All Things Gypsum: The "Corridor Lid" Rule
One of the most basic principles of fire-rated assembly application is addressed in the Fire Resistance Design Manual (GA-600) with a simple phrase: “systems tested vertically shall not be permitted to be arbitrarily used in horizontal orientation.”
Since heat rises and a wall system and ceiling system are on different planes, the rule makes sense. Reviewing the ASTM E119 test method further reinforces the logic of the rule, since the furnace orientation for fire testing a wall and a ceiling is different.
Enforcement of the concept precludes the use of a wall system as a substitute for a ceiling system when fire-resistant construction is required. And in nearly every situation, that concept is iron-clad. The text of GA-600 contains no exceptions to the rule.
The only real relief from the rule is found in Section 708.4 of the International Building Code and is commonly known as the “corridor lid” rule. The specific language in the IBC allows for a wall to be used as a corridor ceiling when the ceiling is constructed “as required for the corridor walls.”
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 …”
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