What Are Floor Joists and How Do They Work? | BigRentz (2023)

Floor joists are horizontal structural members that span an open space, often between beams, which subsequently transfer the load to vertical structural members. These joists, part of the floor system, carry the weight of everything inside a room, including walls, furniture, appliances, and even people.

Most often, floor joist spacing is 16 inches apart on center, but this can vary depending on building codes and the requirements of the structure in the blueprint.

Floor joists help distribute the load of a structure. When weight is applied to the floor and the joist, wood fibers on the bottom of the joist go into what is known as tension. The top fibers go into compression, and this helps distribute the load evenly.

The Anatomy of a Floor Frame

What Are Floor Joists and How Do They Work? | BigRentz (1)

Prior to building your frame make sure that the ground is compacted and the foundation is sturdy. These are the major components of a floor frame (note: this is not a comprehensive list of everything that goes into a floor frame):

  • Blocking – Bridging is made up of smaller wood sections fastened between joists. This provides what is known as lateral stability.
  • Header joists – A header, or rim joist, is used to frame an opening in the floor. It provides lateral stability.
  • Floor joists – Floor joists are large wood framing members that are positioned on their narrow ends. They are evenly spaced and are connected to the sill plate. A subfloor is attached to these joists, which carry the weight of the loads to the walls.
  • Sill plate – The sill plate attaches to the top of the foundation, usually made from treated wood. Joists fasten to the sill plate.
  • Support beam – The support beam, also known as the center beam, bears the first-floor joists if the joists aren’t long enough to reach between foundation walls.

Types of Floor Joists

What Are Floor Joists and How Do They Work? | BigRentz (2)

(Video) Science of Simple Spans of Floor Joists

There are several different sizes and types of floor joists. The most common sizes are 2×8, 2×10, and 2×12.

Solid Lumber Joists

Solid lumber joists are contiguous boards, usually made from old-growth trees. Their span distances are affected by things such as species, board size, spacing, and deflection. Solid lumber joists on a job site are still common, but the supply of trees is being exhausted, and using younger trees for the joist can result in warped wood.


  • Last longer during a fire
  • Less expensive than engineered solutions


  • Not environmentally friendly
  • Limited span distance

I-Joists (TJI Joists)

I-joists, also known as TJI joists, get their name because they look like the capital letter I. Different sections of the I are made with different materials. The tops and bottoms are often made of wood or a laminated veneer. The center support is usually plywood or oriented strand board (OSB).


  • Longer spans than solid wood
  • Lighter for easier maneuvering


  • Fail more quickly in a fire
  • Cost more

Open-Web Floor Trusses

Open-web floor trusses make use of wood pieces cantilevered to help support floor loads. Many builders like these trusses because they don’t have to measure and make holes for things such as pipes and electrical wires.


(Video) The Big Build 17. Fitting the new floor joists. Posi Joists Floors are brilliant!! An introduction

  • More rugged construction than I-joists
  • Even longer spans
  • Can accommodate HVAC, plumbing, and electrical without cutting


  • Have specific lengths — can’t be trimmed
  • Higher cost

Joist Span Considerations

A joist span is the distance covered between supporting structures. Usually, structural engineers calculate the spans to ensure accuracy.

As a rule, the larger the structure, the larger the joists that are used. However, considerations such as the type of floor joist and the dimensional lumber used to construct them, as well as building codes, factor into floor joist spans.

It’s important to realize that there are no standard joist sizes. That means many factors need to be considered when determining the correct joist size and span.

Wood Species

The common types of wood used to make floor joists are redwood, hemlock, Douglas fir, and southern yellow pine. However, not every type of wood product is available across the country.

It’s important to understand the strength differences between wood types when choosing joist material. The bending strength of a wood reveals the load the lumber can withstand perpendicular to the grain. This is usually measured by low, medium, or high bending strength.

Stronger wood can span longer distances without needing additional support. For example, redwood has a medium bending strength, so it can’t take as much weight as fir. So while one type of wood may look better to you, you have to consider its strength. Staining is always an option if you wish to change the appearance of an exposed joist.


Another strength factor to consider when selecting wood floor joists is the lumber grade. The grade is determined by how many knots and other defects are found in the wood. Higher grades of lumber have fewer defects and are stronger than the lower grades. However, the higher the grade, the higher the price.

The most common lumber for floor joists is #2 grade. This lumber has more knots and defects than higher grades, but it doesn’t lose much in the way of strength.

(Video) How To Install Floor Joists

Lumber Width

The width of lumber plays a large role in determining how far a joist can span — the distance a piece of lumber can cover before it needs to be supported by a foundation or support post. The strength of a joist board is affected by its top-to-bottom width. This is much more important than the thickness of the board.

Load Capacity

Load capacity is critical when using floor joists. Determining the load capacity of a floor joist involves expert knowledge of wood’s structural properties as well as an understanding of building code requirements.

There are two types of weight a floor must bear: live and dead loads.

Live Loads

The live load is basically the weight of anything that isn’t connected to the structure, such as furniture, appliances, and people. Commonly, non-sleeping rooms must support a minimum live load of 40 pounds per square foot and sleeping rooms 30 pounds per square foot, according to the International Residential Code.

Dead Loads

The weight of the floor structure, and structures permanently attached to the floor, is known as dead load. This load is determined by what materials make up the floor. A normal wood-frame floor has a dead load of about 8 pounds per square foot. Flooring materials that weigh more increase the dead load.

Deflection Limits

Deflection is defined as the bend or “sag” in a floor caused by loads. The maximum allowable deflection is usually given as a fraction of the span length (L) in inches. For floor joists, dead and live loads shouldn’t exceed L/240, as specified by the 2012 International Building Code (IBC), Section 1604.3.

Joist Sizing Calculations

Sizing the depth needed for a floor joist can be done with a simple calculation: half the span plus two. For example, if you had a floor in a room that spanned 16 feet, you would divide that number in half (eight), and then add two, to get 10. Therefore, the depth of the joist will need to be 10 inches.


Floor joists do the heavy lifting in a room, holding up the floor and whatever is in the room. It’s important to choose the right floor joists (material, size, etc.) for your specific needs. However, while there are span tables and calculators to help determine joist needs, a structural engineer should calculate the required joist specifications in accordance with the building’s engineering as well as local building codes.

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How do joists work? ›

Roof joists generally join opposing walls and support the ceiling below and/or the floor above. Joists are typically made of timber, and come in a variety of thicknesses and lengths depending on the specifications of the building. Steel, concrete and manufactured wood are also commonly used to create roof joists.

What is the difference between a joist and a stud? ›

Joists – are supportive timbers which the flooring is fixed to. You can see the joists here on this image – they run horizontally at floor level. Studs – are a vertical framing usually made of timber or steel which forms part of a wall or partition.

How strong is a floor joist? ›

Floor Joists Building Codes

Local building codes requires wood joists to support at least 10 pounds per square foot for a dead load and 30 pounds per square foot for a live load. Different wood, grades, span length, and so on affect the support per square foot.

Do floor joists need support? ›

With timber or metal frame walls it is normally acceptable to support them on the existing floor joists (not the floor boards), either by providing a double floor joist underneath, if they run parallel, or across the existing joists if the floor joists run at 90° to the wall.

What purpose do floor joists serve? ›

Floor joists are used to support a floor that spans over an open area, normally forming the upper storey in a house. They are placed equidistant and parallel to one another and span between loadbearing walls or where large open spaces are required below between walls and / or structural beams.

What is the purpose of floor joists? ›

Floor joists are horizontal structural members that span an open space, often between beams, which subsequently transfer the load to vertical structural members. These joists, part of the floor system, carry the weight of everything inside a room, including walls, furniture, appliances, and even people.

Are joists load-bearing? ›

Located between walls, beams, and foundations, floor joists are structures that support floors and most easily identified in a building's basement or attic. Walls that run parallel to joists are not typically load bearing, whereas walls that run perpendicular to the joists are most likely load bearing.

Which direction do floor joists run? ›

An important thing to know about joists is that they usually run in the same direction throughout a house. If the visible joists in a basement or attic run east to west, for example, you can be fairly certain the invisible joists under the bedroom floor also run east to west.

Can a stud finder find floor joists? ›

Use a stud finder to locate the joists. A stud finder is an electronic gizmo used to determine the location of wall studs or floor joists. They're available at hardware stores and home centers. Move the stud finder along the floor to determine the possible location of a floor joist at the center point of the squeak.

How much do floor joists cost? ›

Floor Joist Installation Cost Per Joist

Expect to pay anywhere from $100 to $2,000 per joist, depending on their accessibility and whether additional repair or replacement work is required. You may pay only $100 to $300 per joist if your contractor can easily access the joists from below.

Do floor joists have to sit on a beam? ›

Floor joists spread the loads bearing from above, and must be framed adequately to complete the load paths. Ideally, if a load-bearing wall runs parallel to floor joists, then it should sit directly over a beam or a joist supported by a load-bearing wall below.

Can you replace floor joist without removing floor? ›

You can replace floor joists without removing the whole floor. If you need to, you can use a reciprocating saw to cut vertically. It is important to use a reciprocating saw to cut through nails. It's possible to replace floor joists in this manner.

Why does my floor shake when I walk? ›

Wood-framed floors are the most typical culprit of floor vibrations due to small deflections up and down as the joists are loaded and unloaded. While this type of floor is designed to support people, furniture, and large appliances, certain movements can cause a floor to vibrate over time.

Can you lay hardwood straight to floor joists? ›

Wood floors should always be laid perpendicular to floor joists—across rather that in between them. This will make the floors structurally sound and will help prevent the planks from separating, sagging or buckling. So, there is no right or wrong way to lay your wood flooring.

How do I know if my floor joists are sagging? ›

Some of the common indicators that you have sagging floor joists include:
  1. Cracks or Crumbling on the Walls Inside the House. Sagging floor joists can also cause issues for your walls. ...
  2. Windows and Doors Problems. ...
  3. Lack of Support. ...
  4. Sloping Floors. ...
  5. Bouncy Floors. ...
  6. Joist Sistering. ...
  7. Beam Replacement. ...
  8. Structural Jacks & Beams.
12 Sept 2020

Are floor joists considered structural? ›

A floor joist is a horizontal structural component that spans an open space between beams that transmit the load to vertical structural components. These joists, a component of the floor system, support the weight of the walls, furnishings, appliances, and even humans inside a room.

How far apart are floor joists in old houses? ›

Building codes specify the minimum joist depth and spacing (typically 12 or 16 inches on center) for spans up to 20 feet, but those requirements are intended to prevent plaster ceilings from cracking, not to eliminate springy floors.

Can floor joists fail? ›

Unfortunately, floor joists can rot and decay and this can lead to serious structural problems, including: Slopping, sagging, or uneven floors. Skewed or uneven window and door frames. Tilting or sinking crawl space supports.

What causes rotten floor joists? ›

Wet Rot in floors

In the past, the beams supporting the floors (floor joists) and the associated timber wall plates and lintels were all built into, or resting on naked masonry. If the walls get damp, the timber in them or against them will also become moist. Decay then follows.

Do all houses have floor joists? ›

Except for homes with concrete slabs, all houses have joists that hold up the floors.

How much weight can floor joists hold? ›

Construction Code specifies “Live Loads” 40 pounds per square foot. It also specifies a deflection of the floor joist. If your floor has a 16 foot span between load bearing walls, and the floor joists are spaced at 16 inches, and the joists are 2x12's you are allowed at that load to deflect 0.52 inches.

What are two common types of floor joists? ›

Types of Joists

Two types of engineered joists are most frequently used in floor systems today: Wood I-Joists with dimension lumber top and bottom chords and OSB web. Open web trusses with wood webbing held together with metal plates.

How much weight can 1 floor joists hold? ›

That means the joists can support a minimum of 40 pounds per square foot live load. Though, by consulting the 50 pounds per square foot live load/10 pounds per square foot dead load table, you can see the joists' span would need to be reduced to 11 feet 11 inches to support heavier weight safely.

Should floor joists be nailed or screwed? ›

Nails are better in nearly all cases, particularly when installing joists. A regular 3 ½” deck screw has very little sheer strength. “Shear” strength refers to a screw's ability to hold lumber, or other material, together against the force of gravity.

How can I find a floor joist without a stud finder? ›

Another method that doesn't require stud finders is measuring the distance, but this will only work if you know the distance between the joists. Keeping this distance in mind, start measuring from the edge of the floor, and use a hammer and the sound test to locate joists.

Do floor joists run same direction as rafters? ›

Most of the time, floor joists will run parallel to the rafters, but occasionally they will be perpendicular. There are many custom features and other aspects of a home's construction that can impact how your framing members are oriented.

Should I worry about squeaky floors? ›

Are squeaky floors a structural problem? There's no need to panic. In real life, a creak or squeak is no big deal—that is, they don't signal structural damage, like termites, that could cause your floor or joist to collapse. And fixing creaky floors is fairly simple.

Why does my floor squeak under carpet? ›

Floor squeaks are caused by gaps between the sub-floor and the floor joists which have separated over time and can be fixed by simply reattaching that sub-floor back to the framing. The trick, however, is to not damage your wall-to- wall carpet and to locate where the joists in your floor are located.

Can I use a magnet to find a stud? ›

If you slowly slide a magnet along the surface of your wall, it will be attracted to the steel drywall screws that come up very close to the surface of the drywall. Even better, you can leave the magnet stuck to the screw to mark the stud throughout your project.

What happens if a floor joist breaks? ›

When one or more joists fail, you can experience floor bounce, or notice sagging or heaving in the floor. Loud squeaks also may signify floor joist troubles. Ceiling joists, like floor joists, can suffer from the same structural issues, and methods to repair them are the same.

How far apart are floor joists on a floor? ›

450mm tends to be the normal spacing adopted.

What is the last floor joist called? ›

The outermost floor joists are called the end joists; they are not rim joists. Only the joists that run perpendicular to the floor joists are called rim joists.

What goes on top of floor joists? ›

The subfloor sits on top of the joists. This allows your floors to have a continuous structural surface over the floor joists. There are different materials that can be used for the subflooring. Those are wood planks, plywood, OSB (Oriented Strand Board) and concrete.

What kind of wood is used for floor joists? ›

Lumber Grade

Lumber graded as #2 is the most common choice for floor joists and other framing lumber. It has more knots and defects than higher grades, but usually not enough to cause significant loss of bending strength.

Does homeowners insurance cover rotten floor joists? ›

Foundation damage caused by shifting or settling earth or sagging floors caused by rotting floor joists are typically not covered by homeowners insurance. If the damage is caused by flooding or an earthquake, you'll typically require separate coverage.

How much does it cost to replace rotten floor joists? ›

Floor Joist Repair Cost for a 500-Square-Foot Room
Cost to sister floor joists$100 - $300 per joist
Cost to replace floor joists$6,000 - $10,000+
Cost to replace floor joists, jack required$20,000+
25 Jul 2022

Can sagging floors collapse? ›

Sagging floors are not only deformed and unappealing but they can be dangerous. They can collapse and cause injuries if they're not fixed quickly.

Are bouncy floors normal? ›

If your floors are bouncy and also noticeably sagging, they probably are too weak. You may need the help of a structural engineer to solve this problem, but begin with a call to your local building inspector, whose advice is free.

What does a bouncy floor mean? ›

If the joists or columns are spaced too far apart, the joists will sag and bounce. Home remodeling projects, such as adding an addition, installing new appliances, granite countertops or large furniture pieces can add extra weight and stress to the floor joists– leading to bouncy floors.

How can I make my floor more sturdy? ›

Plywood or OSB scraps make great shims for small gaps. Large gaps require 2x offcuts with vertically oriented grain. Don't use cedar shims; they are soft and crush too easily under pressure. Adding a second joist of the same size alongside each existing joist, also known as sistering the joists, stiffens a floor.

When should you not install hardwood floors? ›

Replacing your flooring in the winter may be tempting. However, winter is one of the worst times of year to install hardwood floors. No matter which part of the United States you call home, winter is the driest time of year. The air naturally has less moisture during the winter than in other seasons.

Which direction should wood floors run? ›

The most common way to lay hardwood flooring is by aligning the planks parallel to the longest wall. Apart from a few exceptions like sagging joists, this is the preferred direction to lay wood floors because it aesthetically provides the best result.

Do you need plywood under hardwood floors? ›

Is the subfloor rigid or strong enough to support your hardwood flooring? A wood subfloor should be OSB or plywood of 3/4″ or thicker for a hardwood floor installation. Particle board and chipboard are unacceptable for solid hardwood installations, but may be used in an engineered flooring install.

How much does it cost to fix sagging joists? ›

The typical costs for repairing sagging floors start at $1000 and can go up to $10,000, with the average rate being around $300 per square foot. But this can vary depending on the extent of the damage and materials needed to get the job done.

What is acceptable floor sagging? ›

The degree to which your floor slopes or sags indicates whether or not you have reason for concern. Typically, floors that slope 1-1/2 inches or less in 20 feet is not a problem. Floors that sag 2 inches or more in 20 feet, though, are a cause for concern.

How do I know if my floor will collapse? ›

Sometimes areas of the floor will be sunken, heaved or sloped. Look for bowing floors or areas where floors are separating from walls. Cracks that start at the top corners of doors or windows and extend toward the ceiling can be a sign of a shifting foundation.

How are joists attached to beams? ›

The best things to use to attach the joist hangers to the beam are structural screws or bolts. Never use roofing nails. I've seen people do this. You can buy structural nails that are rated for the weight, but trust me, the structural screws or bolts are far better.

Can I board directly onto joists? ›

Can I Board directly onto joists? The simple answer is no. The most common joist heights in use in modern properties are between 75mm and 100mm.

Is it OK to cut into floor joists? ›

You're not allowed to do any notches in the middle third of a floor joist span, and you're not allowed to notch the bottom side of members over 4″ in nominal thickness. For anything else, you're allowed to notch 1/6 of the joist depth deep, and 1/3 of the joist depth wide.

Do floor joists run the same way on both floors? ›

They range from 2-by-6s to 2-by-12s in most houses, depending on their span and the load that must be carried, and they normally are spaced on 16-inch centers. An important thing to know about joists is that they usually run in the same direction throughout a house.

Can you lay hardwood with the floor joists? ›

Hardwood flooring must be installed perpendicular to the floor joists or on a diagonal for any single layer subfloor. To run parallel to the joists, you'll need to add a 1/2” plywood underlayment or brace every 16” between joists with a nominal 2”x 6” SPF nailed in place.

What wood goes over floor joists? ›

OSB is your best bet for a subfloor material if you plan to install over wood joists. It's more economical than plywood and will resist moisture and expansion. However, neither OSB nor plywood are water-resistant. They can swell if exposed to water making the finished floor uneven.

Can plumbing go through floor joists? ›

You can safely drill joists for electrical and plumbing runs without weakening the joists, if you follow the rules. By the DIY experts of The Family Handyman Magazine. Where and how to drill joists for electrical cables or plumbing runs depends on what type of floor framing you have. Keep the hole at least 2 in.

Does screwing into a joist weaken it? ›

As long as the top fibers and the bottom resist these stresses, the joist will do its job of keeping your floor strong, straight and solid. But when you notch or drill a joist, you cut some of those fibers and reduce the joist's ability to withstand compression or tension.


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