How Far Apart Should Floor Joists Be? (Complete Guide)

Joists are very commonly used in most woodworks. If you are building a wooden floor, chances are you will need to add joists for additional support for your floor. While making the floor, you might be confused about the spacing of joists.

The spacing of joists may sometimes get a bit confusing. But there are some general rules about it. Let’s take a look and find out more about the spacing of floor joints below.

How far apart should floor joists be?

In the USA, 12-in, 16-in, and 24-in spacings are allowed for floor joists depending on the builds. However, the most common spacing between joists is 16-in. This gap is standard as it can support the amount of load typical in usual houses. Some use 19.2-in spacing, but they are not common.

Floor joists are usually used to provide additional support to the floor framework—this helps retain the structural integrity and keep the flooring from wrapping or twisting.

According to the construction code, the most common spacing between joists in floors in the United States is 16 inches. However, some also use 12-in and 24-in spacing in their builds.

How far apart should floor joists be

Floor joists are commonly used in most wooden floors. However, for different types of constructions, the spacings used in these joists vary. The list below shows the required spacing in floor joists for different builds:

In a house:

In the USA, the building code approves three different spacings for floor joints. These are 12-in, 16-in, and 24-in. The most common spacing, however, is the 16-in gap.

On a deck:

For floors of the decks, the joist spacing is not very different from houses. The most common spacing on decks is 12-in and the 16-in gap.

For a shed:

The joist spacing for sheds is commonly 16-in. However, if you want to maximize the load-bearing capacity, then a 24-in gap is preferable.

In a mobile home:

Like regular homes, the floor joists spacing of mobile homes are 12-in, 16-in, and 24-in. However, 16-in is the standard spacing for this purpose.

Engineered floor joists span:

Typically, the engineered floor joist spans have the standard 16-in gap between them. However, 12-in, 20-in, and 24-in gaps are also permissible.

How far can these floor joists span?

In the woodwork line or construction, joist span usually refers to the length or measurement that a joist covers between the beams or other supporting structures. The spans are an essential measurement in any build.

That is why most carpenters use pre-calculated measuring charts to find the appropriate spans for the joists.

Joist span depends on a number of variables. These variables include the type of wood, lumber grade, on the center gap between the joists, the load, and the joists’ size. The list below shows various spans for joists of different sizes and wood.

Here the o.c. is the standard 16-in, and the grade of timber is grade 2.

2×4:

When it comes to floor joists, 2-in by 4-in joists are a bit unusual. However, there are still pre-calculated values for this size of joists. For Yellow Pine, the span will be 6-ft 4-in, while for Douglas Fir, the span measures 6-ft 2-in. 

2×6:

In 2-in by 6-in joists, the span lengths differently. The span for Yellow Pine joists is 9-ft 4-in, while for Douglas Fir, the span length measures around 9-ft 11-in. For Spruce, this length is around 9-ft 2-in.

2×8:

2-in by 8-in joists are a bit bigger. That’s why they have a bit larger spans in general. Yellow Pine joist of this size spans around 12-ft 3-in. Douglas Fir, on the other hand, measures around 13-ft 1-in.

2×10:

2-in by 10-in joists can span a lot more than the previous sizes. Yellow Pine wood spans around 15-ft 8-in, but the Douglas Fir spans even more, which is 16-ft 9-in.

2×12:

2-in by 12-in joists are the biggest size of joists used for floors. The Yellow Pine joists span around 19-ft 1-in, while the Douglas Fir joists span around 20-ft 4-in.

How far can floor joists be without support?

Floor joists are an essential part while building a wooden floor. And choosing the correct number of joists and spacing between them is quite a difficult task. In some cases, it may even be more complicated than it meets the eyes.

To decide how far your floor joists can last without support needs various deciding factors. Some of the common factors are the type of wood, grade of the timber, width and the thickness of the joists, spacing between them, and the amount of estimated load.

So, it is safe to say that there is no definite number for this question. However, the general rule of thumb is that for every 1.5 times of a board’s depth in feet, 1m extra span is added. But these numbers can change based on the type and grade of the timber used.

How many floor joists do I need?

Joists are usually added to wooden floors for additional support and to increase structural firmness.

However, the number of joists that one needs to add to their floor depends on the dimensions of their base, the load it will need to bear, and the distances they will keep between each joist.

Finding the required number of joists is a more challenging task than many think it is.

For a standard 11-foot by 12-foot room, the number of floor joists you will need will be around 12 when a standard spacing is 16 inches between them. Other factors like the type of timber used and its grade also affect this number. 

To find the exact number of joists that you will need, you can do simple math. Divide the width of your floor by the center spacing of the floor. Then round the number, you got to the nearest integer. That will be your required number of joists for your floor.

Do I need noggins in floor joists?

The function of noggins is to provide additional support for your wooden structure. So, it is always better to add noggins to your floor joists. In this way, you will have a stronger and structurally more sturdy floor.

You can find whether you need to add noggins to your floor joists or avoid them by checking the joist spans. For joists with a span of 2.5m, you will not need any noggins at all. If the joists span around 4.5m, you will need one row of noggings on the middle spans.

In the case of joists that spans above 4.5m, you will need two rows of noggins for every third point. As already mentioned, it is good to have noggins in your floor joists.

However, you can skip them or cut their numbers to a bare minimum if you are on a budget.

Do noggins strengthen joists?

Noggins are instrumental pieces of wooden support that strengthens the joists. They are used often in walls and floor joists as they provide structural balance and robustness.

Noggings are sometimes called bridging. Their primary function is to strengthen the joists or studs in a wooden wall or floor. They also provide additional fixing for separate structures of a primary framework.

In addition to providing extra support, noggings also stiffen the woodwork. As a result, the overall structure becomes stiff and less likely to get bent in the future.

In the case of walls, noggins are vital as they help to prevent the joints from cracking. So, all in all, it is definitely safe to say that noggins strengthen joints.

Should floor joists be bracing/cross bracing?

The purpose of the floor joists is to provide additional support and load-bearing strength. They are horizontally placed wooden supports for the frame of the floor. For providing extra support, joists are often placed in different patterns.

Cross bracing makes joists even more robust and provides further support. According to the code for residential buildings, floor joists must have cross bracing.

Cross bracing makes the floor joists stronger, stiffer and thus increases the overall integrity of the structure. In contrast, unbraced joists tend to wrap or flex over time. That is why you will need to implement cross bracing to your floor joists.

In some states, it is mandatory to use cross bracing in floor joists. So, if you are building a new home, check your state’s building code and preferably use cross bracing to your floor joists.

Final Thoughts:

The standard spacing which is allowed according to the building codes in the USA is 16-in. Furthermore, sometimes depending on the size of the joists and the expected load on the floor, 12-in or 24-in gaps are also used. However, you can tweak these numbers a bit based on your specific needs.