The purpose of fire blocking is to prevent fire from spreading through the concealed spaces of a building. It works by dividing framing cavities into separate compartments, slowing the passage of flames and combustion air.
In an unblocked balloon-framed building, for example, a fire that starts in the basement can rapidly travel up the stud bays and spread into the joist bays and attic. In a platform-framed structure, though, the top plates separate stud bays from joist bays. This means a fire that starts in a wall cavity would have to burn through the plate to enter the joist bay above. But if something short-circuits the fire blocking — if, for instance, you install a soffit without separating it from the wall cavity — fire is free to travel up the stud bay, through the soffit, and into the joist bays.
As a framer, I have to install fire blocking regularly. Doing this correctly can get tricky, depending on the complexity of the building. To avoid the delay and expense of inspection tags, I always pay special attention to fire blocking, making it a point to look for places where the inspector might require it.
I work under the IRC. Unless otherwise noted, all code references in this story are from the 2006 IRC.
Interpreting the Code
Section R602.8 of the IRC states, “Fire blocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space.” The section goes on to list a number of locations where horizontal and vertical cavities might connect — in the enclosed area around stair stringers, for instance, and at cove ceilings, drop ceilings, and soffits. Chimneys and fireplaces are also cited. Listing every single place where fire blocking might be required is impossible; the best strategy for builders is to understand the code’s intent well enough to figure out where fire blocking is needed — before the inspector has to flag it.
Approved materials. Only certain materials can be used for fire blocking. Of the items listed in the code (R602.8.1), the ones we use most commonly are 2-by lumber, 3/4-inch structural panels, and 1/2-inch drywall. Unfaced fiberglass and mineral fiber batts are also approved if they “fill the entire cross-section of the wall cavity to a minimum height of 16 inches” and are securely installed so they can’t move. (The 16 inches would be measured down from a soffit and up from a tub deck.) But in my experience, inspectors tend to prefer solid materials and would not be likely to accept the use of batts.
Types of Fire Blocking
Most carpenters think of fire blocking as short scraps of 2x4 or 2x6 nailed horizontally in stud bays. While we do use lots of short blocks, there are also places where a single piece of lumber or plywood will achieve the same result. Since the cost of labor goes up with the number of pieces installed, we always try to use as few individual members as possible.
A good example of this is soffits, which we always frame before the drywall is installed: Instead of installing an individual block in every stud bay along the wall, we run long pieces of 2-by or 3/4-inch sheet material across the face of the studs before framing the soffit. As long as this piece is wide enough to reach all the way to the top plate, there’s no need to install blocks in the bays.
The author uses a single 2x10 fire block nailed to the face of the studs for this kitchen soffit — much faster than installing individual blocks in the stud bays.
Scraps of OSB rim board isolate a dropped ceiling from the stud cavities on its left and the stair stringers beyond.
The same is true for dropped ceilings. Even if the finished ceiling is a foot or more below the floor joists above, we find that running a band of sheathing or drywall around the perimeter before hanging the ceiling joists is more efficient than installing individual blocks in the stud bays.
As I mentioned above, the top plates used in standard platform framing act as fire blocking, so for the most part we don’t need additional blocks in walls. One exception is rake walls, which we typically frame using full-height studs running from the floor deck to the sloped top plate. Under an earlier state code, we had to install fire blocking every 10 vertical feet in these tall bays. That rule has been dropped, but we still have to block where a flat ceiling or attic floor meets the rake wall. With cathedral ceilings, the double top plates serve as fire blocking.