Resolve fire-resistance rating questions before they arise

by Katie Daniel | March 6, 2018 4:03 pm

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By Pamela M. Shinkoda, P.Eng., CSC
In this author’s role as a technical solutions manager with a gypsum wallboard manufacturer, a lot of questions are received about fire-resistance-rated wall partitions and floor-ceilings. (This article was written with the assistance of Abbas Nanji, M.S., P.Eng., a senior staff engineer at Underwriters Laboratories [UL] who has been with the organization for nearly 19 years. In his role, Nanji interacts with manufacturers to ascertain their code conformance requirements for building construction materials and assemblies. He is a member of ASTM E5 Committee on Fire Standards.) Inquiries usually begin with “Can I do this?” or “Do we have to do that?” Some of the questions require a lot of research to answer, while others get quick responses. Regardless, over many years in this position, it has become clear not everyone is familiar with the Guide Information for Fire-resistance Ratings section of the Underwriters Laboratories (UL) and Underwriters Laboratories of Canada (ULC) Online Certification Directories.

To that end, this article summarizes the answers to common questions, all of which can be found in either the ULC or UL Guide Information for Fire-resistance Ratings. (In previous printed versions of the Fire Resistance Directory, this section was at the front of the guide.) (The answers provided in this document are not UL’s official position but selected extracts from UL’s publication referenced herein. One should refer to UL’s website—[2]—for the most updated and complete information.)

Figure 1: The figure above shows the time-temperature curve for controlling furnace operation during fire-resistance classification tests (standard exposures). The assemblies are tested in accordance with CAN/ULC-S101, Standard Methods of Fire Endurance Tests of Building Constructions and Materials, and are subjected to a fire exposure defined by the temperature-versus-time relationship depicted by the curve.
Images courtesy ULC

Learning the ULC guide
A method of establishing a fire-resistance rating of a wall partition or floor-ceiling, complying with the National Building Code of Canada (NBC) requirements, is determined by the assembly’s hourly performance. The evaluation is based on the results of tests conducted in conformance with CAN/ULC-S101, Fire Endurance Tests of Building Construction and Materials (Figure 1). (If a design is labelled as ‘Certified for Canada,’ this means it can be used in Canada because it was tested to CAN/ULC-S101, Fire Endurance Tests of Building Construction and Materials.)

As shown in Figure 2, the ULC and UL designs are a letter/number combination. There are more comprehensive tables that explain this in greater detail. For example, tables are available showing the ULC R200 series describes designs for roof assemblies with an exposed ceiling grid, while ULC R500 series designs describe gypsum board ceiling assemblies.

The answers to the following questions are all ‘yes:’

Although the language is not simple in the Guide Information for Fire-resistance Ratings[4], it is all there:

With the exception of support (e.g. studs) and fastener (e.g. nails, screws) spacings, the dimensions indicated in the following designs are intended to be construed as the minimum allowable for each rated assembly. Support and fastener spacings stated are the maximum allowable. Additional layers of gypsum board are permitted to be added to any design.

Figure 2: UL designs are a letter/number combination indicating the kind of assembly.

Gaining efficiencies on the jobsite
In reference to ULC wall designs, the guide includes the limitations and acceptable allowances for adding insulation to meet acoustical requirements when a ULC wall design does not include insulation, explaining which type of fibrous insulation is permitted and under what conditions. For wall assemblies, wood stud depth can be increased, as can steel stud dimensions and gauges, along with chase wall depths.

For floor-ceiling systems, the guide offers similar information about increasing dimensions. It also provides clarification that older designs often cite use of nails, but screws “may be substituted for nails, one for one, when the head diameter, length, and spacing [are] equal or exceed the requirements for the specified nails.” Further, the guide includes a recent addition explaining the difference between Type X and proprietary Type X gypsum board products (often referred to as Type C).

Most gypsum board products fall into one of three general categories for fire resistance: regular, Type X, or proprietary Type X (or Type C).
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The UL Guide Info outlines instances when fire taping is acceptable and whether base layers in a multi-layer system need to be finished:

Except where specified otherwise under VI. WALLS AND PARTITIONS, and where otherwise specified in the individual designs, all gypsum board systems except those with predecorated or metal-covered surfaces have joints taped and joints and fastener heads covered with one coat of joint compound (fire taped). Base layers in multi-layer systems are not required to have joints or fastener heads taped or covered with joint compound.

The guide also discusses the requirement for hold-down clips in UL designs:

Designs incorporating lay-in acoustical ceiling panels specify the use of hold-down clips. Hold-down clips are required for assemblies incorporating ceiling panels weighing less than 1 lb. per square foot.

There is explanation of when and where insulation can be added to UL floor-ceiling and roof-ceiling designs (if not listed in the tested UL design):

Insulation in G500, L500, M500-, and P500-Series designs: for 1-hour-rated G500-, L500, M500-, and P500Series assemblies, fibreglass insulation, either loose-fill, or faced or unfaced batts or blankets may be added to the plenum or joist space above the gypsum board, provided an additional layer of gypsum board is added to the assembly.

The guide also provides explanation of cases when boards can be installed horizontally (perpendicular to wall framing) and whether or not staggering of joints is required in these instances:

Except when gypsum board is allowed to be applied horizontally in the individual wall design, horizontal butt joints of vertically-applied gypsum board should be backed by the same type studs as specified in the design.

Horizontal butt joints on opposite sides of the studs in single-layer applications should be staggered a minimum of 12 in. unless otherwise stated in the individual designs. Horizontal butt joints in adjacent layers on the same face of the assembly in multiple-layer applications should be staggered a minimum of 12 in. unless otherwise stated in the individual designs.

The UL guide can be referenced for clarification about the skim coating of gypsum panels and its effect on fire-resistance ratings:

Interior finish materials including paint and wall coverings less than 0.036 in. thick and applied directly to the surfaces of walls and ceilings may be added to the exposed surfaces of fire-resistance-rated assemblies without restriction.

It also expands on circumstances where joints in a board applied to wood or steel studs may be left unfinished. Assuming joints occur on framing and the partition is not part of a smoke or sound control system, it states “the joints in gypsum board applied to wood or steel studs may be left unfinished for that portion of the joint above a suspended ceiling which is part of a fire-resistive floor-ceiling or roof-ceiling assembly.”

Additional, helpful material covered in the guide also includes:

Most gypsum board products fall into one of three general categories of fire resistance:

  • regular;
  • Type X; and
  • proprietary Type X (often referred to as Type C).

These three categories each provide different levels of fire resistance.

Regular and Type X board are described in ASTM C1396, Standard Specification for Gypsum Board. Where fire-resistive performance is required, either a Type X board or a proprietary product is typically specified.

Type X gypsum board is defined as:

gypsum board that provides not less than a fire-resistance rating for boards 5⁄8 in. thick and not less than 3⁄4 hr. fire-resistance rating for boards ½ in. thick when applied parallel with and on each side of load-bearing 2 by 4 wood studs spaced 16 in. oc applied with 6d-coated nails spaced 7 in. oc and with joints staggered 16 in. between sides when tested in accordance with ANSI/UL 263 [Standard for Fire Tests of Building Construction and Materials].

Proprietary Type C boards have a better fire performance achieved through the use of proprietary modifiers in the gypsum core which reduces shrinkage under fire conditions, thereby allowing the boards to remain attached to the structural element for a longer period of time. Since proprietary Type C boards have a better fire performance than Type X boards, they also meet the ASTM C1396 definition for Type X board.

The ULC guide contains explanations of when and where insulation can be added to ULC floor-ceiling and roof-ceiling designs.

Implementing the guide as part of your daily routine
The UL Guide Information for Fire-resistance Ratings explains topics such as:

The information in this article only touches on sections of the UL and ULC Guide Information for Fire-resistance Ratings. These resources themselves should be read in their entirety to understand the context of the sections to which this article has brought attention. The guide is frequently updated and revised, so it is important to view the most current information available associated with one’s design.

The author of this article is highlighting this information because when these questions or concerns are raised at a jobsite, they can delay work. The hope is arming design/construction professionals with valuable information from the UL and ULC Guide Information for Fire-resistance Ratings allows projects to run more smoothly.

When it comes to the use of floor-ceilings as roof-ceilings, Class A, B, or C prepared roofcoverings may be used on wood floor designs without reducing the fire-resistance rating, provided a nailer of equal thickness to the length of the mechanical fasteners is added to the flooring. This only applies to residential roofing with no insulation on the deck. For further details, one should refer to Section 24 of the Guide Information for Fire-resistance Ratings.

Pamela M. Shinkoda, P.Eng, CSC, is the manager of technical solutions and quality with CGC Inc. She has been part of the company for three years and in the building products industry for more than two decades. Shinkoda is a member of the Ontario Building Envelope Council (OBEC), CSC, the Canadian Fire Safety Association (CFSA), and the Gypsum Association (GA). She also chairs ASTM International’s C11 committee. Shinkoda can be reached via e-mail by writing to[8].

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