Understanding Firewall Construction And Fire Rated Joist Hangers
When designing and framing a building, proper fire separation is a critical component of structural safety. Firewalls are engineered to prevent the spread of fire from one area of a structure to another, providing vital time for evacuation and firefighting efforts. Building a reliable firewall often involves choosing between traditional wood framing methods and modern, specialized hardware like fire-rated joist hangers.
Traditional Firewall Framing And Blocking
One traditional approach to creating a firewall involves resting the floor joists directly on top of the bearing wall. To maintain the fire rating in the spaces between the joists, builders install solid wood blocking. Solid lumber has a predictable char rate of approximately one and a half inches per hour. Therefore, stacking multiple blocks can theoretically increase the duration it takes for a fire to burn through the cavity.
In conjunction with blocking, fire-rated gypsum board must be applied to the wall surfaces. A single layer of five-eighths inch Type X drywall typically provides a one-hour fire resistance rating. Installing two layers can achieve a two-hour rating. When using the traditional blocking method, the drywall must be carefully notched and fitted around the wood joists to maintain a seamless fire barrier, which can be highly labor-intensive.
The Role Of Top Flange Fire Rated Hangers
To streamline the installation of the drywall, many engineers specify top flange fire-rated joist hangers. These specialized brackets attach to the top of the wall plate and hang downward, allowing the floor joist to be secured to the bottom of the hanger. The primary advantage of this hardware is that it creates a gap between the end of the joist and the wall framing. This allows the drywall crews to slide the sheets continuously behind the joists after the initial structural framing is complete.
Maintaining a continuous plane of drywall eliminates the need for tedious notching and sealing around individual joists, which can significantly speed up the construction schedule. However, this efficiency comes at a substantial material cost. Fire-rated top flange hangers are expensive specialty items, and requiring them for large beams or open web floor trusses can heavily impact the overall framing budget.
Structural Independence During A Fire
A major engineering intent behind fire-rated top flange hangers is to allow the floor assembly to collapse during a severe fire without pulling the vertical firewall down with it. In theory, if a joist burns through the center and sags, it should tear away from the bottom hanger fasteners while leaving the top flange intact on the wall.
While this is the intended design, practical field mechanics suggest that the heavy fastening of the floor sheathing to the top of both the joists and the wall plates can still create complicated tension dynamics during a collapse. Regardless, builders must follow the engineered plans and local municipal codes when selecting a firewall connection method.
THREE KEY TIPS
Wood blocking can serve as an effective fire barrier due to the predictable char rate of solid lumber, but it requires meticulous drywall fitting to be up to code.
Top flange fire hangers allow for a continuous layer of drywall to pass behind the floor structure, eliminating the need for notching and improving the integrity of the fire barrier.
Always verify material costs before committing to hardware-heavy firewall designs, as specialized fire hangers for large-scale joists and beams can drastically increase the project budget.
BONUS QUESTIONS AND ANSWERS
To further expand on proper fire separation techniques, here are a few common questions regarding firewall framing and safety codes.
What Is The Difference Between A Fire Partition And A Firewall?
According to the International Building Code, a firewall is a highly specific structural element built to restrict the spread of fire and is designed to remain standing even if the structure on either side completely collapses. A fire partition is a less rigorous interior wall that simply separates spaces, like dwelling units or corridors, to slow the spread of fire but does not require the same structural independence as a true firewall.
Why Is Type X Drywall Specified For Fire Separations?
Standard drywall provides limited fire protection because the gypsum core calcines and crumbles when exposed to high heat. Type X drywall contains glass fibers mixed into the gypsum core. When exposed to fire, these fibers act as a reinforcing mesh that holds the calcined gypsum together, allowing the drywall to remain intact much longer and effectively delay the spread of flames.
Are Fire Rated Joist Hangers Required By Building Codes?
Fire-rated joist hangers are not universally mandated by standard building codes for every project. Builders can often achieve the required fire separation using traditional platform framing and approved fire blocking methods. However, specific architectural designs, especially in attached townhomes where independent firewall performance is required, may necessitate these hangers to comply with local structural engineering specifications.
