This Is Why Some Floor Joists Are Doubled

Floor joists are an important feature in a construction. They allow the support of larger floor plans without needing too many columns by transferring the load from walls to subfloor and beams. Look under your floorboards, and you might find that some joists are thicker or even doubled, but why?

Double joists are added to increase the span of a floor, as they provide more support to bear the additional weight. They also come in handy to build a second floor without the need for adding extra columns from the roof to the foundation. This frees up space under the deck as much as possible. 

Keep reading to find out more about doubling floor joists when this practice is necessary. Also, you will understand the purpose of a joist. 

This Is Why Some Floor Joists Are Doubled

Where Are Double Joists Necessary?

Double joists are necessary whenever you want to increase the load on the floor by adding a roof, staircase, tub, or second story without the addition of columns. They are also required if you want the floor to span a larger area, especially in rim joists.

Multiple factors determine how much load a floor frame can carry.

These include:

  • The grade of lumber used.
  • The type of wood used in making the frame.
  • The width of the joists.
  • The thickness of the joists.
  • How the joists are spaced.
  • The Joist span.

Taking all these factors into consideration can help you decide whether or not double joists are necessary to add to your floor frame. 

If you are building a deck, for example, double joists will free up a lot of space under the deck because there is no need to add an extra column to support the weight of the roof. 

Joist span is particularly important when deciding to use double joists. The rim joist is the perpendicularly attached plank that provides lateral support. If the rim joist’s span is planned for more than 4 feet (121.92 cm), doubling is necessary to provide sufficient lateral support to the rest of the floor. 

Whenever doing any renovations, builders need to consider how much additional load the renovations will bring. 

For example, when installing a heavy bathtub (cast iron tubs are particularly heavy), doubling the joist underneath the tub will ensure that sufficient support is available to carry the tub.

Single Joists vs. Double Joists

The simplest way to describe a double joist is that it is two single joists screwed together along their length to form a thicker, stronger joist. While single joists are standard in all door frames, double joists are usually added under load-bearing walls or when additional load is expected. 

Generally speaking, joists provide stiffness to a floor frame. Doubling (or even tripling) joists increases this stiffness significantly. Single and double joists are distributed throughout a floor frame to provide adequate support. The joist will transfer loads bearing on the floor to the foundation underneath.

The Difference Between Joists and Beams

Many people confuse joists and beams or mistake the header joist (rim joist) for beams. It’s important to understand that there is a clear difference between them, as each serves a specific purpose.

A beam is the main load-bearing structure of a floor. It runs perpendicular to the joists that are placed on top of it. Beams are larger in size when compared to joists because they must support a lot more weight. A typical beam is 2 by 10 inches (50.8 x 254 mm).

The purpose of a beam is to transfer the load from the joists to the column or studs, which is then transferred to the foundation of the building. 

In addition to the beams found under every floor, there is always a different (stronger) beam that carries the additional load from a roof. The roof beam must bear the weight of roof trusses and additional structures that are specific to the roof. 

On the other hand, joists are generally smaller than the beam supporting them. When looking at a floor frame, you will see many more joists than beams, with the joists leaning into the beams. The rim joist is different from the beam because it is not underneath the joists but perpendicular and on the same plane. 

While a beam supports the load transferred from all the joists, rim joists provide lateral support only. Note that rim joists are the same size as other joists. Contrastingly, beams are much larger and more noticeable. 

The Anatomy of a Floor

Before we move on to further elaborate on the role of joists (doubled or not), here’s a quick overview of the anatomy of a floor frame. These are the components of a raised floor, as opposed to those floors built flat onto a concrete slab.

  • Floor joists: Planks of wood (usually 2 x 8 inches /50.8 x 203.2 mm or 2 x 6 inches /50.8 x 152.4 mm) running parallel to each other at regularly spaced intervals, spanning the entire floor frame. Usually single joists.
  • Rim joists: These are joists placed perpendicular to the rest of the floor, providing lateral support.
  • Doubled joists: Two single joists screwed together wherever additional support is required to distribute the load.
  • Sill plate: Preservative-treated sill used to prevent rotting of joists at the foundation level.
  • Subfloor: Plywood or other panels laid perpendicularly on top of joists to provide a base for the finished floor.

Span Calculation for Floor Construction

I’ve already mentioned that double joists are used to increase the joist span. That is, the distance that a joist can transfer the load bearing on it. Span calculation is a complex equation that architects and DIYers need to keep in mind when building a floor. 

Without correct calculations, building a structurally sound floor is not likely. Here are some of the factors that a considered in joist span calculation:

Dead Load vs. Live Load

The dead load is usually set at 10 pounds per square foot. It is the weight of all materials used in the construction of the floor, divided by square footage. Dead load is a number that will not change over time, as it accounts for permanent components of the construction. 

For a building to be structurally sound, the floor frame needs to account for the dead load (permanent) and additional live load. To be clear, live load refers to the changing weight of people, furniture, and other items that will exert a downward force on the floor. 

The norm for residential buildings is to count live loads as 40 pounds per square foot. Some buildings account for less (not less than 30 pounds per square foot). So the total load capacity of most residential buildings is 50 pounds per square foot (10 dead load + 40 live load)

Species of Wood

Because different species of wood provide varying levels of strength, the type of wood used must be factored into the span calculations. Even within the same species, slow growth leads to stronger wood that is favored over wood from faster-grown trees. 

The most commonly used wood species in floor frames are:

  • Douglas Fir
  • White Oak
  • Cypress 
  • Eastern White Pine.

Of course, other species may be used depending on the specifics of a construction project. 


Fitting joists closer together provides more strength and stiffness than when they are installed far apart. However, spacing them too closely can be expensive, along with other problems. That is why most builders try not to fit joists too close to each other.

It’s better to add double joists at regular intervals than to reduce the space between joists overall. Not only does this save on time, but proper spacing also saves on the cost of installation because you use fewer joists. Also, proper spacing (with doubled joists) allows for more space underneath that can be used to accommodate plumbing and other internal structures.

Lumber Grade

In the lumber industry, wood is rated based on the number of defects in a plank of wood. Higher-grade lumber contains fewer defects. Consequently, high-grade lumber is much stronger and will provide better support when used as a joist. 

When calculating joist span, high-grade lumber will increase the overall span

Size of the Wood

Another important factor that is considered when calculating joist span is the actual size of the joists. While most joists measure 2 inches by 6 inches (50.8 x 152.4 mm), some construction projects require the use of 2 by 8 inch (50.8 x 203.2 mm) joists. 

Many DIYers ask whether using 2 by 4 inch (50.8 x 101.6 mm) wood planks as joists is feasible. While it’s certainly possible, I wouldn’t recommend it because the joist span is very low. 

Only use 2 by 4 inch (50.8 x 101.6 mm) joists in smaller-sized decks and never in flooring with a considerably large load to bear. Otherwise, the floor would be prone to bending and even breaking.

These factors also affect your ability to notch the bottom of the joist when you want to add wiring because the joist affects your building’s structural integrity. [Can You Notch the Bottom of a Joist?]

Final Thoughts

Joists are an important part of the structure of flooring in any building, and doubling is necessary to support the additional load. Joist span is a complex calculation dependent on multiple factors. But doubled joists generally increase span and support additional load by increasing the stiffness of a floor frame.

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