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I-Joist vs Floor Truss: Which is Better?



V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com


I-Joists vs trusses.

An I-joist is a single horizontal load-bearing structural member that transfers loads from floors and ceilings to beams. Conversely, trusses are triangular configurations of beam-like components that support floors and roofs for structural stability. Additionally, they can span longer distances but are more expensive than I-joists.

Here are the main differences between I-joists and floor trusses:

PropertyI-JoistsFloor Trusses
Structural ConfigurationA horizontal beam-like structure in I-shapeTriangular configurations of beam-like components.
AssemblyCan be assembled and installed directly on-site.Needs to be assembled before delivering to the site for installation.
Load Bearing CapacityLow load-bearing capacityVery high load-bearing strength
FrequencyMore joist panels with closer spacing.Fewer trusses with wider spacing
HolesNeed to drill holesOpenings are available
SpanShorter than trussesLonger spans
CostCheaper Expensive
Roof ShapeFlat roofSloped roof
AlterationsEasier to alter after installationDifficult to alter the design

I-Joist vs. Truss

Both I-joists and floor trusses come under the category of engineered floor joists and are sometimes used together in construction.

What are I-Joists?

I-joists are beam-like structural members that carry the load of everything above them and transfer it to beams.

They span the length of a room or section and support the weight of roofs, ceilings, and floors.

I-joists are also smaller in size than normal beams, so a large building requires a greater number of joists.

Generally, joists are made from wood or metals such as steel.

Among many types of joists, I-joists are considered the most desirable option in residential construction.

Engineered I-Joists.
Engineered I-Joists.

I-Joist Applications

I-joists are quite desirable as floor or roof supports due to their versatility and efficiency.

For instance, consider wood-engineered I-joists.

They offer spans greater than 2 x 10 lumber and are rigid enough to allow the drilling of holes without affecting the structural integrity.

Holes in Engineered Joist
Holes in Engineered Joist

This makes installing electrical wiring and plumbing through joists very easy.

Wooden Roof Joist with small spacing.
Wooden Roof Joists.

What are Floor Trusses?

Trusses are a popular choice for many engineers, especially for large-scale projects.

They consist of triangular units made of beam-like components; the simplest configuration includes two diagonal beams connected to a horizontal beam.

However, new variations are now available to enhance its properties.

Roof Trusses.

Additionally, floor trusses squeak less than I-joists due to the wider nailing surface offered by floor trusses and offer more strength compared to joists.

Furthermore, when constructing floor trusses, shrinkage and warping are considered in advance to avoid the wastage of materials.

Floor trusses are also prefabricated and then shipped to the site, reducing the construction time of the project.

Floor Truss Applications

They are common in residential construction as they can span longer, around 30 feet, without the need for intermediate support.

The spans allowed in a traditional lumber joist are much lower than I-joists or floor trusses.

Long Floor Truss Spans.
Long Floor Truss Spans

Trusses are also an excellent choice for constructing roofs due to their pitch (the angle between the horizontal and diagonal beams of a truss).

Pitch of Truss.
Pitch of Truss

Moreover, roof trusses can withstand various weather conditions like rain or snow.

Additionally, trusses are an integral part of bridge construction due to their efficient load distribution capabilities. They effectively resist shear forces and buckling.

Their structural efficiency also makes them suitable for commercial buildings.

Furthermore, they have openings that eliminate the need for drilling holes to run the utilities.

Trusses With Openings.
Floor Trusses With Openings

Difference in Load Bearing Action: I-Joist vs Truss

Floor joists can deform significantly over long periods under heavy loading.

In contrast, trusses do not undergo major deformation even when subjected to heavy loading.

Floor trusses also have a self-support system, whereas I-joists require intermediate supports like beams, posts, or columns for load transfer.

Since I-joists need columns at regular intervals, this can reduce the amount of usable floor area.

After years of use, I-joists also need further strengthening, such as sistering.

This makes floor trusses a better choice for resisting lateral as well as vertical loads.

Are Floor Trusses Stronger than I-Joists?

Floor trusses provide greater strength compared to I-joists due to their larger surface area and longer spans.

They can distribute the loads more efficiently, thus increasing the overall stability of a roof or floor.

Trusses can handle additional loads of floor tiles, kitchen shelves, and high-traffic areas, while I-joists offer limited advantages.

I-joists are sometimes further strengthened by blocking or bridging, but that’s not an option with floor trusses.

Cost Comparison: I-Joist System vs Floor Truss

I-joists are a cost-effective option than floor trusses.

Furthermore, if design modifications are necessary, joists can be easily altered, whereas trusses need to be replaced and reinstalled to meet the new specifications.

This results in an increased project cost.

For most projects, I-joists are the best choice, except for those with long spans or heavy loads.

Which Member Makes Matching Floor Heights Easier?

Multiple bearing conditions are available with floor trusses, which are utilized in various scenarios.

The bottom chord bearing, top chord bearing, bottom chord with trimmable ends, and even a middle block bearing are accessible for construction.

This range of bearing conditions allows for easier matching of heights.

I-joists, on the other hand, offer a single bottom chord bearing.

Frequently Asked Questions (FAQ)

How do I-Joists Differ from Beams?

Beams are horizontal structural members that carry loads from joists and slabs and transfer them to columns.

Unlike I-joists, they require larger sizes for the same span.

Why is Bracing Necessary for Floor Trusses?

Bracing is essential to prevent lateral movement or buckling.

Without proper bracing, trusses can weaken the structural integrity of the building.

Why are Fixed Supports Not Used in Floor/Roof Trusses?

Trusses are designed to act under pure compression and tension, which is possible with pin supports only.

Using fixed supports will induce unwanted moments into the frame of a truss.

V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com

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