When building a mobile home, you must use sturdy floor joists to support the weight that will be on top of it. You might be wondering how far apart your floor joists should be. We researched the answer for you, and here's what we found.
The typical spacing of floor joists is 16 inches apart from the center of the joist. This is the standard spacing for mobile homes.
For various lumber sizes and spans, here is a list of the appropriate spacing:
| Spacing (Inches) | Lumber Size | Span (feet) |
| 16 | 2x4 | 6, 7 and 8 |
| 16 | 2x6 | 10 |
| 12 | 2x6 | 12 |
| 16 | 2x8 | 15 |
| 12 | 2x8 | 16 |
In dealing with the spacing of floor joists, there are a number of factors to consider. To learn more about the subject, keep on reading. We'll discuss the specifics of spacing for floor joists and the different types of floor joists.
Factors That Impact Floor Joist Spacing
Several factors should be taken into account when determining the spacing of floor joists. These can affect the strength and stability of the floor, so they are important. These factors include the joist span and joist size.
Keep in mind that the standard spacing of floor joists is 16 inches. This is the most common spacing used by homebuilders as approved by the International Residential Code.
Joist Span
A joist span is a length between two supporting structures like beams, walls, or other framing elements. In determining the span of the joists, the design span must be considered, not the total span.
Other factors that affect the determination of the joist span include load capacity and the grade of the lumber.
Joist Size
Given that floor joists make up the support of a house or building, you must correctly choose their size. As with the span, the joist's size depends on wood type, load capacity, joist spacing, and length of the span.
Joist size, span, and spacing are three factors that are intertwined. The size, space, and span of each depend on one another. These three factors are critical in building floor support.
Wood Type
Different wood species have their own characteristics. Only certain types of wood are suitable for making floor joists. These common wood species are Southern yellow pine, Douglas fir, hem fir, and Sitka spruce.
Douglas fir is the most common wood type used for floor joists. The wood is known for its strength and ability to withstand moderate fires. In cases where there is no Douglas fir, the hem fir is a good substitute.
The Southern yellow pine is also a good choice for floor joists because of its load-bearing strength and resistance to shrinkage. Sitka spruce is the cheapest of the four. It is soft and flexible.
Grade Of Lumber
The typical choice of homebuilders is 2-grade lumber. Lumber with few defects tends to be stronger, and this lumber is labeled as clear, select, or one.
Although 2-grade lumber has more flaws than higher-grade lumber, it is the most common lumber grade because its flaws are insufficient to affect the strength of the lumber.
Load Capacity
Load capacity is another factor affecting the determination of joist span, size, and spacing. The weight situated on the floor has two categories: live and dead loads.
A dead load is a weight of construction materials integrated into the building, like walls, floors, ceilings, and other structural objects. A live load is a weight placed on top of the floor, including people and furniture.
The dead load ranges from 10psf to 20psf, and the live load is set between 30psf and 40psf by the International Residential Code.
Different Types Of Floor Joists
In a mobile home, floor joists keep the floor steady and strong and provide weight-bearing support. Three types of floor joists are widely used and available: solid lumber, I-joists, and open-web floor trusses.
I-Joists
I-joists are sometimes called TJIs because they resemble the capital letter I. These joists have different parts composed of various materials.
The top and bottom of an I-joist are commonly wood or laminated veneer, and the center connecting the top and bottom is composed of plywood or oriented strand board.
This type of joist has a longer span distance that can reach up to 26 feet. It has a load capacity that is higher than solid lumber joists and also has little flex. I-joists can pre-scored or already have knockouts.
The combination of durable structural materials makes this type of joist an economically friendly option.
Advantages
I-joists have longer spans and are easy to install because of the knockouts. In addition, they are lightweight and easy to maneuver.
Disadvantages
Unlike solid lumber joists, I-joists can't withstand fire for long periods. Another disadvantage is that they are costly.
Open-Web Floor Trusses
An open-web floor truss is made with 2 x 4 or 2 x 3 lumber with a web-like structure in the middle. The web-like section is comprised of metal plates that provide steadiness and support. This type of floor joist uses less wood than the other types, making it environmentally friendly.
This type of floor joist is low in moisture capacity. It also has an excellent load-bearing capacity and is constructed to resist bowing and twisting.
The open area is perfect for wires, plumbing, electricity, and utilities. Homebuilders prefer this joist because they do not need to make holes and measurements for the electrical wires and pipes.
Advantages
Open-web floor trusses have longer spans that can be up to 30 feet. It also allows electrical lines, water pipes, heating, ventilation, and air conditioning systems to be installed without cutting.
Disadvantages
This type of joist is heavy and more expensive than the other types. Also, it isn't easy to trim and cannot always be adjusted. Additionally, the metal plates tend to strip electrical wirings when installed.
Solid Lumber Joists
Solid lumber joists were the most widely used type of floor joist in the past. But because of technological advancements, modern building materials have been produced, and this type of joist has become less common.
Solid lumber joists are made of old-growth trees and strong wood.
The price of this joist varies depending on wood species, lumber grade, and board size. This type of joist is said to be the most economical choice because it's affordable. However, from an environmental point of view, they can be thought of as expensive because the supply can be exhausted.
Advantages
One of the advantages of solid lumber joists is that they can survive floods and fires. Another is that they are cheaper than other types of joists.
Disadvantages
Some disadvantages are that old-growth wood supplies are quickly depleted. Moreover, these joists have a limited span distance.
Can You Use Joist Hangers For Floor Joists?
A joist hanger is commonly used to hold and attach floor joists to beam structures. It makes a connection between the joist and the beam or ledger board for building a floor surface. These hangers are made of strong steel and kept in place by nails or screws.
While you can use joist hangers for floor joists, they are not always required. Some floor joists can be attached to beams without joist hangers.
Check out this joist hanger on Amazon.
In Conclusion
The most commonly used spacing for floor joists is 16 inches. Factors like floor joist span and size determine floor joist spacing.
You will need to take these factors into account when installing floor joists because the strength and safety of the flooring structure depend on them.
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