I-Joists and Floor Trusses are commonly used structural members that take the load from the floor to vertical members.
The primary difference between a floor truss and an I-joist is that an I-joist is a single horizontal load-bearing structural member. It is used to transfer a load of floors and ceilings to beams, while trusses are triangular configurations of beam-like components. Trusses are also used for the same purpose of supporting roofs and floors.
It is of paramount importance that the design not only looks great but is structurally sound as well.
There are many situations where a designer or an engineer is presented with multiple structural components to choose from.
The selection between an I-joist and a truss can be made by comparing the pros and cons in using either.
|Structural Configuration||Triangular configurations of beam-like components.||A horizontal beam-like structure in I-shape.|
|Assembly||Need to be assembled before being delivered to the site for installation.||Can be assembled and installed directly on site.|
|Load Bearing Capacity||Very high load-bearing strength.||Low load-bearing capacity.|
|Frequency||The lower the number of floor or roof trusses are required to provide stability. The spacing between each member is relatively greater.||The higher the number of floor and roof joists panels are required to provide stability. The spacing between each member is small.|
|Holes||Openings are already present to allow passage for plumbing ducts and electrical wires.||Holes need to be drilled into joists to allow passage for ducts and wires.|
|Span||With trusses, large spans of a room are possible to construct||Spans created with I-joists are smaller in comparison.|
|Cost||Floor and roof trusses are very expensive.||I-joists are cheaper in comparison.|
|Roof Shape||Trusses provide a slope when used in constructing roofs.||I-joists provide a flat roof.|
|Alterations||It is not easy to alter truss systems after they have been installed. In some cases, they may need to be replaced entirely.||I-joist panels are easy to alter even after they have been installed.|
I-Joist vs. Truss
I-Joists vs Floor Trusses: Which is Better?
Floor Trusses offer more strength, and flexibility compared to I-Joists.
They allow the construction of larger spans and have openings that eliminate the need for drilling any holes.
Moreover, roof trusses provide stronger resistance against the accumulation of rain or snow.
However, certain restrictions come along with the use of trusses, such as cost, and on-site construction changes.
Truss systems are considerably more expensive than I-joists.
If cost is not an issue and major design changes later in the construction phase are unlikely, then trusses are the better option.
Both I-joists and floor trusses come under the category of engineered floor joists.
What are I-Joists?
Joists are beam-like structural members that carry the load of everything above them and transfer them to beams.
They span the length of a room or section and support the weight of roofs, ceilings, and floors.
They are smaller in size as compared to normal beams and as such a large building will typically be equipped with a larger number of joists.
Joists are made from wood or metal, such as steel which makes them quite versatile.
Among many types of joists, I-joists are considered the most desirable option in residential construction.
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 x10 lumber and are rigid enough to allow the drilling of holes.
Holes can be drilled without causing any major loss in structural resistance.
This makes installing electrical wiring and plumbing through joists very easy.
Another major factor for their popularity is their low cost.
What are Floor Trusses?
A floor or roof truss is a triangular unit constructed out of at least three beams or beam-like components.
Each beam is connected at the nodes to form a triangle.
The simplest configuration includes two diagonal beams connected to a horizontal beam.
But, many new variations have been made that build upon the basic design.
Their distinct arrangement is the key reason for their preference in residential construction. They are utilized in constructing roofs and floors.
Trusses are the popular choice for many engineers as their design distributes weight more efficiently compared to other structural systems.
Trusses are commonly used in large-scale projects.
Floor Truss Applications
Due to their strength and flexibility in design, they have several applications. They are primarily utilized in residential construction and bridge construction.
Trusses are used to support floors due to their heavy load-bearing capacity.
Another important aspect of trusses is their high tensile strength.
Trusses are an integral part of bridge construction due to their efficient load and weight distribution capabilities. They effectively resist shear forces and buckling.
In houses, they make an excellent choice for constructing roofs due to their pitch (the angle between the horizontal and diagonal beam of a truss).
The pitch of a truss prevents the accumulation of rain water and snow.
Difference in Load Bearing Action: I-Joist vs Truss
Trusses carry and distribute loads much more efficiently, due to their triangular configuration. As a consequence, they do not undergo major deformation even when subjected to heavy loading.
In contrast, floor joists can deform significantly over long periods under 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.
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 weights and loads more efficiently, thus increasing the overall stability of a roof or floor.
Trusses can accommodate extra loads that may be caused by floor tiles, kitchen shelves, high traffic areas, etc. I-joists in comparison offer limited benefits.
I-joists are sometimes further strengthened by blocking or bridging but that's not an option with floor trusses.
Which Member Makes Construction of Larger Spans More Feasible?
With floor trusses, spans of up to 30ft can easily be constructed. But there is no theoretical limit to the span length in codes.
Floor trusses allow for large open rooms to be constructed easily because they can span farther than I-joists.
This also helps reduce the costs of the project by eliminating the need for bearing posts and support beams.
Designers may increase the length of trusses using appropriate materials and configurations.
The spans allowed in a traditional lumber joist are much lower than either I-joists or floor trusses.
Cost Comparison: I-Joist System vs Floor Truss
Trusses are usually quite expensive if used in supporting floors and roofs. Joists on the other hand are quite cheaper in comparison.
Furthermore, in cases where changes in the design are required, joists can be easily altered.
Whereas, floor and roof trusses need to be replaced and reinstalled with new specifications. This increases the cost of the project.
Unless there is a project requirement, such as the construction of long spans or provision of heavy load resistance, it is much cheaper to use I-joists instead of trusses.
What are the Advantages of Floor Truss Systems?
Floor trusses squeak less frequently as compared to I-joists due to the wider nailing surface offered by floor trusses.
While constructing floor trusses, phenomena like shrinkage and warping are considered beforehand thus wastage of material is avoided.
Electrical wiring and plumbing pipes are easily installed through trusses due to the presence of openings. These ducts can be shifted later on as well.
Roof trusses provide better resistance against rain and snow accumulation due to their pitch.
Floor trusses are prefabricated and then shipped to the site, reducing the construction time of the project.
Trusses allow longer spans to be constructed and offer more strength compared to joists.
Which Member Makes Matching Floor Heights Easier?
Multiple bearing conditions are available with floor trusses, which are utilized in various scenarios.
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 when trusses are incorporated into floors.
I-Joists on the other hand, offer a single bottom chord bearing.
Which Member is More Useful in Constructing Roofs? I-Joist or Roof Truss?
Trusses when incorporated into roofs provide a slope to them.
Consequently, a trussed roof behaves more efficiently in regions with a lot of rainfall and snow.
However, if your house does not lie in such a weather-restricted region, and you want to economize your project, I-joists can prove to be a more ideal option.
Is I-Joist a Part of a Floor Truss?
I-joists and trusses are separate structures. I-joists are beam-like singular units used to carry the weight of ceilings and floors.
Trusses are triangular configurations of beam-like members. Both are used in supporting floors and roofs.
While I-joists and trusses differ from each other, they are sometimes used together in construction projects.
Are Floor/Roof Trusses Worth It?
Trusses offer a variety of advantages that make their use in construction very worthwhile.
They provide high strength, easy installation, longer spans, sloping roofs, and easy passage for wires and ducts.
Trusses, coupled together with prefabricated walls minimize the construction time of a house.
Thus, it is safe to say that truss systems are worth it if the economy is not an issue.
Both I-joists and trusses have their own applications in construction projects.
Trusses offer more variety as they can be used in residential construction as well as bridge construction.
They act as high-strength supports and are an excellent choice for roofs due to their slope.
Trusses also enable the construction of longer spans. Consequently, for large spanning houses and buildings, trusses can be an ideal choice.
Houses that need protection against excessive rain or snow may also incorporate trusses.
The use of trusses, however, is not cheap and should only be utilized if there is a specific design requirement.
In contrast, in most residential construction projects, I-joists offer sufficient strength and are cheaper.
I-joists can also be altered more easily than trusses during and after construction.
The only hurdle with using joists is the need for drilling holes to allow passage for plumbing ducts and electrical wiring.
Both these structural members can meet a project's requirements. In the end, the decision lies with the designer and the project constraints.
Frequently Asked Questions (FAQ)
Can Holes be Drilled into I-Joists and Floor Trusses?
Holes can be drilled into I-joists anywhere along the middle of its span.
According to the International Residential Code (IRC) 2021, a hole of up to 1-1/2 in. diameter can be drilled into the web of a joist panel.
Trusses already have openings that allow the passing of plumbing ducts and electrical wiring, so no holes do not need to be drilled into trusses.
How do I-Joists Differ from Beams?
The primary function of I-joists is to carry loads of roofs and floors and transfer them onto the beams.
Beams are horizontal structural members that carry loads forms joists and slabs and transfer them to columns.
I-joists are similar to beams in shape but are smaller in size.
Are Floor and Roof Trusses the Same?
Floor trusses are arranged to resist loads from flatter surfaces, while roof trusses are designed to resist loads from sloping roofs.
Their arrangement is different because their load-bearing function is different.
Why are Fixed Supports Not Used in Floor/Roof Trusses?
Trusses are designed to act under pure compression and pure tension, which is possible with pin supports only.
Using fixed supports will induce unwanted moments into the frame of a truss.