Floor Joists Explained

Floor Joists Explained – Residential floors require proper support to carry the weight of everything and everyone inside a room, providing a secure and stable space. However, for the floor system to provide proper support, it must include the correct implementation of floor joists.

Floor joists are horizontal structural elements, spreading between foundation walls or structural beams. Their purpose is to transfer the load to vertical structural elements, helping distribute the structure’s load.

Long story short, floor joists are an essential part of the floor and an entire structure. This is why builders need to ensure that floor joists provide appropriate support. Several factors may impact the floor joists’ load capacity, including floor joist spacing, size, and wood species used.

If you’re going through the home building or remodelling process, the following article will provide you with all the answers regarding floor joists.

Different Types of Floor Joists

The first thing to know is that there are several types of floor joists you can opt for – solid lumber joists, TJI joists (I-joists), and open-web trusses. Here’s a quick look at how they differ and what are their pros and cons:

Solid Lumber Joists

Solid lumber floor joists are compact boards made of old-growth trees. They are known for their fire-resistance properties, as they can last longer during a fire than other types of joists. They are also less expensive.

On the other hand, solid lumber joists do come with a limited span distance, which is affected by several factors, including joist board size, wood species, deflection, and spacing. They are also not environmentally friendly and using younger trees to make them might result in warped wood.

TJI Joists

LJI joists are also referred to as I-joists (they resemble capital I). This type of floor joist is made from different materials. The tops and bottoms are usually manufactured from wood or laminated veneer, and the centre is made of plywood or OSB (oriented strand board).

To their pros, we can include longer spans compared to solid wood. These floor joists are also lighter for more straightforward maneuvering. However, I-joists are more expensive and fail quicker during a fire.

Open-Web Trusses

Open-web trusses are made of lumber, with metal connectors holding diagonal and vertical elements to two horizontal wood frames. Because of their unique construction, they provide increased support and even longer spans. What’s more, due to their construction, they can accommodate plumbing pipes, HVAC, and electrical wires easier as they don’t require cutting.

However, as you probably suspect, these floor joists are more expensive. They also cannot be trimmed, as they come in specific lengths.

Floor Joist Span

Floor joist span refers to the distance between the supporting structures covered by joists, for instance, structural beams. In general, larger floor joists can carry more load. The same goes for floor joists spaced closer together. However, as mentioned, several factors can impact the load capacity and, therefore, the size of joists used. The most common floor joist sizes are 2×8, 2×10, and 2×12, with contractors adjusting spacing depending on the circumstances.

Wood Species for Floor Joists

Floor joists can be made from several various types of wood, with the most common being hemlock, redwood, Douglas fir, Southern yellow pine, Western red cedar, ponderosa pine, and spruce. Each comes with its unique properties, with some being bendier than others.

The bending strength refers to how much load the lumber can withstand and is typically measured by high, medium, and low bending strength. As a rule, the stronger the wood, the more it can withstand without any additional support.

In general, the wood derived from slow-growing trees is significantly stronger than the wood from fast-growing species. When it comes to woods and their strengths, your best options are:

  • For high bending strength: Southern yellow pine and Douglas fir,
  • For medium bending strength: redwood, hemlock, and spruce,
  • For low bending strength: Western red cedar and ponderosa pine.

However, keep in mind that not all types of wood may be available in Australia.

Lumber Grade and Size

Other considerations when choosing floor joists are lumber grade and lumber width. The first is another strength factor, whereas the latter refers to the joist board’s top-to-bottom width.

Starting with the lumber grade, the grades are given based on the wood’s quality. The fewer defects it has, the stronger it will be. Stronger pieces of lumber will be given higher grades – clear, select, or 1. However, when it comes to floor joists, these are mostly made of 2-grade lumber. 2-grade lumber may not be as flawless as the one with higher grades, but it’s still robust enough to provide proper and secure support. As for the 3 and 4-grade lumber, it’s best to avoid it.

Now, when it comes to the lumber width, it heavily determines how far a joist can span. It all comes down to the board’s top-to-bottom width, which is much more important than the board’s thickness.

Floor Loads

Determining the floor load capacity is essential when constructing durable and secure floors. This, however, requires expert knowledge of wood’s properties and building code requirements. This is why it should always be calculated by a professional.

In general, there are two types of floor loads – dead and live load. More on them below.

Dead Load

Dead load refers to the weight of the entire floor structure, as well as structures permanently attached to the floor. It’s calculated by adding together the weight of the materials used and then dividing it by the square footage. The dead load should be around 8-10 pounds per square foot for residential constructions.

Live Load

In essence, the live load is the total weight the floor has to carry, such as people, furniture, appliances, and other objects. Live load for residential floors should support between 30 and 40 pounds per square foot (psf). However, the psf may vary depending on location in a house.

For example, the first-floor live load should be higher than the second-floor one, with the first having to be at least 40 psf and the latter 30. Only sleeping rooms also need to carry just 30 pounds per square foot. However, if you have, let’s say, a garage over your basement, the psf should be at least 50 or higher.

Key Takeaways

Floor joists play an essential role in supporting the entire home structure. They carry the weight of everything and everyone within a room, which is why it’s so vital to ensure you get all your measurements right. Not just that, floor joists should also be made of top-quality wood. Otherwise, they may lose their properties sooner, forcing you to replace them, which can get costly.

And if you want to rest assured your flooring needs are met, choose ID Flooring. We will help you with everything, starting from choosing your perfect floor and ending with professional installation services. Give us a call today to get a free quote or contact us via our website if you seek advice regarding your project.