Floor Joist Sizing and Span In Residential Construction

I had a good debate with a friend the other day. He claimed his floor joists were too small for his home, which made his floor wobbly. I took a look and said they looked fine but just needed some added support. I then mentioned that I could prove my point because there are specific load tables outlined by the building code that dictate floor joist sizing.

Proper floor joist sizing is specified by the International Residential Code – the IRC. It outlines max span lengths for wood joists of all sizes – from 2×6 to 2×12 and greater. The code also identifies joist sizing and spans for various joist spacing – from 12” up to 24” apart.

Whether you are building a new home or already own one, understanding the function of your floor joists is critical. Why? For one, they support the weight of your home, along with the exterior walls. The other reason is that knowing what role they play in the framing of your house can help you make repairs or assess issues with your floor framing down the road.

In this article, we’ll go over everything you need to know about floor joist sizing and spans in residential construction. We’ll cover standard wood joists as well as engineered joists such as I-beams and LVLs. Finally, we’ll go over ways to assess your floor joists and give you some tips as to how to go about fixing undersized joists.

Floor Joist Sizing and Spans In Residential Construction

What Is Floor Joist Span and Spacing?

Floor joists sit beneath your floor and are the pieces of lumber that support the weight of everything inside your house. They are supported by a beam, or beams, which are planted in the concrete of your basement slab.

Floor joist span is the length of an individual joist, from the foundation walls to an interior beam.

The spacing between joists is typically 16”, but can also be 12”, 20”, and 24”. The spacing and ability to span a greater – or smaller – distance is dependent on the size of lumber used for the joists.

Floor Joist Code Requirements

Building code, specifically the International Residential Code, clearly dictates the allowable spans for floor joists based on lumber type and spacing. Code also outlines the type of lumber, fastening requirements, and any other requirement you would need to install a joist in a home.

Factors that Affecting Floor Joist Spacing and Span

Standard joist spacing

Many factors are involved in determining how far the joist will span. As well, the spacing between joists depends on a variety of factors. The anticipated live load on the floor framing, the species of wood used, the quality of the wood, and the size of the lumber all play a role in the length and spacing of floor joists.

Lumber Size

The size of the lumber used for floor joists is the single greatest determining factor in terms of how far a joist can span. The joist span tables start with 2×6 lumber and it goes all the way up to 2×12.

For example, a 2×6 Southern Pine joist can span 12’. A 2×12 joist of the same species and quality can span nearly 25’. The spans listed in the span tables of the IRC indicate max span, so many contractors will often overbuild a floor for the sake of stability. So, if there is a span of 11’, 2x8s are often used even though 2x6s are technically acceptable.

Wood Species

The species of framing lumber you will be able to access will depend on where you live. For much of the US, Southern Yellow Pine (SYP) is the main type of lumber you will be able to access. Other species, such as spruce, pine, and/or fir are also quite common. Douglas fir, hemlock, and larch round out the species found on the floor joist span tables.

An SYP 2×12 at 16” spacing can span about 23’. A spruce/pine/fir 2×12 at the same spacing can only span around 21.5’. With a difference of nearly 18”, knowing the type of species you are building with is critical. Overall, Douglas fir and larch are the strongest, followed by SYP.

Lumber Grade

Another aspect of floor joists is the quality of the wood used. All wood produced for commercial sale is graded. The highest grade – select – is usually free of any blemishes, knots, and other imperfections that can weaken the lumber. Select lumber can bear the greatest loads.

After select grade, lumber carries a grade of either 1,2 or 3. Lumber graded as 1 is of higher quality than 2 or 3, and so on. Joist span tables include max span lengths for all four grades for each species of lumber.

If you are unclear as to the grade of lumber you are purchasing, there should be both a white tag stapled to the wood indicating the grade and a black stamp on the lumber itself with the grade and species.

Floor Joist Loads

Potential loads for the floor supported by your joists are taken under consideration by the joist span tables, too. When you look at the IRC, you’ll notice two separate span tables for floor joists. One is for 30 pounds per square foot (psf) of live load, and the other is for 40 psf.

There is a reason for this. Bedrooms, or the joists supporting bedrooms, are allowed to be built within the specs for a 30 psf live load. Why? Bedrooms are perceived as not having the same load levels as a more trafficked area like a living room. Other living spaces must have joists that can support 40 psf, hence the need for two different joist load tables.

While a bedroom can potentially have smaller-sized joists than a living room due to the differing load requirements, most homes are built with the same-sized joists throughout the home. So a home that requires 2×10 joists will feature only 2x10s, even if it has bedrooms that can have 2×8 joists due to the lower load requirements.

Typical Floor Joist Size In Residential Construction

Floor joist size

Floor joists range from 2×8 to 2×12. It is less common to see joists that are 2×6, even though they are included in joist span tables. Older homes are more likely to use 2×6 for joist framing.

There is no “typical” floor joist size because there are so many variables regarding the floor framing. However, if you are concerned that your floor joists are undersized, then a quick look at the IRC joist span tables can tell you if they are the proper size or not.

Sizing Floor Joists – Example

When sizing floor joists, you measure the distance between the faces of the supports. That means the span indicated in the table does not include the length the joist will bear on the beam, concrete block, or masonry wall on either side. Bearing is not included because the length a joist bears on a surface is dependent on the bearing surface.

A joist must bear at least 1.5” on a wood or metal surface and at least 3” on a concrete or masonry surface. Knowing this, let’s say you are building a new house with concrete block walls and one main beam running down the center of the house, which is a rectangle.

For example, the distance from the inner face of the concrete wall to the face of the wood beam is 17’ 6”. Your joists will bear directly on the concrete and wood beam. You also plan to use the same sized floor joists for the entire structure, so you will first lookup the joist span table for 40 psf loads, not 30.

Next, you will look in the “spacing” column for 16” on center, which is the spacing for your joists. You will be using grade 1 SYP lumber. With this information in hand, the joist span table tells me that 2×12 joists are necessary for the house.

Sizing Floor Joists in an Existing Home

Wonder if your floor joists are sized properly in your home? Go downstairs and have a look at your joists. This could be difficult for those with fully finished basements, although if you have a drop ceiling, you can simply push a panel up and use a flashlight to have a look.

You are first looking for your floor joist size. Measure the width of the joist. Remember that 2×8 actual width is 7 ¼”, 2×10’s width is 9 ¼”, and 2x12s are 11 ¼”. Next, you need to know your joist spacing. If you can see one joist, then you can probably get a tape measure and see the distance to the next joist.

Once you’ve measured, see if you can measure your span. If there is a wall in the way, you’ll have to estimate. A framed wall with drywall is 4” wide. Add a gap between the framing and concrete foundation wall, and you are looking at about 5”.

You’ll also need to see if you can determine the type of lumber used for your floor joists. The black label on the lumber will indicate the tree species used, as well as the grade. This will help you look at the right column on the floor joist span tables.

If your existing joists are 2×10 at 16” on center and are a select grade of SPF (spruce, pine, fir). You measured from the edge of your beam to a finished basement wall and added 5 extra inches to that span for a total span of 15’ 8”.

A glance at the floor joist span tables tells me that a select grade SPF 2×10 at 16” on center can span a max distance of 16’. Since your span is only 15’ 8”, then your joists are properly sized for your home.

Floor Joist Spacing: How Far Apart Are Floor Joists in a House?

Lumber spans for floor joists

Floor joists are typically 12”, 16”, or 24” apart in a home. Most common spacing for floor joists is 16” o.c.

You’ll notice that the joist span tables also include 19.2” on center as an option. This spacing is typically for I-beam and truss joists since the tops of those types of joists are wider. However, you will find standard lumber joists spaced at 19.2” in newer homes, as this spacing is becoming more common.

Notice that floor joist span tables do not include spacing options for anything beyond 24”. Thus, if you are planning a new build and want to space joists at 30”, for instance, then you do so at your own risk. Also, know that no building permit office will ever allow a plan for joists spaced anything other than what is found in the span tables.

How Far Can Floor Joists Span?

A floor joist can span nearly 26’ in the most extreme cases. A select douglas fir 2×12 supporting a 30 psf live load and 10 psf dead load, spaced at 12” will have the greatest span length. However, you would only be able to use this beneath a sleeping area since it is rated for 30 psf, not 40.

The maximum span for a floor joist that needs to support a 40 psf live load would be just over 23’. Again, this would be for a select douglas fir 2×12 spaced at 12”.

Standard Floor Joist Span

A quick glance at the floor joist span tables will tell you that any span beyond 20’ is not common. It is difficult to source framing lumber longer than 20’, which is usually much more expensive when available. Many builders who face spans of greater than 20’ choose to install a beam to break up longer spans.

There is no standard floor joist span length simply because every home is unique. It just takes one look at a floor joist span table to tell you that there are many combinations of variables when it comes to floor joists.

Maximum Floor Joist Span

There are maximum spans for floor joists. We’ll take a look at those spans below. Note that all max spans are for select douglas fir lumber at 12” on center spacing.

Max Joist Spans for 30 PSF Live Loads

Joist Size Max Span
2×6 12’6”
2×8 16’6”
2×10 21’
2×12 25’7”

Max Joist Spans for 40 PSF Live Loads

Joist Size Max Span
2×6 11’4”
2×8 15’
2×10 19’1”
2×12 23’3”

Floor Joist Span Calculator

If you don’t have the time to look up the joist span tables, then using a floor joist span calculator can help you determine how far a joist of any species, width, quality, and spacing can span.

This calculator also takes more specific criteria into consideration, such as moisture and species not listed on the IRC span tables. However, it should be noted that the IRC floor joist span tables are the final word in terms of what is and is not allowed for floor joists – not the floor joist calculator.

Other Floor Joist Types

There are many other types of floor joist options beyond traditional framing lumber. Most new home construction projects favor the use of truss-style or I-beam floor joists over standard lumber as they can span greater lengths at a lower cost.

I-Beam Floor Joists

Engineered floor joist

I beam floor joists feature a top and bottom “flange”, which is either a 2×3, 2×4, or an LVL. Between the flanges is engineered wood – typically OSB (orientated strand board) – called a “web”.

I-beams are often wider than standard framing lumber and can span longer distances. I-beams are not costly, particularly when compared to 2x12s. Since I-beams only use smaller lumber for the flanges and mass-produced engineered lumber for the webs, they are substantially cheaper, which is why they are preferred in new construction.

Truss Floor Joists

Maximum floor joist span

Truss floor joists are just as their name implies – trusses. Truss floor joists use either 2×3 or 2×4 lumber exclusively to form a series of webs to make up one floor joist. Typically there is a top chord and a bottom chord. Between the chords are “webs” of the same sized wood that run diagonally, top to bottom, repeating this pattern over the course of the joist.

The intersections of the webbing with the chords are held together with steel nailing plates. Trusses have benefits that I-beams and traditional lumber joists do not, such as having open webbing that makes it much easier to run plumbing, electrical, and HVAC services through.

I-Joists Span Chart

The span of an I-beam floor joist in residential construction is entirely dependent on the manufacturer of the I-beam. That is why there are no I-beam floor joist span tables in the IRC – because they couldn’t possibly account for all the different types of I-beams on the market.

Manufacturers will produce an I-beam span chart to guide installers for the installation of their specific product. They are not to be used in the installation of any other type of I-beam. Therefore, if you opt for I-beam floor joists, be sure you have the proper literature outlining proper installation techniques for that manufacturer because you won’t find it in the IRC.

Conclusion

The length of your floor joists depends on many factors, and simply glancing at your joists and assuming they are too small is not a good way to determine the efficacy of your joists. You must know all the variables that determine joist span length to assess your joists.

If you are building a new house, then your home’s plan will be stamped by an engineer and your joists will be sized properly. But if you are building an addition yourself or another structure, the joist span tables in the IRC must be followed.

Remember, failure to size floor joists properly can result in the wobble, vibration, and even outright failure. Consult a professional before you build, and understand that overbuilding is always better than underbuilding.

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