Fire blocking is necessary to restrict the passage of fire flames inside concealed spaces, like the walls of a house. Most walls in a house have a double top plate and drywall on both sides so if fire gets inside the wall it can't get past the top plates, these walls are naturally fire blocked. Fire blocking is needed when wood and steel stud walls only have drywall on one side because the fire is now able to go around the top plate and up into the framing above.
Sometimes it is hard to see where you need fire blocking. Think of like this, if fire got inside the wall would it be able to get out? Are there any openings, holes or other spaces that fire could travel out of the space into? Walls with drywall on both sides are simple. Fire blocking questions arise when framing things like soffits, columns that are more than 1 stud wide, furdowns around hvac and plumbing and the most common one, framing against a concrete wall so you can insulate and install drywall.
This article will show you how to install fire blocking at basement walls, stairways, furdowns, and architectural framing. Anytime this article talks about building codes it is refering to 09 IRC 302.11
In this article, you will find information about:
Basement walls that are framed up against the concrete foundation walls need to be fire blocked because there is a gap between the framing and the concrete wall.
Vertical Basement Wall Fire Blocking Fire blocking basement walls is installed by nailing on strips of 5x8" or thicker O.S.B. or Drywall to the top plate and over the gap between the concrete wall and the framed wall.
Horizontal Basement Wall Fire Blocking Fire blocking must also be installed every 10' horizontally along the wall, IRC 302.11.1.2. This means that every 10 feet along a wall you will need to close the gap between the wall stud and the concrete wall, from floor to ceiling. Install this fire blocking by nailing a piece of 5/8" O.S.B. to the side of a wall stud an making sure the other edge of the O.S.B. is tight against the concrete wall. (insulating these these walls sometimes is approved under the IRC 302.1.1.1 rule. Ask your building inspector)
Fire blocking is required inside concealed spaces both horizontally and vertically just like basement walls. When framing soffits or columns that will have drywall installed on them or any other architectural feature like an arch it seems that fire blocking is necessary because there is almost always a cavity on the top side that goes up into the ceiling or floor above.
These passage ways are sometimes hard to see when the drywall is not yet installed. To figure out where to install the fireblocking just imagine a fire inside of the framed space and then look for ways that it can get out after the drywall is installed.
This fur down around the air duct needs fire blocking installed in the wall cavity so that fire cannot go up inside the wall and into the duct cavity.
Stairways are another space that fire blocking is required. The main reason for fire blocking at stairways is to protect the strength of the stairway so it will remain strong as long as possible during a fire so people can get out of the basement or down from up stairs. There are two main steps to fire blocking a stairway:
Install wood blocking in the cells between studs along the side of the stair stringer
Install fire rated 1/2" drywall as a fireblocking on the under side of the stairway and any walls enclosing the space under the stairway. IRC 302.7
All penetrations through the plates on a wall must be sealed with a foam, caulk or insulation that is fire rated. Any time a duct, plumbing pipe or electrical wire goes through a floor or ceiling it must be sealed around it with some type of fire stop. The most often used product is a expanding foam but regular caulking or unfaced batt insulation works as well. The horizontal holes drilled for electrical wiring do not need to be fire blocked.
The caulk at these openings does not need to be fire rated IRC 302.11. But if you use expandable foam it should be.
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