Floor Joist Sizing,Span,Spacing: IRC Requirements

The sizing, span, and spacing of floor joists are crucial aspects of house framing.

The span length of joists refers to the length of a single joist in between the supporting beams.

The sizing of the joist refers to the depth and width of the floor joists.

The most common spacing for wooden floor joists is 16". According to the 2021 International Residential Code, spacing variations such as 12", 19.2", and 24" are also acceptable. For a particular span, this spacing distance can vary based on the species of lumber, grade, and load conditions.

This article explains different floor joist sizing and spans, as well as how to select them.

Floor Joist Sizing, Span, and Spacing Requirements

The most used code for determining floor joist sizing, span, and spacing is the International Residential Code. The code requirements are based on a number of factors.

The limits are given based on the type of wood used, grades of the wood, and spacing.

The following tables are typical examples of the requirements and resulting spacing and length spans recommended by the IRC.

The tables provided in the code are based on the type of wood the joists are made from, the grade of the joists, and dead and live loading conditions.

Joist Spacing	Lumber	Grade	2x6	2x8	2x10
(inches)			Spans (ft-in)	Spans (ft-in)	Spans (ft-in)
12	Douglas fir-larch	#1	12-0	15-10	20-3
12	Douglas fir-larch	#2	11-10	15-7	19-10
12	Hem-fir	#1	11-7	15-3	19-5
12	Hem-fir	#2	11-0	14-6	18-6
12	Southern pine	#1	11-10	15-7	19-10
12	Southern pine	#2	11-3	14-11	18-1
12	Spruce-pine-fir	#1	11-3	14-11	19-0
12	Spruce-pine-fir	#2	11-3	14-11	19-0

Joist Spacing	Lumber	Grade	2x6	2x8	2x10
(inches)			Spans (ft-in)	Spans (ft-in)	Spans (ft-in)
24	Douglas fir-larch	#1	9-7	12-4	15-0
24	Douglas fir-larch	#2	9-3	11-8	14-3
24	Hem-fir	#1	9-2	12-1	14-10
24	Hem-fir	#2	8-9	11-4	13-10
24	Southern pine	#1	9-4	12-4	14-8
24	Southern pine	#2	8-6	10-10	12-10
24	Spruce-pine-fir	#1	8-11	11-6	14-1
24	Spruce-pine-fir	#2	8-11	11-6	14-1

The above tables provide span and sizing information for joists under a dead load of 10 psf and a residential sleeping space live load of 30 psf.

Factors such as fastening details and load capacity have also been factored in while developing the standard values.

Similarly, span and sizing information under more grade types and loading conditions are provided in the International Residential Code.

Table R502.3.1 (1) and Table R502.3.1(2) of the International Residential Code can be used to determine the maximum lengths and sizing for joists.

Provided that the dead load does not exceed 20psf and the residential live load is under 40 psf.

The IRC also dictates the standard span lengths for cantilever floor joists used for supporting a light frame exterior load-bearing wall in Table R502.3.3

Moreover, the size limits for drilling and notching in accordance with floor spans is also mentioned in the IRC.

For joists larger than 12" in width, IRC recommends blocking or bridging the joists.

Maximum Floor Joist Span

The maximum floor joist span for SS grade southern pine lumber at 16" spacing and 30 psf live loads according to the 2021 International Residential Code is as follows.

Joist Size	Maximum Span Allowed (Feet-Inches)
2x6	11'-2"
2x8	14'-8"
2x10	18'-9"
2x12	22'-10"

Exceeding the maximum allowed span can lead to sagging of the joist.

To repair sagging joists, you'll need to strengthen the joists from below.

Sistering of joists is a common technique that can be used to strengthen, stiffen, and support a building.

Size and Spans for Other Floor joist types

Engineered Wood Joists-Size and Span

In addition to traditional lumber, joists are also made from engineered wood.

Engineered wood offers more durability for lesser self-weight.

Engineered I-joists are normally available in lengths of up to 48’. However, the maximum length they offer is around 60’.

 The depths of I-joists range between 9 ½” to 16”, whereas the widths range from 2 ½” to 3 ½”.

The depths of engineered I-joist are usually kept at 9 ½” but can be increased up to 16”, likewise, the web can also be made thicker.

The lengths can be cut according to the house dimensions as required. There is not a standard code for engineered joists.

Instead, manufacturers provide user manuals dictating the proportion limits and spacing of engineered joists.

Engineered wood joists are typically more common in larger buildings as compared to residential blocks.

Truss Joists-Size and Span

Truss joists are also an efficient variation of joists that offer advantages over the traditional lumber joists.

Truss joists provide greater strengths, are required in lesser ratios, and have built-in openings that eliminate the need for drilling.

Moreover, greater span lengths are possible with truss joists relative to traditional joists.

Like engineered joists, there are no international codes available for truss joists. Instead, user manuals are generated by the manufacturers.

The following truss span and sizing data is provided by Alpine Engineered Products.

Center Spacing	Total Load(dead+Live)	Truss Depth	Span	Deflection Limit
16”	55psf	12”	22’2”	L/360
16”	55psf	14”	24’11”	L/360
19.2”	55psf	12”	20’9”	L/360
19.2”	55psf	14”	22’8”	L/360

I-shaped Joists: Size and Span

I-joists are designed so because their shape is best at resisting both bending and shear forces.

The webs transfer bending moments and the flanges transfer the shear forces.

As in the case of truss joists, there are no standard codes for I-joists to determine their spans and size. This is due to the many variations of I-beams that exist in the market.

As such, manufacturers are bound to provide an I-beam span chart indicating its specifications and proportioning limits.

Thus, when opting for I-beams, a detailed literature review and professional consulting are a must.

What are Common Sizes of Joists Used in Construction?

Generally, a 2x8 or a 2x10 floor joist is the most popular option. There are no set sizes that are to be used in residential construction.

2x10 lumber joists are increasingly being replaced by engineered floor joists in newer constructions.

Compared to 2x10 joists, engineered joists can span much longer although they are more expensive.

The specifications of various sizes of Joists are available in the IRC 2018.

Older homes used to be built with 2x6 joists. 2x6 are not as common anymore but you can still find their specifications in the residential code.

How to Select Floor Joist Span and Sizing

If you are an engineer, builder, or a common person trying to build your own house, then you will need to follow a set criterion to select the size and span of your joists.

Firstly, you will need to see the joists available in your local area. As the joist available will depend upon the material available in your locality.

It is important that you select joists of good material and grades to avoid any chances of collapse later on.

Next, you will need to determine the dead and variable load of your environment.

This can be determined by consulting other builders in your community or checking with local construction companies.

After you will need to determine the proportions of the room you want to build and select the size of your joists accordingly.

Many sizes can suffice your requirements, so you have to select based on economy and availability.

After taking into account the above requirements, you will make an initial selection from the IRC or your local construction manual.

It is important that you factor in the lengths required for drilling holes in the joists and for resting joists onto beams.

A joist should bear 1.5” on a wooden surface.

It is also to be noted that the actual dimensions of joists are somewhat smaller than mentioned due to finishing processes.

Consider that your local joists are made from southern pine wood and are of grade 2.

Next, ensure that the dead and live load would not exceed 10psf and 30psf respectively.

Consider that you chose 2x10 joists for your project.

Then, from the tables provided in the IRC, the length of your joists will be 18’ 1” and the spacing will be 12”.

What is the most common Spacing Provided Between Joists?

As evident by the IRC, the typical spacing for joists includes 12”,16”,19.2”, or 24”.

Out of the four, 19.2” spacing is gaining more popularity and is being used in truss joists, I joist as well as traditional joists these days.

The codes do not provide guidelines for spacing beyond 24”. If you want to go beyond this, say 30” or 35” then you should do so at your own risk.

However, most residential authorities do not permit incorporating spacings that are not found in the local user manual and codes.

Factors Affecting Floor Joist Spans and Spacing

There are various factors that need to be incorporated before determining the length and size of joist members.

These include the type of wood, the grade of the wood, dead and live loading conditions, and other dimensions of joists (web and flange thickness, etc.).

Other minor factors that may or may not be consulted are drilling requirements, fastening conditions, and the shape of the floor to be supported (simple or cantilever, etc.).

It is obvious that the larger the size of a joist and the closer the spacing, the more load it would be able to bear.

However, this concept is not always efficient. This is because the spans and sizing become limited based on the size of the room being built.

Thus engineers have to carefully select optimal joist sizes, lengths, and spacing to carry the required load within the required room size.

Factors that are critical for determining the right floor joist sizes and spans include

• Type of wood
• Grade of wood
• Width and depth of the wood
• Loading conditions

Wood Species

Wood types are abundant in nature. Every type of wood specie has its own characteristic strength. Some are less flexible than others.

Generally, the wood that grows slower is considerably stronger than that which grows faster.

Alternatively, weaker wood can be regrown more quickly.

Common species of wood found in the west include southern pine, redwood, and spruce among others.

When making joists traditionally local wood is used for which there are standard codes available dictating the span and spacing.

However, these days engineered wood has gained significant popularity as well.

Engineered wood is made from a number of wood derivatives making it lighter in weight and stronger in strength.

Such wood allows for even greater span lengths to be utilized in construction.

For very long spans, steel floor joists are used instead of wooden floor joists.

Lumber Grade

Grade refers to the quality of the wood. It categorizes the number of defects in lumber.

The defects include bows, checks, crooks, cups, knots, splits and twists, etc.

The lesser the number of flaws in a wood the greater its strength will be. Usually, grade1 or grade2 lumber is used for making joists.

Out of the two, grade2 is more widely used. It has few defects and knots are allowed if they are well spaced.

Grade3 and grade4 lumber are generally not recommended as joists make up the support for floor and ceiling systems.

Wood Dimensioning

Joist panel lengths are also dependent on the width and the depth of the joist.

The width is significantly more important than the thickness of the panel.

For example, a 2x6 lumber joist spans about 12’ whereas a 2x10 joist spans more than 20’.

The dimensions also depend upon the shape of the joist.

In addition to rectangular solid panels, joists commonly come in I-shape as this shape is considered more effective.

Loads

Another major factor in determining joist lengths and spacing is the amount of load that it has to bear.

The load under discussion refers to the floor loads that joists have to bear. Floor loads are then categorized as live and dead loads.

For both these categories, there are limits that influence the span lengths and spacing of joist members.

The dead load value for residential blocks is set between 10psf to 20psf in the code. Whereas the live load varies between 30psf to 40psf.

Different lengths and spacings are required for different values of loadings.

Conclusion

The length spans and sizing of joists are crucial to determine beforehand so that you can avoid any sort of failure or collapse afterward.

To do this it is essential to understand what are the factors that affect the spans, sizing and spacing of joists.

Once you have understood the critical parameters, you can move on to selecting the right size and span of joists for your construction project.

To aid you with your design there a number of codes like the IRC available at your disposal to consult and construct.

However, these codes do not take into account all types of joists.

Truss joists, I-joists, and all types of engineered joists need to be dealt with separately.

To do this you can consult the literature available on their spanning and spacing or consult with your local or international manufacturers.

Manufacturers are bound to provide a manual incorporating the different specifications and sizing limits of their engineered products (like joists).

Frequently Asked Questions (FAQ)

How far can you span a 2x6 floor joist?

A typical 2x6 joist can span a maximum of 10' 9" at 16" center to center spacing. This is assuming 2nd grade douglas fir larch as the lumber and 30 psf load.

How long can a 2x8 floor joist span?

The span for a 2x8 wood joist ranges between 16’ to 23’.The span for a 2x8 varies depending upon the species and grade of the wood as well as the loading conditions.

Are 2x10 good as floor joists?

2x10s are still the most common option for making floor and ceiling joists in residential construction. In addition to offering long spans, they remain the cheapest option that provides the best value for money.