Floor joists are essential structural components used to secure floors in a home.
They can majorly impact many aspects of a floor plan, such as span lengths, fire protection, air quality, type of floor surface, and finishing.
Floor joists are horizontal load-bearing structures that span between walls or between a beam and a wall. They provide secondary support to the floor systems. There are several types of floor joists available, each with its own merits. These include traditional lumber joists, engineered joists, and open web trusses.
This article discusses the various types of floor joists and compares them to provide homeowners with the necessary information for selecting the correct kind of floor joists.
Going over the different types of floor trusses available in the industry helps designers and homeowners decide when to select a joist type for their projects.
There are many different types of floor joists, providing designers with more construction freedom and accessibility.
3 Types of Floor Joists: Most Used
Three kinds of floor joists are most popular in the USA:
- Lumber Joists
- Engineered Joists
- Open Web Trusses
|Criteria||Traditional Lumber Joists||Engineered Joists||Open Web Trusses|
|Structural Configuration||Horizontal solid rectangular block||A horizontal beam-like structure in rectangular or I-shapes||Triangular configurations of beam-like components|
|Assembly||Direct installation on-site||Direct installation on-site||Need to be assembled before being delivered to the site for installation|
|Load Bearing Capacity||Low load-bearing capacity||High load-bearing capacity||Very high load-bearing strength|
|Frequency||The highest number of traditional joists are required||A relatively high number of floor-engineered joist panels are required||A small number of floor trusses are required|
|Holes||Holes are drilled||Holes are drilled||Already present|
|Span||Least span lengths||Smaller than trusses but higher than traditional joists||Possible to construct large spans of a room|
|Cost||Cheapest||Cheaper than trusses||Floor and roof trusses are very costly|
|Alterations||Easy to alter and manage||Easy to alter even after installation||Hard to alter truss systems after installation|
2x10 Lumber Floor Joists
2x10 wood joists are the traditional standard floor joists that have been widely used in residential construction for a long time.
They are made of various native kinds of wood such as spruce, fir, pine, and so on.
2x10 floor joists are horizontal, solid wood blocks used to support floors.
Their actual size is smaller than the claimed 2x10.
Due to the finishing processes, the final size of a 2x10 joist comes out, to be 1 ½”x 9 ½” to be exact.
They can have a maximum span length of 16'.
These joists offer sufficient strength for a relatively low cost, thus making them durable and cost-effective.
2x10s are also very easy to install, handle and replace.
However, these joists can only span up to 16’, a relatively smaller span length compared to engineered joists and open web trusses.
Engineered Floor Joists
Engineered floor joists are increasingly replacing traditional lumber joists for some time now.
Compared to traditional 2x10s, they dissipate floor loads onto beams, but more efficiently.
They are made from a mixture of wood-based materials and conventional woods that form a new compound.
This new compound is thus more potent and lighter than traditional lumber.
Engineered joists are available in solid rectangular blocks and as I-shaped joists.
Typically, engineered I-joists are incorporated more in residential construction due to their superior design.
Just like an I beam, an I-joist consists of two flanges and a web which provide extra resistance against shear and bending phenomena.
This shape also uses the least amount of material in its fabrication.
Engineered joists have longer lengths as they are more durable and lightweight.
Typically engineered I-joists span up to 48’ but spans as long as 60’ are also possible.
The I-joist's depth ranges between 9 ½” to 16”, whereas the widths range from 2 ½” to 3 ½”.
Engineered I-joists are usually 9 ½” deep, but the depth can be increased to 16", and the web can also be thicker.
Open Web Trusses
Open web floor trusses are superior engineered joists capable of spanning greater lengths than other joists.
Placing floor trusses can meet complex design requirements as they provide the highest strength and durability.
Trusses have no drilling requirements as they have openings in their design.
These openings allow easy passage for plumbing ducts and electrical wiring.
Floor trusses are more expensive than traditional floor joists and are therefore only used in large projects.
They are usually more common in large-scale projects than in residential construction.
A composite floor deck can also be used to strengthen a floor system without adding extra weight.
Types of Floor Trusses
Three types of open web trusses can be used as floor joists.
These are Warren Trusses, Howe Trusses, and Prat Trusses.
The Warren truss is the most common type of open web truss, which are easy to make but have poor performance under concentrated loads.
Pratt trusses have an N-shaped configuration with diagonals sloping backward.
The vertical members in this truss system carry the compressive loads, while the diagonal members carry the tensile loads.
Lastly, there are Howe truss systems, which are the opposites of Prat truss systems.
It has a reversed N shape configuration where its vertical members carry the tensile forces, and its diagonal members carry the compressive forces.
Which Floor Joist Should You Use?
Choosing the right type of beam for your project depends on many factors, including the size of the room, applied loads, and budget.
Floor plan, loading values, joist availability, and, most important economy, all factor into making the final floor joist selection.
In contrast to engineered I joists,open web trusses offer greater strength, feasibility, and flexibility in design.
Longer spans are made using floor trusses than with other joists.
Moreover, there is no drilling required for open web trusses.
But all these advantages mean that open web trusses have a very high price range, which means they can break your budget.
This is the main reason why trusses are usually not used in residential construction, but in large projects.
In contrast, engineered joists offer sufficient strength and span lengths for a much more reasonable price.
They span longer than traditional joists and are cheaper than open web trusses, making them the ideal choice for most residential projects.
How Do You Strengthen Floor Joists?
Floor joists tend to degrade over time and lose their strength.
The floors may thus end up becoming bouncy or saggy and cause the surfaces to become uneven.
There are many ways to strengthen floor joists used in floor systems to avoid such problems.
One way is to provide beam-column supports at the mid-spans of the joist panels used under the floors of homes.
This is an effective method for improving joist performance, but it reduces the free space in a room.
Another technique is to use sistering joist panels to secure already present joists.
Sister joists are added on either or both sides of a joist.
Metal wrapping is also a popular method for increasing joist strength.
The wrapping transfers the loads from the center to the ends of a joist.
Steel can also be directly used to reinforce the joist panels, increasing their strength and durability.
How Much Do Floor Joists Cost?
Normally, a traditional 2x10 joist costs about $30 for a 16’ span, whereas an engineered 2x10 joist, costs around $40 for a span of 16’.
Floor trusses, on the other hand, cost around $4.40 per foot.
The cost of floor joists changes with the variety in their design, specifically their lengths and sizes.
A floor joist's cost also depends largely on the material from which it is constructed.
Wooden floor joists are relatively cheaper than steel floor joists.
For example, a 2x3 flange I-joist is more common and readily available.
Meanwhile, a 2x4 flange I-joist is rare and thus would cost more.
Floor joists are an integral part of house framing.
Many types of floor joists are available to choose from when securing floor systems.
The primary types of floor joists include traditional 2x10 lumber joists, engineered joists, and open web truss joists.
Traditional lumber joists are made using local wood and span a maximum of 16'.
Engineered joists are an improved version of traditional joists, made using a mix of local and engineered wood.
They offer better durability and strength and can span longer distances than lumber joists.
Engineered joists are available as solid rectangular blocks or as I-shaped members.
The third type of floor joist is the open web truss joist.
This member offers the largest span length options and the best strength values.
Open web trusses are available with various end-bearing conditions, allowing for more flexibility in the floor system design.
Although they offer great durability, open web trusses are quite expensive.
They are more suitable for large-scale construction projects instead of residential projects.
Frequently Asked Questions (FAQ)
Why are Engineered Joists Shaped like I-Beams?
The I-shape is the preferred shape given to engineered wood joists due to its efficient load-dissipating capability.
It consists of an upper and a lower flange that resists bending stresses and a web in between that resists shear forces.
What is the Common Spacing Provided Between Floor Joists?
The typical spacing provided in-between floor joists includes 12”,16”,19.2”, or 24”, according to the International Residential Code (IRC).
19.2” spacing is gaining more popularity among these four spacing options.
The codes do not provide guidelines for spacing above 24”.
If you want to go beyond this, say 30” or 35”, then you should consult local codes.
Can you Drill Holes in Engineered Floor Joists?
Yes, you can easily drill engineered floor joists to provide passage for plumbing ducts and electrical wiring.
As they have a large surface area, drilling should follow drilling code requirements.
Engineered floor trusses do not require drilling, as they already have built-in openings.
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