Answers to questions frequently asked by Architectural, Engineering, and Construction (AEC) professionals. Click on the question to view the answer.


How can I search for UL certified Wall-opening Protective Materials that can be used to protect electrical boxes that create a membrane penetration in fire resistance-rated wall Designs?

There is an exception for certain size and number of electrical boxes that can penetrate the membrane of a fire resistance-rated wall Design that can be found in the BXUV GuideInfo:

Metallic Electrical Outlet Boxes

Certified single- and double-gang metallic outlet and switch boxes with metallic or nonmetallic cover plates may be used in bearing and nonbearing wood stud and steel stud walls with ratings not exceeding 2 h. The metallic outlet or switch boxes should be securely fastened to the studs and the opening in the gypsum board facing should be cut so that the clearance between the box and the wallboard does not exceed 1/8 in. The surface area of individual metallic outlet or switch boxes should not exceed 16 sq in. The aggregate surface area of the boxes should not exceed 100 sq in. per 100 sq ft of wall surface. The aggregate surface area of the boxes may be exceeded when Wall-opening Protective Materials (CLIV) are installed according to the requirements of their certification.

Metallic boxes located on opposite sides of walls or partitions should be separated by a minimum horizontal distance of 24 in. This minimum separation distance between metallic boxes may be reduced when Wall-opening Protective Materials (CLIV) are installed according to the requirements of their certification.

Metallic boxes should not be installed on opposite side of walls or partitions of staggered stud construction unless wall-opening protective materials are installed with the metallic boxes in accordance with certification requirements for the protective materials.

Where the size, number or concentration of electrical boxes is outside the specifications described above, a certified Wall-opening Protective Material can be specified to meet applicable code requirements. These materials can be found by searching UL product categories CLIV and QCSN

Can the fastener size specified in a UL fire resistance-rated assembly be increase?

Yes. Unless a limitation of tolerance is noted in the individual design, the fasteners identified in UL designs are considered minimums.

If I add an additional layer of gypsum, or increase the thickness, can I achieve a higher fire resistance rating?

According to section II, Subsection 9 of the BXUV Guide Information, Gypsum board thicknesses specified in specific UL designs are the minimums. Additional layers of gypsum or greater thicknesses of gypsum board are permitted in any UL design, provided the fastener length is increased to provide penetration into framing that is equal to or greater than that achieved with the specified gypsum board thickness and fasteners.
There is no corresponding increase in the hourly fire resistance-rating for adding layer(s) or increasing thickness of gypsum. Fire resistance-ratings are determined by the specified construction in the design based on the UL 263 fire test.

Can I substitute Fire-retardant-Treated Wood for Gypsum in a UL Design?

No. Fire-retardant-Treated Wood cannot be substituted for gypsum in any UL Design. Fire-retardant-Treated Wood may be added to a UL Design following the requirements for adding wood structural panels found in the UL BXUV Guide Information for Wood Structural Panels (General) and Wood Structural Panels in Wall Assemblies.

Can I add structural wood panels like plywood to a wall assembly?

Yes. The addition of wood structural panels in fire rated gypsum board wall assemblies is permitted as follows:

  • Wood structural panels that are 4 ft. wide, minimum 7/16 in. thick oriented strand board (OSB) or 15/32 in. thick structural sheathing (plywood) complying with DOC PS1 or PS2, or APA Standard PRP-108, manufactured with exterior glue, may be applied horizontally or vertically to the framing members.
  • Vertical joints should be centered on studs, and staggered one stud space from the gypsum board joints.
  • The wood structural panels are permitted to be applied either as (1) a base layer (directly to the wall framing and under the gypsum board), (2) in between gypsum board layers, or (3) over the top of the completed gypsum board wall.
  • When wood structural panels are used on top of the gypsum board layers of exterior wall assemblies, the wood structural panel should be protected from the exterior environment either as specified in the design or as specified here.
  • When wood structural panels are added to wall assemblies that include furring channels, there should be no more than two layers (either gypsum board or wood structural panel or combination thereof) attached to the furring channel.
  • When wood structural panels are added to the wall assembly, the length of the fastener used for the outermost layer (either gypsum board or wood structural panel) should be sized appropriately to accommodate the additional thickness of the wall panel.

Can a larger wood or metal stud be used than what is specified for a specific UL wall assembly?

Wood and/or metal studs specified in UL Rated Designs are minimum sizes. Larger dimensional studs can be substituted.

What is an engineering judgment?

An engineering judgment is a method or process used to resolve field needs when a tested design, system or assembly is not available. The engineering judgment results in a letter or report being issued by some knowledgeable party, which evaluates the impact of modifications to published fire resistance designs, firestop systems, etc. To be effective the engineering judgment must be acceptable to the code authority and should be site specific.

What modifications can be made to a UL certified assembly?

UL certified assemblies are intended to be constructed as shown in individual designs. If a construction element is identified as ‘optional’ or ‘may be provided’ it is not mandatory and doesn’t need to be included in the construction. Also, if a design specification is indicated as a minimum or maximum, the construction can include greater or lesser thicknesses or dimensions respectfully. Additional variances for designs are included in the BXUV Guide Information for Fire-resistance Ratings. Modifications not covered above may be evaluated using an engineering judgment.


If the UL Wall Design (assembly) does not specify that the fire resistance-rating is from interior or exterior, does that mean it is rated for fire exposure from both sides?

Yes. That information is covered in the BXUV.GuideInfo – Fire-resistance Ratings – ANSI/UL 263.

Specifically in section VI. WALLS AND PARTITIONS of that guide, where it states that the ratings for walls and partitions apply when either face of the assembly is exposed to the fire unless indicated otherwise in a specific design.

How does the cross bracing get installed in a wall assembly when the Design specifies “Cross braced at mid-height” ?

Details on cross-bracing can be found in the Supplement to the 1991 Edition of the “National Design Specifications” for wood construction. (PDF Available for purchase from AWC (Click Hear). Most structural engineers are well aware of the practice.

In basic terms, cross-bracing refers to the practice of bracing stud to stud in order to reduce the effective length of the stud and as such increasing the load capacity. In most cases, the bracing is done at the mid-height of the wall as that is the most effective location of the bracing. This may not be the only method specified by the National Design Specifications, but contractors typically use 2 X 4 (or 2 X 6) members spanning stud-to-stud, located at essentially the mid-height of the wall, orientated with the 1.5 in. dimension vertically, offset vertically 1.5 in. between cavities to allowing for nailing. The cross bracing is not intended as backing for horizontal joints of the gypsum board.

What is a “Finish Rating” found on some fire-rated assemblies?

A finish rating is established for assemblies containing combustible (wood) structural supports. The finish rating is defined as the time at which the wood stud or wood joist reaches an average temperature rise of 250°F or an individual temperature rise of 325°F as measured on the plane of the wood nearest the fire during the fire test using the standard time-temperature curve. The temperature reading is measured between the gypsum and the stud.

What does an * (asterisk) mean when its denoted next to a product specified in a UL Design

The “ * ” (asterisk) indicates such products must bear the UL Certification Mark. This means the UL mark, or the cUL certification mark for Designs applicable for use in Canada.

The use of the correct UL Certified product is essential for compliance with the UL design specifications and to ensure the construction of the assembly matches the assembly that was tested and certified in accordance with UL 263.

The UL Code Authority newsletter article UL Critical Component Identification in Fire Resistance-rated Designs contains more information on this subject.

To easily search for UL certified products for use in UL fire resistance-rated designs visit the free UL online search tool UL Product iQTM.

What is the difference between a “Restrained” and “Unrestrained” rating?

Restrained and Unrestrained are structural conditions. “Restrained” pertains to a structural system that takes into account thermal expansion, whereas, “Unrestrained” (more common) does not. UL recommends that you confer with your structural engineer to confirm the system appropriate for your project.

The UL Code Authority newsletter article Restrained and Unrestrained Assemblies – What Designers and AHJs Need to Know contains more information on this subject.

Are there any tested floor-ceiling or roof-ceiling designs that specify a wood joist (2×10, “I”-shaped or parallel chord) that incorporate a wood beam (like an LVL) or steel beam?

No; UL is aware of the IBC requirement “Protection of the primary structural frame other than columns”. To date, a manufacturer has not come to UL to test these materials as an assembly. NFPA 5000, Section 8.6 Horizontal Assemblies, allows the condition.

Understanding the Contents of a Tested Design

hy do some items have an asterisk*? What does the asterisk mean? Can I specify a similar product even though it is not specified in the design?
Every tested design contains a series of item numbers. The following are examples of how to understand what the asterisk means when used or not used:

  • Example A:
    “1. Nails – 6d cement coated nails 1-7/8 in. long, 0.0915 in. shank d…………………….”
    In this example there is no asterisk. Any nail meeting these specifications can be used.
  • Example B:
    “4Q. Gypsum Board* – Any 5/8 in. thick, 4 ft. wide, Gypsum Board UL Classified for Fire Resistance (CKNX) eligible for use in…………………….”
    In this example the asterisk is used after ‘Gypsum Board”. Any gypsum board meeting these specifications AND bears the UL or ULC Certification Mark can be used.
  • Example C:
    “8. Batts and Blankets* – Required for use with resilient channels, Item 7, min. 3 in. thick mineral wool batts, placed to fill interior of wall, attached to the nom 4 in. face of the studs with staples placed 24 in. OC.
    The asterisk is used after “Batts and Blankets” AND “Rockwool (manufacturer name) – Type SAFEnSound” (product name) precedes the item specification. In this case, the Rockwool product (bearing the UL or ULC Certification Mark) must be used, substitutions are not allowed. Rockwool is a sponsor of the test.

Why are dampers and exhaust fans specified in some designs and not others?

The manufacturers who sponsor a test determine which products will be part of the tested assembly and not every construction scenario is tested.  So some assemblies are tested with dampers or exhaust fans while others are not. 

What is a ceiling radiation damper? 

  • Ceiling radiation dampers are used to protect penetrations through the ceiling membrane of fire rated floor-ceiling designs.  

Can I add a damper to a design that was not tested with one?

  • If a design has not been tested with a damper, you cannot add one without code authority approval.

What type of equipment can be used in conjunction with a damper?

  • UL certified equipment like exhaust fans and ductwork can be used in conjunction with a certified damper. These specifications are contained within the tested design. Manufacturers are always interested to know if a product, based upon demand, needs to be included in a test. Design professionals should contact manufacturers when they cannot find a tested construction scenario.

If a UL floor-ceiling Design shows a generic hinged door damper in the construction specifications, what type of listed and labeled damper is permitted to be installed in this assembly?

Refer to the Damper Marking and Application Guide for even more information on this subject.

I need designs that include wood joist 2 x 8’s and 2 x 10’s. Can you help?

I cannot find a floor-ceiling design or a roof-ceiling design that specify 2 x 8 wood joist. Why? Are there any roof-ceiling designs that specify 2 x 10’s?
Floor-Ceiling designs that specify 2 x 8’s (or 2 x 10’s) can be found using Product iQTM “Search with Specific Parameters”: Assembly type: Floor-Ceilings; Construct group: Wood Joist or Combination Wood and Steel Assemblies; Protection type: Gypsum Board; Keyword: “wood joists”. There are two 2 x 8 (L540 and L556) and many 2 x 10 designs. There are no roof-ceiling designs that specify dimensional lumber.

Why can’t I find a design? What happened to the design I used to specify?

If you cannot find a design using the online search tool Product iQ® but were able to find it in a previous publication such as the hard copy of the UL Directory, that may be due to the fact that Certifications for tested designs are periodically “withdrawn” or removed from our records. Additionally there may designs you can find in Product iQ® that were not previously in the hard copy version of the directory. This can occur for several reasons:

  • One of the test sponsors (a manufacturer of a product listed within the design) is no longer in business. Since one of the products is no longer available the design cannot be replicated as originally tested.
  • The building codes mandate that the products listed within tested designs must be audited annually. The test sponsor may no longer want to maintain the audit.
  • A test sponsoring company may get bought by another company; sometimes the new company is unaware of the contractual relationship with UL and the certification will lapse.
  • UL is constantly testing new designs and since the hard copy directory is a “snap-shot” in time, any design tested and currently listed can only be found using the online search tool Product iQ®

Can a wall design be used in a horizontal application, for example, as a soffit? If not, is there a tested design for this application?

No, all walls are tested for vertical applications. A possible solution for this application can be found “Searching with Parameters”:
Assembly type: Floor-Ceilings; Construct group: Non-load-bearing Horizontal Barrier; Protection type: Gypsum Board.

Are there fire-resistant wall designs that include gypsum board on one side of a metal stud?

OR, I am working on an existing project that requires a fire rated tenant demising partition and I do not have access to the other side of the wall/tenant space. How would I search for designs that could be used to increase the fire resistance of the wall?
Go to, click on “Find Walls, Floor, Roofs, Beams and Columns” image. Search with parameters: Assembly type: Walls and Partitions; Protection type: Metal Stud, Gypsum Board, Lath &/or Plaster; Keyword: “one side of steel studs”. Type this exactly with quotes and leave the hourly rating blank.

Where do I find information regarding the fire resistance of archaic materials?

Where do I find information regarding the fire resistance for common materials like wood, concrete, etc.?


How do I determine the hourly rating of an existing “UL Labeled” door?

First – Go to, input whatever information that is available on the door or label to see if product information is contained on our database.
Second – Go to and submit a request electronically.

Can the ceiling suspension system be lowered?

What you are proposing is an approved design variance. Many design variations can found by going to Scroll down to “Design Criteria and Allowable Variances” and click on the type of system you need to research. The variation you propose is allowed per Section III, Item 11: “Where a ceiling is supported directly from structural members, it may be lowered”.

Where is the sound transmission class (STC) rating listed for a design?

The STC rating can be found in the heading of the design. Manufacturers tell us what test to perform and most do not require this test; therefore, very few designs include this information. UL recommends that you contact the gypsum board manufacturers listed in the design to see if they have performed this test.


How do I find fire rated walls, floors beams and columns?

Most assemblies can be found by searching with specific parameters using the UL Product iQTM online tool. Guidelines for finding designs can be locatedHERE.

How do I reduce the number of assemblies found within a search?

  1. Leave the hourly rating blank. Sponsors of the tested designs typically want to achieve the highest rating possible: the materials you would like to specify might have a higher hourly rating than you require.
  2. You must be precise when entering the manufacturer’s name. If you are unsure, try using the keyword box.
  3. Keywords are only useful if the word or its derivative is contained within the design. Trade names or abbreviations like, “drywall”, “TJI”, “CMU”, “wide flange”, etc. will not work.
  4. Examples of useful keywords: “siding”, “stucco” “exterior” (walls with exterior finishes); “damper” (designs tested with dampers); U300, U400, etc (finding a fire rated joint based on a wall series type).

Are wall assemblies tested for exposure to fire on both sides?

Yes. The ratings for walls and partitions apply when either face of the assembly is exposed to the fire unless indicated otherwise in a specific design.

How do I find an assembly for exterior use?

Assemblies are investigated with the understanding that their use is limited to interior applications unless otherwise specified in the individual designs. Where an exterior application of a UL-certified design is desired, the code should be reviewed to ensure compliance with other code requirements applicable to exterior walls such as weather protection, water resistance, and vapor retarders.



How do I know if a tested design is tested for acceptance in the United States or Canada or both?

  • All designs found on the online search tool Product iQTM have been tested to US ANSI/UL 263 and CA ULC-S101. Therefore accepted in both countries. This is documented in the heading of every design.
  • Designs found on the ULC (UL Canada) Online Directories have been tested to CA ULC-S101, and therefore accepted only in Canada.

How do I know if a fire-resistance rated assembly is certified to Canadian standards?

cUL assemblies – Many US designs have also been certified for use in Canada. If the title of a design includes BXUV7 – Fire Resistance Ratings – CAN/ULC-S101 Certified for Canada, it has been investigated to Canadian standards and is intended for installation in accordance with Canadian building codes. Any loading or other restrictions noted on the design should be considered in the installation. For additional information see the General Information for Fire Resistance Ratings – CAN/ULC-S101 Certified for Canada link on the top of the design. To locate Canadian cUL designs in Product Spec, enter the desired design parameters, along with keyword “BXUV7”.

ULC assemblies – ULC fire-resistance rated assemblies can be found in the ULC Online Certifications Directory under Fire-Resistance Rated Assemblies. To find an assembly that meets your project specifications, scroll down to the NUMBERING SYSTEM FOR FIRE-RATED ASSEMBLIES table and use the hyperlinks in the table to locate the types of construction needed (e.g. floor-ceiling, beams, roof-ceilings, walls and partitions or columns) and the type of protection anticipated in the installation. Clicking on the corresponding hyperlinks in the table will result in a list of available ULC designs that can be opened and reviewed individually.