A joist block is a piece of material, typically wood, pre-cut to fit securely between floor joists. Its primary function is to provide structural support and stability to the joist system.
Understanding Joist Blocks
In construction, especially in floor framing, joists are parallel beams that support a floor or ceiling. While joists bear vertical loads, they can be susceptible to sideways movement or twisting, particularly when under heavy load or spanning long distances.
This twisting or rotation is known as lateral torsional buckling. The reference states that joist blocking is used specifically "to prevent joist rotation against lateral torsional buckling". By fitting snugly between adjacent joists, blocks create a rigid web that connects the joists together, significantly reducing their tendency to twist or buckle sideways.
These blocks are essentially short pieces of the same lumber used for the joists, cut to the exact distance between them and installed perpendicular to the joists.
Why Use Joist Blocks?
Blocking between joists serves several important purposes in a framed structure:
- Prevents Buckling: As highlighted by the reference, their main role is to stop joists from rotating or twisting sideways, which prevents lateral torsional buckling and maintains the structural integrity of the floor or deck.
- Distributes Load: They help distribute loads more evenly across multiple joists, especially when concentrated loads are applied above a single joist.
- Increases Floor Stiffness: By tying the joists together, blocking makes the entire floor system more rigid, reducing bounce and vibration.
- Provides Nailing/Attachment Points: Blocking can serve as backing for attaching various elements like drywall, trim, or even plumbing/electrical components.
- Fire Stopping: In some cases, solid blocking can act as a fire stop, preventing the spread of flames and smoke through the joist bay.
- Sound Dampening: While not their primary purpose, solid blocking can offer some minimal sound dampening by reducing the open air space between joists.
Types and Installation
Joist blocking is typically made from dimensional lumber that matches the size of the joists (e.g., 2x8 blocks for 2x8 joists). They are cut precisely to the clear span between joists and installed by nailing or screwing them into the sides of the adjacent joists. They are often installed in rows at intervals along the joist span, or at specific locations like under walls running parallel to the joists.
Key Functions of Joist Blocking
| Function | Description | Primary Benefit |
|---|---|---|
| Prevent Lateral Buckling | Stops joists from twisting or rotating sideways. | Structural stability, prevents failure |
| Load Distribution | Helps spread weight across multiple joists. | Reduces stress on individual joists |
| Increase Stiffness | Connects joists to form a more rigid system. | Reduces floor bounce and vibration |
| Provide Attachment Points | Creates solid backing for fastening other building materials. | Easier and more secure installation of finishes |
| Fire Stopping (Solid Blocking) | Blocks open cavities to slow the spread of fire and smoke. | Enhanced building safety |
In summary, joist blocks are crucial components in floor and deck framing, acting as lateral bracing to prevent twisting and improve the overall strength and stiffness of the structure. They are pre-cut specifically for their purpose and installed between floor joists to prevent joist rotation against lateral torsional buckling, as well as offering other benefits like load distribution and increased stiffness.
