Joists Vs. Beams Vs. Trusses (What Are The Differences?)

Last Updated on October 9, 2022 by Barry Gray

Modern engineering has transformed the landscape. Structures like the Empire State Building, the Burj Khalifa rises 829.8m above the city of Dubai, and Sky Gate Bridge (connected to Kansai International Airport) in Japan spans 3.75km are examples of 21st-century engineering which have changed the face of the world.

The difference between joists, beams, and trusses is the loads and lengths they can carry. Joists are used for short distances and may be attached to beams; beams support higher loads across longer spans, and trusses hold even higher loads over greater distances.

Whether they are roof trusses securing a home or spanning 3,75km to Kansai airport, trusses have the highest load-carrying capacity. Beams are used for building construction; for example, they can be used as the support for upstairs floors, and joists serve a similar function for lighter loads over shorter distances.

The Differences Between Beams, Trusses, And Joists

To adequately explain the differences between joists, beams, and trusses, let’s first discuss what each component is and its use.

What Are Beams?

Beams are long structural elements designed to carry a load applied at an angle of 90 degrees (perpendicular) to the direction of the beam. Beams transfer their loads vertically to columns or foundations.

Imagine a balance beam used by gymnasts. It consists of a beam of wood laid horizontally between two supports. When the gymnast mounts the beam, the gymnast’s weight, expressed as a vertical force, is supported by the part of the horizontal beam on which the gymnast is performing.

Another illustration involves several beams being installed from the top of one wall, across an open space, to the top of the opposite wall. 

The engineers then install an upstairs floor on top of the beam. The upstairs floor imposes a vertical (90-degree) load onto the horizontal beam. Without the beams, the engineers would have to steal space from the bottom level and build multiple columns up from the lower-level floor to support the upstairs structure.

Therefore, the beam enables floors, ceilings, and roofs to be supported, over an open space, without the need for additional supporting structures to be built from below.

What Are Trusses?

Trusses consist of several structural elements joined together by pin joints.

Trusses apply any load equally to all the members of the truss. It is contrasted to a beam that only carries the load at the point the force is applied. Using an example of a motor moving across a bridge.

  1. If a bridge was constructed using a beam. Assume a vehicle moves across the span; the load is applied to the point where the vehicle’s wheels touch the bridge as the car moves across the bridge, so the weight moves along the beam.
  2. Assume the engineers had decided to use a truss. As the vehicle moves across the span, the truss will spread the load through the pin joints and across each truss member. Therefore, with a truss, no single point of the bridge span carries the load, and instead, it is shared equally across the whole bridge. 

Trusses serve a similar purpose as beams; however, the design enables them to carry structural loads over longer distances.

What Are Joists?

Joists are essentially small beams installed across full beams, at right angles to the direction of the beams,  typically arranged in parallel series and are used to support horizontal structures such as floors, ceilings, or decks.

Each beam will support many joists.

Because they are installed at right angles across the beams, the gaps between them are less than the gaps between the beams.

The joists can transfer the vertical loads more evenly across the beams. It spreads the loads across at least two beams, enabling the joist/beam spread the loads similarly to trusses.

What Are Trusses, Beams, And Joists Used For?

The difference between Trusses, Beams, and Joists is their ability to carry the load.

What Are Trusses Used For?

Trusses are manufactured structures consisting of several elements designed to spread the load evenly across the frame. It makes them capable of spanning large distances (think the Golden Gate Bridge in San Francisco).

On a much smaller scale,  roof trusses are prefabricated offsite and manufactured into various shapes used to carry the internal loads of a domestic home roof. Because roof trusses work by spreading the load across the truss, manufacturers can use lighter-weight timber.

It generally makes trusses cheaper than their counterparts and easier to install.

When you look at a roof that has not yet been tiled, you will notice the roof trusses installed on the new building’s walls. They are constructed in the shape of a triangle. The purpose is to evenly spread the forces that the roof is subject to across the truss, rather than a single point on a wooden beam.

The triangular shape of roof trusses serves two functions.

  1. The most apparent purpose is that they provide support for the roof
  2. They offer a second structural purpose to tie the home’s outside walls together.

What Are Beams Used For?

Beams are a single extended lateral structural component. Unlike trusses, beams are not constructed, and therefore they take up significantly less space than a truss.

Although they don’t have the structural ability of a truss, they serve a useful purpose by acting as the support for constructions such as floors, ceilings, and decks.

These applications are not suited to trusses because a truss would take up to much volume.

Beams attach from support (column or wall)  to support. Although beams do not have the structural ability of trusses, they can still span substantial areas, leaving the room they cross free of columns or walls.

Like trusses, roof beams also connect the outside walls and stop them from spreading or leaning in.

What Are Joists Used For?

What Are Joists Used For?

As mentioned previously, joists are essentially mini beams made of the same material, typically timber, as the beams.

Joists provide support over shorter distances or spread the load across multiple beams, as would be the case in installing an upstairs floor.

Joists are also used to support ceilings and even decks.

Conclusion

The different capabilities and uses of Joists, Beams, and Trusses can be summed up with the phrase, horses for courses. The primary difference between each component is the length they can support loads. Trusses can support the heaviest loads, for the longest distance, beams follow and 

Joists round the pack up.

Each component has an essential part to play in home construction and will be chosen by the engineering team based on its optimum capability.

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Barry Gray

Hi, I’m Barry. I’ve loved woodworking and bringing things back to life for more years than I care to remember. I hope my passion for tools comes across loud and clear in everything you read here on The Tool Square.