Floor Joist Blocking and Bridging (Bracing): Compared

Floor joist blocking and bridging are methods to solidify a floor structure. Reinforcing connections are installed between joists to correct the bounce and wobble of a flooring system. 

Blocking and bridging create a better weight distribution among all the joists and reduce the pressure on the joist.

What is Floor Joist Blocking?

Floor Joist blocking refers to installing solid lateral supports between two adjacent floor joists to evenly distribute loads imposed on joists from the floor above. Lumber with the same width of the joist is used for solid blocking between joists at mid-span of joist or at a spacing, not more than 8 feet.

Blocking is done either in a straight line or staggered form.

These blocks, sometimes termed floor joist spacers, are installed between two adjacent joists perpendicular to the direction of the joists and are properly fastened with nails.

What does Joist Blocking do?

A joist is highly susceptible to horizontal and vertical movement in the absence of blocking.

A properly installed solid woodblock or floor joist spacer will distribute vertical loads across all joists. As a result, the movement of joists will be reduced.

These wooden blocks prevent floor bouncing by distributing weight evenly across all joists.

Lateral strength and floor stability can be increased by blocking the floor joist.

Pros

• Reduces floor wobble
• Installing process is easy and simple
• Low-cost solution for floor movement.

Cons

• Blocking may create a 'Hump' in the floor due to different drying rates of joist and blocking.
• Require a high precision measurement to perfectly fit between joists.
• Existing electric and plumbing ducts can make the installation process strenuous.

Joist Blocking Methods

A. Straight-line Joist Blocking

Straight-line blocking requires some expertise for correct installation. It is also good aesthetically, especially when the floor is exposed from the bottom.

This works best if you can cut the blocks into shapes first. With that, you can initiate your installation from one side for blocking joists.

Because the spacing between joists is often insufficient for a hammer swing, you will need to use a nailer.

Nails are the best option for blocking or bridging. When compared to screws, nails provide significantly greater resistance to shear forces.

Straight-line blocking provides one face of blocks for straight nail driving through joists.

Toe-nailing of blocks is required, however, to connect the second face of the block to the joist. Two nails should be driven at a 45-degree angle from both sides when toe-nailing.

Toe nailing reinforces the joints between joists and blocks. Consequently, straight-line joist blocking provides slightly more strength than alternate joist blocking.

B. Staggered blocking

In staggered joist blocking, also called altering, the blocking is laid asymmetrically.

This makes it ideal for unexposed floors.

In this method of joist blocking, you will get both faces for straight nail driving. No toe-nailing is required for staggered blocking of floor joists.

Most people prefer alternate blocking of joists because it is easier, faster, and requires less degree of expertise in comparison with straight joist blocking.

In case you want to perform altering, you have to simply draw a straight line across the joists. Now you have to place and fasten your blocks on either side of that line alternatively.

This will allow straight penetration of nails instead of toe-nailing. 

Joist Blocking Spacing

According to the International Residential Code (IRC), joists with dimensions less than 2”×12” (thickness× width) are exempt from blocking.

However, joists that are larger than this need blocking at a distance of not more than 8 feet.

Blocking at 4 to 6 feet is generally preferred by various contractors.

If you have an engineered floor joist or an I-joist, the floor joist bridging code of IRC recommends that you implement the manufacturer's instructions.

Compared to conventional 2x10 joists, engineered joists are lighter and simpler to install.

Alternatives to Joist Blocking to Reinforce a Floor

If you notice that your floor is not firm and has some bounce, you can use other methods to reinforce it.

Adding a sister joist is one of the options. In sister joisting, a second joist runs alongside the first joist and provides additional support and resistance to bounce.

The only disadvantage of sistering floor joists is that you will require more space to install a new joist.

Sometimes the structure beneath might be fine, but the subfloor could be causing issues in your structure.

Subfloor panels in old houses are prone to deterioration.

To rectify this issue, the best option is to lay a new plywood panel to replace the existing subfloor.

The new layer of plywood will add extra thickness and significantly reduce floor bounce.

You can also add a mid-span beam and some support columns to the joists. Placing a mid-span beam through half of your joists reduces the loads they were previously experiencing by half.

If you're planning to build a new home, then consider a floor truss instead of a joist if you can afford the higher initial investment.

Is Joist Sistering the same thing as Blocking?

No, joist sistering and joist blocking are not the same. In joist blocking, an extra wooden piece is installed perpendicularly between two adjacent joists.

This provides resistance against joists twisting which in turn reduces any bouncing of the floor. 

In joist sistering, an additional joist is installed just adjacent to a joist that is damaged.

It is an additional identical joist for adding additional strength to the existing joist to hold floor weight efficiently.

If there is a significant sag in the floor then various joists need to be sistered together.

Both joist sistering and joist blocking are popular ways to strengthen floor joists.

What is Floor Joist Bridging?

Joist bridging refers to a brace or combination of braces installed between floor joists to hold them tighly in place. Joist bridging is primarily used to improve the stability of a floor structure. These braces can be made of wood, metal strips, or strappings. 

Floor bridging prevents joist rotation, provides sufficient strength, and offers resistance against bounces to the floor when someone walks across it. 

Due to joist bridging, each joist shares its loads with an adjacent joist. Consequently, the deflection of joists or floors can be reduced by up to 50%. 

Bridging joists is accomplished by connecting the top of the joist's width with the bottom of the adjacent joist's width.

There are two main methods of joist bridging-solid bridging and cross bridging. 

In solid floor bridging, a solid timber having equal depth to the joist's depth is installed between two adjacent joists.

This solid timber floor bracing should run perpendicular to the direction of the joist span. This is similar to straight-line joist blocking.

Joist cross bridging involves forming an 'X' shape with a couple of braces between two adjacent pairs of joists.

This type of bracing is also called as 'Herringbone struts.'

No building code in the USA recommends the standard size of wooden floor bridging.

Joist bridging sizes are adopted based on experience.

Different builders use different sizes of wood bridging.

But no matter how large or small, bridging is an efficient and effective counter for your wobbly or bouncy floors.

Bridging using galvanized steel connectors is now quite popular in the USA due to its low cost and longevity.

Among steel connectors, nailless connectors are preferred due to the simple installation process.

If your floor already has joist bridging and is showing some bounce, you can add more rows of joist bridging on either side.

While bridging is effective in reducing bounce it's not as sturdy as a floor truss system. However floor trusses when compared to floor joists, are more expensive.

The existing joist bridging should be securely fastened with additional nails or screws.

Types of Bridging

A. Solid wood joist bridging

There are no specifications in any code on the sizes for solid wood bridging.

Normally, wooden strips of 1×2 or 1×3 are commonly used by builders for cross bracing. However, 1×4 strips are better because of the low splitting threat and affordable price. 

A major challenge with solid wood joist bridging is precisely cutting every piece at the same angle.

You may have to change the cutting angle of the first and last bay of the joists to make it fit correctly.

B. Steel Joist Bridging (Steel Brace)

Cross bracing or X-bracing is the most suitable method of steel joist bridging. 

Floor joists are connected with diagonally oriented steel straps. 

These steel straps have bendable ends that fit at the top and bottom of adjacent joists tightly. 

They also have holes at both ends that allow nails to drive through these holes for easy fastening and tightly securing them in place. 

Nailless straps are another type of steel joist bridging. These straps have a nailing plate on each end for simple installation.

Both ends of these straps are easily bendable to provide a tight and secure fit. The ends are then hammered into the joists to secure a connection between the joist and the strap.

These nailing plates are not strong enough to properly hold the shear forces. 

Nailing plates will come out when joists will dry out or twist.

C. Wood Composite

Wood composite bridging strips perform the same function as solid wood joist bridgings. 

Wooden composites do not come in standard strip sizes. You have cut them by yourself in the required dimensions. 

Wooden composites provide a strong connection to the joists. 

You can use a single wood composite strip or go for cross bracing. 

Again you have to cut them at an angle similar to the solid wood bridging of joists. 

There is no specific condition in which you have to opt for any specific method out of those methods mentioned above. It is totally on you which method you are going to opt for.

Is cross-bracing the same as joist bridging?

The term ‘joist bridging’ is often confused with ‘cross bracing’ and is frequently used by the general audience interchangeably. However, they are not the same.

Bridging can be termed as an act of fastening strips between two adjacent joists to avoid the bouncing of floors and distributing deflection loads among the joists.

However, joist cross-bracing is a method of joist bridging in which bridging strips form a shape of ‘X’ between two adjacent joists. 

Is Blocking (or Bridging) Required for Floor Joists?

According to clause R502.7.1 of the International Residential Code (IRC), blocking is essential for floor joists having a thickness of more than 2 inches and a width of 12 inches.

Blocking is also required at the supporting end of joists.

If both the ends of a joist system are blocked to rim joists or band joists and the width/thickness ratio of the joist is less than 6:1 then blocking is not necessary.

If joists are fastened properly at both ends then there is no compulsion of proving blocking or bridging systems until it seems required. 

How to Install Blocking Between Joists

Installation of blocking between joists is a labor-intensive process and requires the use of various types of equipment. 

A wrong measurement can waste your entire effort.

If you are great at measurements then you can pull it off. Otherwise, hiring an expert is highly recommended.

To install blocking you need some equipment such as a ladder, a saw a measuring tape, solid wood, a pencil, a hammer, and of course some nails or screws.

After assembling all the listed equipment you are good to start blocking.

Here is a step by step process for installing blocking between joists:

1. Identify the location where your floor is experiencing bounce or wobble. If your joists are wider than 12 inches and span more than 16 feet then you can go for blocking. 

As per the joist blocking code of IRC, blocks should not be placed more than 8 feet apart.

2. Measure the distance between two adjacent joists. The easiest way to measure the gap between joists is to put your measuring tape on the inner face of one joist and spread your tape to the inner face of the next adjacent joist. 

This will tell you which type of lumber or wood you will require for blocking. 

Measure the length of your joists also. Mark two points at 8 feet apart. Suppose you have a joist of 20 feet in length. You can mark one point at 6 feet and another at 14 feet. 

Now both these points are equally spaced from the wall i.e. 6 feet and the distance between these two points is 8 feet, which satisfies the provisions of the building code.

3. Now cut the pieces of wood having a length equal to the spacing between two consecutive joists and their width should be equal to the width of the joists.

Keep these blocks in marked locations and fasten them with at least two nails from one end. For other ends, you will require toe-nailing because you don’t have space for swinging your hammer. 

5. Cut another piece and insert it between another pair of joists same as you have done in step 4. 

Another row of blocking can be installed by following the above steps in the same manner. 

How to Install Bridging Between Joists

You don't have to spend a lot to install a joist bridging system for your structures. 

The key requirement is that you have to cut all the pieces at the same angle.

If you already have a subfloor above the joists, then nailing will be difficult on the top side. You have to take thicker wooden strips because thinner wooden strips are not suitable for toenailing. 

Here is a quick guide on how to install bridging between joists.

1. Determine the location where you want to install your bridging. If your joists are wider than 12 inches and span more than 16 feet, you can install your bridging.

You have to keep a maximum of 8 feet distance between two rows of bridging as per the code requirements. 

2. The next step is to choose your type of bridging. If you have an existing floor then steel strips can be a good option. For old houses, you can install nail-less bridging steel strips. 

If you are installing a new one, then wood or engineered wood can be a good choice as it is easy to install and works perfectly fine. 

3. If you are going with lumber or engineered wood, then you have to cut your pieces at a proper angle.

You can use a radial arm saw to cut your pieces at an angle. If you don't have one, you can go with a normal saw also.

4. Now you have to fasten your bridging system with nails or screws. If you are using steel straps, you can simply bend them downwards to fit the gap. Palm nailers are a good choice for nailing. 

Always prefer 6d or 8d nails for nailing. If you are working on a new floor then only nail the top side of the joist. After installing the floor you can nail the straps from the bottom.

Nailing top and bottom before installation of the floor can move joists out of line.

Joist Blocking vs Joist Bridging – Which is Better?

Due to the solidity of blocks, blocking is a slightly better option as it provides additional strength to the joists. 

Especially when you are installing a new floor, blocking could be a better choice because you will have a lot of scrap lumber leftover that you can use as blocking.

For older homes or wooden structures, bridging can be a better option. The use of solid lumber or metal strips for bridging are the best choice here.

The advantage of bridging is that you can reduce the risk of a ‘hump’ forming if blocks and joists dry out at a different rate.

Floor Joist Blocking	Floor Joist Bridging
A single piece of wood with equal width of the joist is installed perpendicularly to two adjacent joists.	Normally two thin strips are installed diagonally in an ‘X’ shape between two adjacent joists.
It reduces the bounce of the floor.	It also reduces the floor's bounce by leaving more space for electrical and plumbing installations.
It is less costly as it utilizes the leftovers from present construction and low-cost lumber.	More costly in comparison to joist blocking as it requires a new set of wooden or metal strips.
It requires low skills.	It requires a high level of skills as wooden strips need a perfect angle cut. Metal strips are very easy to install.
Joist blocking is normally used in new constructions.	Bridging can be effective for older structures.
Blocking hinders the path of electrical and plumbing runs.	Open space in the bridging system allows smooth installation of electrical and plumbing runs.

Frequently Asked Questions (FAQ)

Do floor joists need cross bracing?

If you notice a bounce or sag on your floor while walking on it, you should think about installing cross bracing floor joists. Cross bracing is typically required in older homes where the strength of the joists has deteriorated over time.

According to building codes, if your joist is wider than 12 inches, you will definitely need cross-bracing in a new home.

What is cross bridging?

Cross bridging refers to the installation of a pair of diagonal bracings in an ‘X’ shape between two adjacent joists to avoid twisting of joists. 

Because of the X shape of floor joist cross bridging, one strip will be in tension and another strip will be in compression which will increase the sturdiness and stability of floors.

Cross-bridging is always necessary when your joists are wider than 12 inches and thicker than 2 inches in new construction.

If you have an old wooden structure and you are willing to correct the movement of the floor, then you can go with floor joist cross bridging. 

When should you block deck joists?

According to the building code, blocking is required to avoid twisting or rotation of joists. Twisting/rotation of the joist may lead to a bouncy or wobbly floor. 

To avoid this problem you will need to block deck joists at a maximum of 8 feet apart. Many builders find that blocking at 4-6 feet apart is more efficient and improves structural health and strength.