet and 11 inches.

If a builder needs a floor joist to span a distance greater than the recommended maximum, he or she will need to install support beams to maintain the structure's stability.

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    • View of second story floor joists from below.
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      View of second story floor joists from below.
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