#133 – Demystifying Firestop

Alejandro Uribe, PE, CFPS, CET

Learn the basics of fire-stopping and how to choose the correct solution

Firestop Applications. Photo courtesy of the International Firestop Council

Nothing is trending more these days than talk about the coronavirus. Did you know that fire protection measures may help contain the spread of diseases? Air resistant firestop sealants and devices can restrict not only the movement of smoke and sound, but also of airborne pathogens. Firestop systems are often required to be rated to restrict the passage of air, a very important feature in healthcare occupancies.

In our last article we talked about the six (6) main strategies for fire protection. Are you able to name them all? Three (3) of them pertain to the construction realm. They are equally important and make up what can be considered The Life Safety Triangle 1: Detection, Suppression and Containment. Firestop is fundamental to guarantee containment, yet it is often the least known subject amongst fire protection professionals who deal with fire sprinklers and fire alarm systems. Meanwhile, third party inspections for firestop systems are being enforced more regularly in new construction projects and, without training, most trades rush to obtain the first red firestop product they can find; however, without understanding any of what is really needed to restore the integrity of fire rated assemblies.

The Life Safety Triangle: Detection, Suppression and Containment. 1

Let’s think about a fire developing in a room inside a building of a determined occupancy where the life safety triangle is being used correctly. First, smoke will be picked up by detectors and notification devices will make occupants aware. If manual suppression is ineffective, sprinklers will activate and help control the growth of fire. At this stage, smoke and toxic gases are expected to be produced. Proper containment will help confine not only fire, but also smoke and toxic gases in the room of origin.


… how important is it, really? Consider the MGM Grand Hotel and Casino fire, Las Vegas, in 1980 where 85 people lost their lives and 650 were injured. 61 deaths were reported to be from the upper floors of the hotel (19th to 24th floors) even though the fire started on the first floor in one of the restaurants. A total of 75 people (out of 85) died from smoke inhalation and carbon monoxide poisoning.2

The severity of the fire was aided by many factors such as lack of sprinklers in the casino and restaurant areas, but mostly because of how easy the smoke was able to spread throughout the building due to joints, shafts and egress stairwells -- allowing the fire to quickly make its way to upper floors.

MGM Grand Hotel and Casino fire, 1980, Las Vegas, Nevada 2

Containment is a component of Passive Fire Protection, in which the aim is to confine a fire using a concept called compartmentation; Compartmentation seeks to restrict the spread of fire by dividing a building into separate smaller “fire” compartments through the use of fire-resistance rated walls and floors.

Requirements of these fire-rated assemblies are given by building codes based on the type of construction and occupancy. For example, fire resistance attributes will differ greatly between a poured concrete and steel commercial high-rise building when compared to a single-family, wood frame home.

Fire rated walls and floors to create fire compartments. Egress components such as exit access and stairways usually call for fire rated assemblies. 3


In order to effectively resist the effects of fire and smoke, a fire-rated assembly must be complete and whole. At a certain point, in all construction projects, trades will start to install ducts, pipes, conduits, trays and cables -- and these must pass from room to room, and from one floor to the next. Gaps (called joints) will start appearing when walls or floors meet other walls or floors. When any of these situations occur, the integrity of the fire rated assembly is compromised. The solution to these problems is Firestopping. Firestop is the fire protection system made of various components used to seal openings and joints that restores the integrity of fire-rated assemblies.

Firestop in a through-penetration and head-of-wall joint. Credits to LSS® and HILTI

Firestop solutions are explicitly required by the 2018 International Building Code for penetrations and joints, as follows:

Section 714.4.1.2 – Through-penetration firestop system

“Through-penetrations shall be protected by an approved penetration firestop system installed as tested in accordance with ASTM E 814 or UL 1479…”

Section 715.3 – Fire resistant joint systems

“Fire-resistant joint systems shall be tested in accordance with the requirements of either ASTM-E1966 or UL 2079…”

Please note how the word “system” is used, indicating that there is more than one factor involved (so it’s not only about a firestop product). This is very important; there is an old misconception among installers which have led them to believe any given red caulk or foam is enough to satisfy code and/or inspectors´ requirements. This is far from the truth and is the origin of many costly construction mistakes. By the end of this article, the reader will be able to identify all components necessary to choose the correct solution for almost any application.


As we saw earlier, firestop systems must be installed as tested in accordance to a certain UL or ASTM standard. It is very impractical to try to replicate a fire test for each construction project or application, so we rely on third party facilities who can test many different configurations of materials and products. Each test lab will then publish its own listing directory of approved firestop system applications.

Sample of popular certification marks from third party agencies used within the building products market. 4

Understanding these test standards and the testing process is key to designing fire resistant systems. In this article we are going to explore through penetrations and joint systems.


Have you ever heard of a 2-hour fire rated slab? How about a 1-hour fire rated drywall? These hourly ratings talk about how a determined assembly has passed a fire test in a lab. If we talk specifically about firestop, there are two possible ratings you can obtain from a firestop system assembly test: An F-rating and a T-rating (F stands for flame and T stands for Temperature).


An F-rating is the duration of time in which flames do NOT pass through the system. For example, if during the test of a firestop system in a wall, according to ASTM E814, the flames were successfully contained from passing from one side to the other for at least 3 hours, the firestop system will obtain an F-rating = 3 hours.


Try to think of the T-rating as more or less the same concept as the F-rating but, instead of flames, you are trying to contain temperature: T-rating is the time period that the penetration firestop system, including the penetrating item, limits the maximum temperature rise to 325°F (163°C) above its initial temperature through the penetration on the non-fire side when tested in accordance with ASTM E814. (The IBC does not require a T-rating for wall penetrations, only on floor penetrations).

Does it really mean that if you have a firestop system with an F-rating of 2 hours, that it will resist the passage of flames for 2 hours in a real-life fire scenario? Not necessarily. Real-life performance may be better or may be worse depending on the actual fire. During the test, all the components are exposed to test conditions that follow the ASTM E814 time-temperature curve (see figure below). So, all test conclusions are according to a fire that behaves just like the test one. It works well in setting a benchmark.

Time-Temperature curve. At 10 minutes, fire reaches a temperature of 1300o F.
Many common construction materials melt below this point. 5


Typical tests will include a certain fire-rated wall or floor assembly and different through penetrating items. A single test can be used to seek approval (and therefore listing) for more than one application: single or multiple items in a single penetration, different size of openings, different materials for penetrating items and different firestop products or devices. A specific system will be awarded a rating only after the required time and after withstanding a Hose Stream Test for integrity. This test is performed by aiming a hose stream of water to the assembly at a fixed psi (pounds per square inch) value and duration to simulate the intense pressure that builds up due to the accumulation of hot air, steam and gases during a fire.

Different through-penetration applications, before fire test. 6

ASTM E814 also gives the parameters to test membrane penetrations. The difference between a through penetration and a membrane penetration is that a through penetration has an opening that passes entirely through the fire rated assembly, while a membrane penetration has an opening that passes only through one (1) side of the assembly (such is the case of a recessed electrical box in a wall, for example). If you are panicking because you have NEVER used a firestop solution with electrical boxes… don’t worry! Most of the time you don’t need to. But please, check the IBC and your local regulations to understand when it’s indeed required.


Typical joint systems are tested to obtain F-ratings and T-ratings. They must undergo a similar hose stream test just as through penetrations. Most joint systems also tested to endure cyclic movement as joints often need to endure regular compressions and extensions.

Floor to floor joint before testing. Notice temperature measurement points. 6



We started this article by mentioning that firestop systems can be used to restrict the movement of air. Indeed, this attribute is determined by an L-rating (L for leakage), which determines the suitability to restrict the passage of air and smoke. The test is performed according to UL 1479, and it measures the amount of air leakage through the firestop system. The test is performed at different temperatures and is measured in CFM (cubic feet per minute). The lower the number, the better the L-rating. This rating is particularly handy also when used in presence of fire suppression systems, which most of the times must guarantee a certain concentration of the suppression agent in the protected room for a determined amount of time for it to work.


This attribute describes the effectiveness of a firestop system to restrict the flow of water. Try to think about how important containing rainwater can be specially during construction in floor penetrations. After the building is occupied, fire stop systems in penetrations or edge of slab with a W-rating come in handy when there are major plumbing leaks or accidents.


Now that we have a grasp on the basics of compartmentation, fire tests and why it’s important to restore the integrity of fire rated assemblies, we can identify all the factors that we need to consider to choose the correct firestop system. The first thing is to identify if we are dealing with a penetration or a joint. After that, the factors we must consider are listed below.

Through Penetrations

  1. Size and type of penetrating item(s): Depending on your trade, you might be installing 4-inch steel pipe with a sleeve, ¾ inch conduit, or a bundle of cables.

  2. Size and shape of opening: 8 inch in diameter, a 4-inch x 4-inch square, or maybe just an irregular hole in the wall.

  3. Annular space: This is the space between the item and the opening. If it’s a single item, you usually have two measurements, a minimum and a maximum. Try to picture an opening that is much larger than the penetrating item, and you will figure out that just a sealant is not enough to fill the space (let alone pass the hose stream test!). A filling material such as mineral wool or a device might be necessary.

  4. Fire rating of the assembly: 1-hour, 2-hour, 3-hour or 4-hours.

  5. Wall or Floor type: pre-cast hollow core floor, or maybe a gypsum shaft wall.

  6. Wall or Floor thickness: 4-1/2 inch, or maybe gypsum with 3-5/8 inch studs.

  7. Secondary attributes: maybe you need an L rating of less than 1 CFM.


  1. Movement requirement: 12.5% required for compression or extension, or maybe not required at all (an architect or engineer should be helping you!)

  2. Joint width: Maybe 1-inch top of wall, or a metal deck with 3-inch flutes.

  3. Fire rating: 1-hour, 2-hour gypsum walls, or 3-hour, 4-hours concrete floors.

  4. Wall or floor type: wood, masonry, concrete or gypsum.

  5. Wall of floor thickness: minimum 2-1/2 inch, for example.


That’s it. All we needed is to understand that firestop protection is a system that considers all the factors we just learned about, and we are ready to choose the correct system for our application. Fortunately, most suppliers have, at our disposition, Firestop Submittal Builders readily available for us online. Once we give them the input of our factors, they will give us various UL listed approved options.

Let’s look at a typical case. Imagine you are in a construction project, you are installing steel pipe, and you know that the maximum size you are using is 2 inches. You run into a 4 ½ inch block wall, you open a hole and you go through. When you measure, you realize you are touching the opening from the top of the pipe, and you have 2 inches of clearance below the pipe. You look at your architectural drawings and you realize the fire rating of the wall is 2-hours. There’s no mention on the notes or specs about secondary attributes. Do you have everything you need? Let’s look at our factors:

Through Penetrations

  1. Size and type of penetrating item(s): Maximum 2-inch steel pipe, no sleeve.

  2. Size and shape of opening: Maximum 4-inch opening.

  3. Annular space: Minimum: 0 inches (point of contact), Maximum: 2 inches.

  4. Fire rating of the assembly: 2-hour fire rated.

  5. Wall or Floor type: Concrete block wall

  6. Wall or Floor thickness: 4-1/2 inches

  7. Secondary attributes: None

So, we have all our factors! Let’s use the firestop submittal generator from HILTI for this example. If we enter our factors one by one, the first option that we get is the UL System No. C-AJ-1421, for metal pipes going through block walls. All we need to do is double check that the drawing satisfies our factors and then we need to follow the steps on the drawing. This solution works with mineral insulation and firestop sealant from HILTI and must be installed as directed by the drawing.


The UL library for different firestop systems from many manufacturers is very large and is ever growing. Nevertheless, it is impossible to have a test that contemplates every single scenario. Maybe the shape of the opening is too irregular, or we have multiple and different penetrating items, and we can’t find a UL listed solution from any manufacturer. This is where Engineering Judgments (EJs) come into play.

An engineering judgment is a drawing usually produced by a firestop manufacturer for a jobsite-specific application. They are a valid solution when a customer's jobsite condition is different from any tested firestop system. Most firestop manufacturers are open to receive requests directly from customers. If they deem the application and the solution as possible, they will produce a drawing that will have the address of the jobsite and will only carry value to that specific job.


Below is information for browsing through some of the UL listed firestop approved systems from HILTI, STI, 3M and METACAULK.

- HILTI: https://www.hilti.com/firestops

  1. “The life safety triangle”
    Firestopping plan review and inspection of joints
  2. “MGM Grand Hotel and Casino fire, 1980, Las Vegas, Nevada”
    - https://en.wikipedia.org/wiki/MGM_Grand_fire
    - http://www.clarkcountynv.gov/fire/PublishingImages/MGM%20Fire%20Images/mgm_fire3.jpg
  4. “Playing with Fire”
    RIBA journal
  5. “Time-Temperature Curve”
    Confinement of Fire in Buildings
    NFPA Fire Protection Handbook, 18-4, Volume II, 20th Edition
  6. “Full-Scale Wall Assembly”
    Firestop Testing – FCIA, in coordination with UL
  7. “Through penetrations vs Membrane penetrations”