Is Blocking Required for Floor Joists?

Skipping the installation of blocking on floor joists can be tempting as the process is somewhat labor-intensive. However, blocking can provide both rigidity and stability to a frame and, depending on the size of the joists, may even be required by code.

Is blocking required for floor joists? Building code requires the use of blocking for floor joists that exceed 2 inches in width by 12 inches in depth. Blocking also needs to be provided at the supporting end of a joist. In addition to providing lateral support, blocking helps transfer weight to adjacent joists, so that the floor acts as a unified system.

There are many different opinions on why to block floor joists, the process behind doing it, and how it helps the floor structure. Let’s take a closer look at this now. 

What is Blocking?

Blocking, when talking about floor joists, is when short wood blocks are used to fill or reinforce floor joists under floors that have a space below them. The wood used when blocking is usually made up of short defective pieces of wood that can’t be used in other pieces of construction. 

Blocking reduces bouncing by distributing the weight to adjacent joists. This means when someone walks on the floor, the blocking distributes the weight to all the other joists instead of just the one beneath the walker’s feet. 

It is essential when framing a building or structure that the blocking scheme is clearly laid out and established with a plan. Blocking, when correctly done, isn’t too expensive when you’re also laying down floor joists, but if you have to install blocking after the fact, it is much more costly and time-consuming. 

Blocking is required in several different scenarios. There must be blocking where joists overlap over a center beam and every eight feet for 2×10 and taller joists. A good rule of thumb is to think if the distance between any two rows with blocking is more than eight feet, you must add some more blocking there. 

This goes a long way in making sure the framing is firm, and the flooring is secure. 

Installation Process for Blocking

When it comes to installing blocking between the joists, you will need a couple of different items to get the job done. You will need lumber (same width as the joist), a hammer, nails, a pencil, a tape measure. You may also need a circular saw to cut the lumber to the length you need and a ladder if you’re operating on a joist that needs a ladder. 

These are the steps to take to install blocking:

1. Measure the length between the two joists. This allows you to find the distance in lumber you need to cut. 
2. Cut the lumber to fit between the two joists. The lumber should be the same depth as the joist. For example, if you have 2×8 joists, make sure to use 2×8 lumber for the blocking.
3. Hammer your nails through the blocking and the joists. Make sure to use two nails per side.
4. Cut another piece and insert it into the next joist. Try to stagger the blocking up and down, so you have room to hammer each blocking piece into the joist. 
5. Continue installing in the staggered path and then do another row approximately 4-8 feet from the first row. 

Most people use nails, but you can use screws. They are usually more expensive but can result in a more secure hold. Obviously, make sure to avoid water and gas lines and electrical wires when installing the blocking. 

Another thing to consider is to make sure you add the blocking before you install the floor sheathing or subflooring. This allows you to ensure proper spacing between the joists and makes the installation of the subflooring much easier. 

Other Ways to Reinforce a Floor

If you are feeling your floor not feel firm or “bounce” while walking on it, there are more options than to add more blocking. 

One of the options is to add sister joists to your floor joists. A sister joist is a second joist that goes alongside the first one and stiffens the floor. The added depth of these two allows extra support and reduces bouncing. The only caveat to this is that you need to make sure you have enough space to add another joist. 

Sometimes it’s the subfloor that’s the problem and not the structure under it. If you’re working on a floor in an older home, you might want to consider adding a new layer of plywood over the decking. Most older homes were laid with 1x planks, which shows lots of weakness and squeaking over time. Adding this new layer of plywood can add thickness and help reduce the bounce of your floor. 

One of the last options you have is adding a mid-span beam and some support columns perpendicular to the joists. Putting a beam halfway through your joists cuts the amount they sag by half, which would make the floor much more resistant and firm. The downside is this usually involves footing that needs to be replaced and even deeper. A project like this usually involves asking an expert or contractor their recommendation when it comes to adding beams.

You Can Use Cross Bracing Instead

Cross bracing or bridging is another system that reinforces a structure of a building just like blocking does. The difference is that the cross-bracing uses two pieces of support in a diagonal shape that creates an X between the joists instead of a solid block in blocking. 

Because of this X shape, one brace will be forced upon and under tension while the other brace will be compressed. This can allow for floors to be sturdier and more stable. 

There is some discussion about what is a better option between cross-bracing and blocking but both have different benefits. Blocking does allow more support and rigidity when it comes to holding the floor flat. You won’t hear as much squeaking as there isn’t much room for a floor to bounce when using blocking. 

However, with cross bracing, because of the X shape, there is more room for plumbing and electrical equipment to pass under the floor and throughout the building. This allows you to easily access this equipment for future repairs and makes these tasks simpler in the future. 

When it comes to bracing, it is crucial to not split the ends of the cross braces when nailing them into the joist. This will create split ends that will weaken the bracing and integrity of the structure of the floor. 

Blocking in Other Types of Construction

The term blocking is often used in other forms of construction as well. Otherwise known as backing, this term can be used when referring to wood that runs between wall studs for hardware mounted into the walls. 

This can be things like cabinets, shelving, bathroom tower bars, and handrails. Once you add drywall, it can be difficult to find a stud to attach these pieces of furniture. If you have a form of blocking in the wall that is already put at a specific height, attachment locations can be found much easier. 

This method is mostly used in places like kitchens and bathrooms where you know where the cabinets and mirrors may be. These are also some of the heaviest forms of furniture that hang off your wall, so secure blocking is needed to keep sturdy.