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Joist vs. Truss: What's the Difference?

Edited by Harlon Moss || By Janet White || Published on March 9, 2024
Joists are horizontal supports in floors and ceilings, while trusses are structured frameworks of timbers or metal used to support roofs and bridges.

Key Differences

Joists are integral components of the structural integrity of floors and ceilings. They run horizontally between foundations, walls, or beams, providing support to the surface above or the ceiling below. In contrast, trusses are designed to support roofs and spans over wide spaces. These frameworks consist of triangular units connected at joints to form a rigid structure.
Joists are typically used within shorter spans to support floors and ceilings directly, trusses are engineered to cover larger spans without needing internal support. This makes trusses ideal for bridges, roofs, and large buildings where wide, open spaces are desired. Joists, on the other hand, are more commonly found in residential construction, offering a straightforward solution for supporting the weight above or below them.
The design of a joist is relatively simple, focusing on strength and the ability to bear loads over a short distance. Trusses, however, are complex structures that distribute weight across a wider area. Engineers design trusses to take advantage of the geometric strength of triangles, thereby efficiently managing the loads they carry.
Another distinction lies in the materials used for each. Joists are often made from wood, steel, or concrete, depending on the application. Trusses, while also utilizing these materials, may incorporate a combination in their design to optimize strength and flexibility for their specific use cases.
The installation process also differs: joists are generally installed parallel to one another at regular intervals, whereas trusses are pre-assembled and lifted into place as a unit. This reflects the differing roles and structural requirements of joists and trusses in construction and architectural design.

Comparison Chart


Supports floors and ceilings
Supports roofs and bridges

Span Coverage

Shorter spans
Larger spans without internal support


Simple design
Complex triangular framework


Wood, steel, or concrete
Often a combination for strength


Installed parallel at regular intervals
Pre-assembled and lifted into place

Joist and Truss Definitions


A horizontal structural member used in framing to span an open space.
The carpenter measured the space between the joists before laying the flooring.


Composed of triangular units to provide stability and support.
The geometric design of the truss makes it both strong and aesthetically pleasing.


Part of the framework of a building, providing support to the structure above or ceiling below.
The inspector checked the joists for signs of wear and tear.


A framework, typically consisting of rafters, posts, and struts, supporting a roof, bridge, or other structure.
The truss was engineered to support the expansive roof of the auditorium.


An element of the skeleton of a building, crucial for maintaining its shape and integrity.
After the fire, the building's joists needed complete replacement.


Used to span large areas without the need for internal supports.
The warehouse was designed with a truss roof to maximize open space.


A beam supporting the floor or ceiling of a building.
We had to reinforce the joists in the attic to convert it into a living space.


A structural unit designed to bear loads over a wide space.
Engineers decided to use a steel truss for the new bridge to ensure its longevity.


A long, thick piece of wood, steel, or concrete used in construction.
Each joist was carefully placed to ensure the deck's stability.


An assembly of beams forming a rigid structure.
The construction team lifted the truss into place with a crane.


Any of the wood, steel, or concrete beams set parallel from wall to wall or across or abutting girders to support a floor or ceiling.


(Medicine) A supportive device, usually a pad with a belt, worn to prevent enlargement of a hernia or the return of a reduced hernia.


To construct with joists.


A rigid framework, as of wooden beams or metal bars, designed to support a structure, such as a roof.


A piece of timber or steel laid horizontally, or nearly so, to which the planks of the floor, or the laths or furring strips of a ceiling, are nailed.


(transitive) To fit or furnish with joists.


To fit or furnish with joists.


Beam used to support floors or roofs


Can joists be used for roofs?

Joists can be used in roof construction, but for smaller spans or as part of the roof decking support.

Are trusses more expensive than joists?

Trusses can be more expensive due to their complexity and the engineering required, especially for large spans.

What materials are trusses made from?

Trusses can be made from wood, steel, or a combination of materials for optimal strength.

What materials are joists made from?

Joists can be made from wood, steel, or concrete.

How do joists and trusses differ in use?

Joists are used for supporting floors and ceilings, while trusses are used for large spans like roofs and bridges.

How are joists installed?

Joists are installed parallel to one another at regular intervals.

What is a joist?

A joist is a horizontal structural element used in construction to support a floor or ceiling.

What is a truss?

A truss is a structural framework of timber or metal, forming a bridge or supporting a roof.

Can I replace a truss with joists?

Not typically; trusses are designed for specific spans and loads, and replacing them with joists might compromise structural integrity.

Can joists be used in all types of buildings?

Yes, joists are versatile and can be used in various types of buildings for floors and ceilings.

How do you maintain a truss?

Regular inspections and maintenance are required to ensure the integrity of the truss, including checking for signs of stress or damage.

How are trusses installed?

Trusses are usually pre-assembled and lifted into place as a single unit.

Are trusses suitable for residential construction?

Trusses are suitable and often used in residential construction, especially for roofs.

What is the maximum span for a joist?

The maximum span for a joist depends on its material and cross-sectional size, but it's generally less than that of a truss.

How do you maintain a joist?

Maintaining a joist involves ensuring it's not overloaded and inspecting it for signs of damage or wear.

Do trusses require special engineering?

Yes, trusses require precise engineering to ensure they can support the intended loads and spans.

What types of trusses are there?

There are many types of trusses, including Pratt, Howe, and Warren trusses, each with specific design characteristics.

Can I add more joists to an existing structure for extra support?

Yes, additional joists can be added to existing structures to provide extra support, assuming the foundation can handle the added load.

Can trusses be customized?

Yes, trusses can be custom-designed for specific architectural needs and spans.

How do environmental factors affect joists and trusses?

Environmental factors like humidity, temperature, and pests can affect the integrity of both joists and trusses, requiring appropriate materials and treatments.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.

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