KEEP
A LID ON IT
The lid rule originated in the Uniform Building Code and was carried into the
first version of the IBC in 2000 when the three predecessor regional codes were
melded into the International family of codes. It has always been codified as
an exception to the basic rule that requires full vertical continuity of a fire
partition.
The rule appears to have been placed into the UBC to facilitate the
installation of corridors in the instance where equipment and wiring would
necessitate the creation of numerous penetrations in the corridor walls above a
ceiling line. Rather than creating walls that are so riddled with penetrations
so as to be almost unsealable, code language took the approach that terminating
the corridor walls at a rated ceiling and creating a safe, U-shaped egress
envelope would not compromise life safety. Thus, the lid rule was
born.
The 1988 Handbook to the Uniform Building Code describes the concept: “In all
probability, typical wall construction, when tested in a horizontal position,
might not pass the one-hour test. However, this arrangement is considered to be
adequate protection for the corridor separating it from the spaces above.”
Notice that the handbook clearly states that the horizontal wall might not pass
a fire test, but does not consider that to be an abridgement of life
safety.
And, to repeat a notion stated earlier, that concept makes sense for it can be
difficult to construct a floor- or roof-ceiling system the width of a corridor
using a conventional tested ceiling system. Since they are more commonly used
as structural elements of a building, floor-ceiling systems generally
incorporate large structural members, an attribute that would make them
unwieldy to construct in a limited or narrow area.
The lid rule is contained in the Section of the IBC that defines requirements
for fire partitions; walls that are generally used to create corridors and
separate dwelling units in the same building. Fire partitions are required to
have a rating of not less than one hour.
In the 2007 IBC Supplement, the lid rule is defined by Exception 3 to Section
708.4 of Chapter 7. As noted previously, Section 708.4 defines continuity
requirements for a fire partition and mandates that a fire partition “extend
from the top of the foundation or floor/ceiling assembly below to the underside
of the floor or roof sheathing, slab, or deck above or to the
fire-resistance-rated floor/ceiling or roof/ceiling assembly above …”
ILLUSTRATIONS
Exception 3 modifies this rule when it allows for a circumstance where “… the
corridor ceiling is constructed as required for the corridor walls, the walls
shall be permitted to terminate at the upper membrane of such ceiling assembly.”
While the grammar may be a bit odd, the concept is pretty clear: build a wall
over the corridor with a rating the same as the corridor walls, and the walls
can terminate at the corridor ceiling.
Figure 1 shows the concept graphically. Note how the walls extend to the
underside of the ceiling and the area above the corridor is open to the
underside of the structure above.
One issue with the lid rule is the language that requires the wall to
“terminate at the underside of … the ceiling assembly.” Neither the code nor
the IBC commentary provides any interpretation or explanation of how that is to
be accomplished, so it can be a cause of dispute at times. In general, what is
typically required, at a minimum, is that the gypsum panels on the outside face
of the corridor walls—the room side of the tunnel—extend up to a corner where
they meet to form an outside angle with the layer of board that is installed on
the top of the corridor ceiling. In addition, the ceiling is constructed as if
it was a separate wall that is placed entirely on the top of the corridor walls
so that the bottom panel on the ceiling extends over and above the top plates
of the walls.
In essence, the junction point where the outside panel on the wall and the panel
on the top of the ceiling meet looks like an outside corner where two rated
one-hour walls meet, except that the entire detail has been rotated 90 degrees
to the vertical. Figure 2 provides a graphic representation of the concept.
Note the positioning of the bottom panel on the ceiling as it rests on the top
plate of the wall and that the outside panel on the wall extends completely
through to the panel that has been installed on the top face of the ceiling.
All joints in the system are sealed with at least one coat of joint treatment
material.
Since creating the head and corner details can be the source of some
interpretation, it is advised that a local code official be consulted prior to
construction of the corridor.
Constructing a rated corridor wall that contains a myriad of penetrations can
be a difficult task. Suggesting an option to incorporate the lid rule concept
as outlined in Section 708 of the IBC might be a viable option on a future
non-residential project. W&C
