Q: Can you help me understand the difference between F and T ratings in firestop design?
A: These ratings reflect completely different performance-based results that indicate the ability of a specific firestop to contain a fire to one side of a floor or wall assembly. The F stands for flame and the T stands for temperature. The contractor should be mindful of which requirement is to be achieved on a project. The F rating is fairly simple to attain while the T rating may require special detailing and materials.
The two ratings come from the same test standard, which is ASTM E814, Standard Test Method for Fire Tests of Penetration Firestop Systems. This test standard is relative to ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, which is fairly new. ASTM E119 is the test procedure that yields the one- and two-hour fire ratings for walls, roofs and floor assemblies.
The two ratings, F and T, also indicate the performance capabilities of head-of-wall designs. That particular test standard is ASTM E1966, Standard Test Method for Fire-Resistive Joint Systems.
The scope of ASTM E 814 reads, “This test method is applicable to through-penetration firestops of various materials and construction. Firestops are intended for use in openings in fire-resistive walls and floors that are evaluated in accordance with Test Methods ASTM E119.” ASTM E814 and ASTM E119 are both comparative in nature and not intended to predict performance in an actual fire event.
It is important to understand some basic definitions in the test itself. The underlying principle starts with a fire-resistive assembly, floor or wall, that is to be breached with a penetrant such steel, copper, plastic, glass piping, cable systems or duct work. There are specific designs for each application that may require special firestop materials and installation techniques. There is not one design that meets all conditions.
The next definition is annular space, which is the hole in the fire-resistive wall or floor that is to be penetrated. Limiting the size of the hole is critical to maintaining the fire resistance of the total assembly. In looking at specific designs, there are minimum and maximum dimensions provided for this space. The penetrating item need not be centered in the hole, but it is equally important that both the minimum and maximum dimensions are met.
Similar to pouring concrete, there are forms termed “forming material” installed in the hole to control the final shape of what UL calls “fill, void or cavity material.” The two common types of fill material are a highly viscous fluid sealant or dry mix mortar. The forming material is typically a mineral wool insulation of a specific density. Some designs call for the use of intumescent-type products. During a fire event, these products will expand and char to effectively seal any penetrating item that otherwise would be consumed by the fire. A firestop is comprised of various components that must act as a system and remain in place during exposure to fire.
ASTM E814 follows some of the same test methods as ASTM E119. There is an exposed and an unexposed side to the assembly. The exposed side is placed in front of the furnace where it experiences the same time-temperature curve as ASTM E119. Also, like ASTM E119, there is a hose stream portion that immediately follows fire exposure.
This all leads up to the F and T ratings. They are both related to time, commonly in hours. To achieve that rating, the firestop must remain in place for the duration of an hour. Further, it must withstand the hose stream such that there is no water projection beyond the unexposed side.
The F stands for flame in that no flame can pass through the firestop or any of the unexposed materials.
The T rating must meet all the above, plus there is a temperature limit placed on the unexposed side of the firestop. This temperature limit is 325 degrees F above the ambient or starting room temperature. When considering materials that exhibit high thermal conductance, such as steel or copper, this T rating is difficult to attain. Special details and materials may be required.
Several items have significant impact to achieve the intended results. It is important for the contractor to understand local requirements for F and T ratings, and what the authority having jurisdiction requires. Firestop designs are not easily value-engineered. There are explicit proprietary designs that cover specific materials with few substitutions. The annular space minimum and maximum dimensions need to be followed. Coordination of trades may be especially more critical in this case. Even if a specialty firestop contractor installs the firestop, there has to be coordination and common knowledge between all trades involved.
Firestop design and installation is essential for life safety. The knowledgeable contractor working with all the trades will help assure a successful installation.
Robert Grupe is AWCI’s director of technical services. Send your questions to [email protected], or call him directly at (703) 538.1611.
