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 creates 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 as the joist is used for solid blocking between joists at the mid-span of the joist or at a spacing not more than 8 feet.
However ,blocking at intervals of 4 to 6 feet is a generally preferred.
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 necessary at the supporting end of joists.
Install these blocks, either in a straight line or in a staggered form between two adjacent joists, perpendicular to the direction of the joists.
Ensure that you properly fasten them with nails.
However, 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 you have an engineered floor joist or an I-joist, the floor joist bridging guidelines in IRC recommends that you implement the manufacturer’s instructions.
Compared to conventional 2×10 joists, engineered joists are lighter and simpler to install.
What does Joist Blocking do?
In the absence of blocking, a joist is highly susceptible to both horizontal and vertical movement.
However, properly installing a solid wood block or floor joist spacer distributes vertical loads across all joists, resulting in reduced joist movement.
Therefore,you can increase lateral strength and floor stability by blocking the floor joist.
Pros
- Reduces floor wobble
- Easy installation
- A 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 fit between joists perfectly.
- Existing electric and plumbing ducts can make the installation process strenuous.
Joist Blocking Methods
| Joist Blocking Method | Description |
|---|---|
| Straight-Line Joist Blocking | Blocks are in straight line between the joists |
| Staggered Blocking | Arranged in alternating or staggered pattern within the joist spacing |
Straight-Line Joist Blocking
Straight-line blocking requires some expertise for correct installation. It also has aesthetic benefits, 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 to block 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.
However,to connect the second face of the block to the joist, it is necessary to toe-nail the blocks.
When toe-nailing, drive two nails at a 45-degree angle from both sides.
Toe nailing reinforces the joints between joists and blocks. Consequently, straight-line joist blocking provides slightly more strength than alternate joist blocking.
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 have both faces available for straight nail driving, and as a result, no toe-nailing is necessary for staggered blocking of floor joists.
Most people prefer staggered 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 this type of blocking, simply draw a straight line across the joists.
Then ,place and fasten your blocks on either side of that line alternatively.
This will allow straight penetration of nails instead of toe-nailing.
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.
What is Floor Joist Bridging?
Joist bridging refers to a brace or combination of braces installed between floor joists to hold them tightly 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 reduces by up to 50%.
Bridging joists involves connecting the top of the joist’s width with the bottom of the adjacent joist’s width.
However, no building code in the USA recommends the standard size of wooden floor bridging.
Instead,choose joist bridging sizes based on experience.
Different builders use different sizes of wood bridging.
But no matter how large or small, bridging is an effective method 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.
Additionally, ensure to securely fasten the existing joist bridging with extra nails or screws.
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.
Types of Bridging
| Types of Bridging | Description |
|---|---|
| Solid Wood Joist Bridging | Use wooden strips for cross bracing |
| Steel Joist Bridging | Diagonally oriented steel straps connect the joists |
Solid Wood Joist 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 risk and affordable price.
However, 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.
Steel Joist Bridging (Steel Brace)
The most suitable method for steel joist bridging is cross-bracing, also referred to as X-bracing or herringbone struts.
This type of bracing connects the floor joists 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.
Like steel straps ,both ends of these straps are easily bendable to provide a tight and secure fit.
Afterward,hammer the ends into the joists to create a secure connection between the joist and the strap.
However,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.
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
- Ladder
- Saw
- Measuring tape
- Solid wood
- Pencil
- Hammer
- Nails or screws
Here is a step-by-step process for installing blocking between joists:
Step 1 : Locate Blocking Locations for Floor Joists
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.
Step 2 : Measure the Distance Between Adjacent Joists
The easiest way to measure the gap between joists is to use a 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.
Additionally,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.
Step 3: Cut the Wood Blocks
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.
Step 4: Secure the Blocks
Keep these blocks in marked locations and fasten them with at least two nails from one end, but for the other ends, use toe-nailing due to limited hammer swinging space.
Repeat the process for all the joists.
How to Install Bridging Between Joists
You don’t have to spend a lot to install a joist bridging system for your structures.
However, if you already have a subfloor above the joists, nailing on the top side can be challenging.
Moreover, thicker wooden strips are necessary because thinner ones are unsuitable for toenailing.
Here is a quick guide on how to install bridging between joists:
Step 1 : Identify the Bridging Location
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.
However, keep a maximum of 8 feet distance between two rows of bridging as per the code requirements.
Step 2: Choose a Suitable Bridging Type
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.
However, 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.
Step 3 :Cut the Bridging
When using lumber or engineered wood, then you have to cut the pieces at a proper angle.
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.
Step 4 : Fasten Bridging
Now, you have to fasten your bridging system with nails or screws. If you are using steel straps, 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 the 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 left over that you can use as blocking.
However,for older homes or wooden structures, bridging can be a better option. The use of solid lumber or metal strips for bridging is 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. |
| 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. |
| 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. |
Differences between Floor Joist Blocking and Bridging
Frequently Asked Questions (FAQ)
Which Wood Types are Suitable for Floor Joists?
Common wood types used for floor joists include Douglas fir, Southern yellow pine, Redwood, Spruce pine fir, and Cedar.
Engineered wood like LVL and I-joists are also excellent choices.
Can a Sagging Floor Collapse?
Sagging floors do not collapse immediately but can be dangerous if ignored.
It can worsen over time, leading to more structural issues and the collapse of your house.
Is Blocking Necessary for Deck Joists?
Blocking is essential for deck joists exceeding 8 feet in length, especially when using 2 x 10 or 2 x 12 joists.
Install blocks after every 2 feet of joists to ensure stability.
