During the recent ICC code change cycle, there was a proposal to eliminate the requirement to install fire-resistant bottom-of-wall joint systems in virtually all cases.
In construction, the most dangerous failures are often the least visible ones. They are the gaps we don’t see, the details assumed to be “minor,” or the conditions dismissed as cosmetic. Few details illustrate this better than the bottom-of-wall joint in fire-resistance-rated assemblies.
For years, the industry has treated the bottom of wall as a gray area, sometimes sealed, sometimes ignored, sometimes covered with a baseboard and assumed to be “good enough.” The recently published International Firestop Council (IFC) research report, “Closing the Gap: The Critical Role of Bottom-of-Wall Joint Protection in Fire and Smoke Containment,” puts that assumption to rest with hard data, full-scale fire testing, and unequivocal conclusions. The message is clear: unprotected bottom-of-wall joints fundamentally undermine compartmentation, compromise life safety, and defeat the intent of the building code.
Just as important, the report highlights why any attempt to relax or remove bottom-of-wall firestopping requirements in the code would be a step backward, not a clarification.
Continuity Is the Foundation of Fire Resistance
The International Building Code is built on a simple but uncompromising principle: fire-resistance-rated assemblies must be continuous. Fire walls, fire barriers, fire partitions, smoke barriers and shaft enclosures are only effective if they form an uninterrupted boundary. That continuity does not stop at the gypsum board edge; it must extend to the supporting floor or slab below.
The bottom-of-wall joint is where continuity is often compromised. Gypsum board typically terminates above the slab due to construction tolerances, track geometry or sequencing. That gap, whether 1/4 inch or 1 inch, creates an opening at the exact location where post-flashover fire pressure remains very high. During a fully developed fire, hot gases, flames and smoke are driven laterally at the base of the wall. If the joint is unprotected, the wall assembly’s rating is effectively meaningless.
This is not theory. It is precisely what the IFC research set out to test, and what it conclusively demonstrated.
What Fire Testing Shows
The IFC study evaluated 19 full-scale bottom-of-wall configurations across five research and testing laboratories, using the same test standards referenced by the code: UL 2079 and ASTM E1966. Both protected and unprotected joints were evaluated under post-flashover fire conditions, including variations in:
- Gap size (1/4 inch to 1 inch and greater)
- Stud type (steel and wood)
- Continuous and discontinuous bottom track
- One- and two-hour wall assemblies
- Baseboards (PVC, MDF, wood)
The results were not ambiguous.
Every protected bottom-of-wall joint, using a listed fire-resistant joint system, passed for the intended rating duration. Every unprotected joint, regardless of gap size, stud type or the presence of baseboards, failed, often catastrophically and early in the test.
In one-hour assemblies, unprotected joints failed in roughly 30 minutes, never approaching the required rating. In two-hour assemblies, some unprotected joints failed in less than five minutes when discontinuous bottom track conditions were present. Temperature measurements showed unprotected joints reaching five times the heat exposure of protected joints, with recorded temperatures exceeding 1,700°F in some cases.
Baseboards provided no meaningful fire resistance. In fact, assemblies with baseboards still failed, with temperatures far exceeding the failure criteria established by the test standards.
The conclusion is unavoidable: an unprotected bottom-of-wall joint is a direct and proven path for fire and smoke spread.
Static Joints Still Require Protection
One of the most persistent misconceptions in the field is that bottom-of-wall joints only require firestopping if they are “movement joints.” The IFC research dismantles that argument entirely.
The IBC defines a joint broadly as an opening created by building tolerances or movement. It further distinguishes between dynamic joints (designed to accommodate movement) and static joints (not designed for movement). Both are recognized by the code. Both are tested. And both require protection when they occur in fire-resistance-rated assemblies.
The testing confirmed that static bottom-of-wall joints fail just as readily as dynamic ones when left unprotected. Whether movement was anticipated or not was irrelevant. The gap itself was the problem. Without a listed fire-resistant joint system, continuity was lost and the wall failed to perform as required.
This is a critical point for designers, contractors, and code officials alike. The absence of anticipated movement does not eliminate the need for protection. The code never intended it to.
Smoke Is the Silent Killer
Fire resistance is only part of the equation. In many occupancies, particularly hospitals, detention facilities, and high-rise buildings, smoke containment is a primary life-safety objective.
The IFC study included smoke leakage testing (L-ratings) in accordance with UL 2079, as required by the IBC for smoke barriers. The results were stark. Protected joints maintained leakage rates well below code limits.
Unprotected joints, even with gaps as small as 1/4 inch, exceeded allowable leakage by more than 10 times.
Visual observations during testing confirmed what the data showed: unprotected bottom-of-wall joints allowed dense smoke to pass freely into adjacent compartments. In defend-in-place occupancies, that failure is not theoretical; it is life-threatening.
Real-World Construction Makes the Problem Worse
The report also addressed what many in the field already know: jobsite conditions are rarely perfect.
Discontinuous bottom track is common, often introduced by mechanical, electrical and plumbing penetrations through the slab, structural elements or coordination conflicts. Larger gaps are routine due to tolerances and sequencing. The testing intentionally replicated these realities and demonstrated that unprotected joints fail even more rapidly under these conditions.
In contrast, protected bottom-of-wall joints performed consistently well, even with discontinuous track and other adverse conditions. That is precisely why listed joint systems exist: to account for real construction, not idealized drawings.
Why Changing the Code Would Be a Mistake
Given this body of evidence, proposals to weaken or eliminate bottom-of-wall joint protection requirements are not only misguided but also dangerous.
The code’s reliance on UL 2079 and ASTM E1966 is intentional. These test methods subject joint systems to post-flashover conditions that mirror real fires. The data shows unequivocally that unprotected bottom-of-wall joints do not meet the performance the code already requires.
Changing the code to permit unprotected conditions would not be a clarification; it would be a rollback of life-safety protections based on convenience rather than science. It would create assemblies that appear compliant on paper but fail when exposed to fire and smoke.
From a liability standpoint, it would also place designers, contractors, and owners in an untenable position: knowingly constructing rated assemblies with a proven weak point at their base.
The Industry’s Responsibility
Firestopping is not an accessory trade. It is an integral part of the wall system, just as critical as studs, gypsum and fasteners. Bottom-of-wall joints are not optional details. They are a required component of continuity.
The IFC research provides the industry with something rare: clear, repeatable, independent data that confirms what many fire protection professionals have long known. Protected bottom-of-wall joints work. Unprotected ones do not.
The path forward should not be a debate over whether these joints matter. That question has now been answered. The focus should be on education, enforcement, and proper detailing—ensuring that bottom-of-wall firestopping is treated with the same seriousness as head-of-wall joints, penetrations and perimeter fire barriers.
Closing the Gap, For Good
In fire protection, gaps kill. The bottom-of-wall joint is a critical gap in the building, and one of the easiest to get right when the code is followed.
The data is in. The testing is done. The conclusion is unavoidable: protect the bottom-of-wall joint or accept a system that will fail when it matters most.
Changing the code to allow failure is not progress. Maintaining and enforcing bottom-of-wall firestopping requirements is.
For detailed fire test results, charts, field photographs, detailed technical analysis and additional supporting documentation, download the full IFC white paper: http://firestop.org/bow.
Don Pilz is the AWCI director of technical services.
