Missing Fire Blocking
multi-level houses have small sections of floor that are
balloon-framed into adjoining walls. The framing in Figure 3 is
typical, and shows good support for the joists where they meet
the tall stud wall.
The missing fire
blocking and edge blocking in this photo can easily be
installed at the top and bottom of joists in the bays without
ductwork (top illustration). Where ducts get in the way,
install a metal collar, which will choke off a fire and prevent
it from spreading into the wall. Where soffits above kitchen
wall cabinets are framed before drywall is installed (lower
illustration), the joint between the wall and ceiling also
needs fire blocking.
The problem, however, is that the fire blocking is missing
where the horizontal floor meets the vertical wall. Without
blocking, flames in the lower area can migrate to upper stories
through the open bay where the joists meet the wall. Solid
blocking is also necessary to provide lateral support when
balloon-framed studs are taller than 10 feet.
The solution in this case is complicated by the ductwork.
It's easy enough to add fire blocking at the bottom of the
joists, and to add blocking along the unsupported edge of the
subfloor. There isn't enough room, however, to add blocking at
the top of the joists in the bays that carry the ductwork. In
this case, I asked the builder to fabricate a metal collar that
would surround the ductwork. Even without sealants, this collar
closes off the air flow enough to reduce the ability of a fire
to sustain itself in these stud bays.
Another common area where fire blocking is omitted is in
soffits built above kitchen wall cabinets. If the ceiling and
wall are drywalled before the soffit is framed, then the
drywall serves as a firestop. When the soffit is framed first,
however, you need to install solid blocking where the ceiling
of the soffit meets the wall.