I-Joist Floor Assemblies

The Benefits of Building with I-Joists

  • Better floor performance means less callbacks: Floors constructed with I-joists help reduce callbacks by eliminating squeaks and vibration.
  • Easy to source, easy to work with: I-joists are readily available and easy to install, especially for long spans, including continuous spans over intermediate supports.
  • Versatility: There are many options when it comes to constructing code-compliant I-joist floor assemblies.

Gypsum board installed on bottom flange of I-joistsFire Protection for Residential Floors

The International Residential Code (IRC) requires some means of fire protection to protect the underside of residential floor assemblies. There are several easy, cost-effective ways to meet these IRC provisions with I-joist floor construction.

APA’s System Report SR-405 details seven different assemblies that can be used in jurisdictions where 2021, 2018 or 2015 IRC Section R302.13 or 2012 IRC Section R501.3 have been adopted. The System Report was developed on the basis of the results of fire tests meeting stringent benchmarks established by the International Code Council Evaluation Service (ICC-ES) Acceptance Criteria for Prefabricated Wood I-Joists, AC14.

While all seven assemblies in APA System Report SR-405 meet code requirements, Options 6 and 7 combine efficient fire protection with quick, easy installation:

  • SR-405 Option 6: 1/2-inch gypsum board installed on top of the bottom flange (used with a maximum joist spacing of 19.2 inches or less on center)
  • SR-405 Option 7: 5/8-inch gypsum board installed on top of the bottom flange (this is the same configuration as Option 6, with slightly thicker gypsum when the I-joist spacing is up to 24 inches)

Both assemblies allow builders to take advantage of all of the performance benefits of I-joist floor assemblies, including improved fire protection. Gypsum panels are simple to install and easy to temporarily remove when necessary to reach wiring, plumbing and mechanical systems. No fasteners or glues are required, which allows quick installation and access to the space above the panels. The systems also hide plumbing, wiring and ductwork in the floor cavity, creating a finished ceiling appearance.

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Design Options that Meet IRC Provisions for Fire-Protective Membranes



Webinar: Fire Protective Membranes for Lightweight Floors

Both the 2012 and 2015 versions of the International Residential Code (IRC) include fire-protective membrane requirements to enhance the fire performance of lightweight floor systems. The IRC includes several alternatives to sprinklers when wood I-joists are used in residential floor systems. This webinar focuses on newly developed assemblies for meeting the fire-protective membrane requirements of the Code, specifically as they relate to I-joist floor systems.

The webinar reviews the options in the IRC, including:

  • Drop-in gypsum board
  • Partial sprinkler systems
  • Fire-protective coatings

Also included is a listing of resources and reference information from APA and others on this important topic.

Originally presented on June 10, 2016. Running time approximately 28 minutes. Note that at this time, no continuing education credits are available for viewing this webinar.

Webinar Participant Questions and Answers

Can you use fire-protective paint?

In the case of field-applied fire protective paints, the same requirements for field-applied fire coatings apply, including the ASTM E119 fire test demonstrating equivalence to 2x10 dimension lumber or structural composite lumber.

Is mineral fiber insulation commonly available, and what does it cost?

Mineral wool insulation is commonly available in most metropolitan areas. Contact material suppliers or distributors that handle the product for costs.

When using a ceramic fiber blanket, do the holes have to be cut prior to final installation of the blanket?

I-joist web holes should be cut prior to the installation of the ceramic blanket. Then the blanket can be cut at the web hole location with a utility knife. Either of these steps can be done before or after the I-joists are installed.

If you are applying 1/2-inch drywall to the web or over the flanges of the joist, does the drywall need to be applied to both sides of the joist?

Yes, because a fire is unlikely to reach a joist from only one side.

Does the ceiling drywall need to be taped or finished?

No, the gypsum membrane covering I-joists is not required to be finished with tape and joint compound for equivalent performance to nominal 2 x 10 lumber floors. In the same way, joints between 5/8-inch wood structural panel membranes are not required to be taped and finished.

Is a suspended ceiling system which is fire rated and uses Class A fire-rated mineral tiles or 1/2-inch gypsum tiles an acceptable alternative?

If the suspended ceiling system is fire-rated as equivalent to the fire performance of 1/2-inch gypsum on the underside of the I-joist framing, it is considered an alternative to the fire protection requirements in accordance with the first paragraph in the 2015 IRC Section R302.13 and 2012 IRC Section R501.3.

Do you work with any of the manufacturers of field-applied fire coatings?

No, APA does not work with any manufacturers of field-applied coatings. However, individual I-joist manufacturers themselves may have such relationships. I-joists coated with an approved field-applied coating are required to demonstrate the same equivalency to 2 x 10 dimension lumber or structural composite lumber as the fire protective membrane methods published in APA System Report SR-405, in addition to several other requirements, such as the effects, if any, of the coating on the structural properties of the I-joists. APA suggests contacting both the coating manufacturer and the I-joist manufacturers that recommend such applications for product approvals.

Does the 2x10 lumber exemption apply to open webbed wood floor trusses that are nominal 2-inch by 10-inch or greater in dimension?

This question has been addressed by the International Code Council (ICC) and some states. The ICC issued a written interpretation indicating that:

Exception 4 permits floor framing of sawn lumber or structural composite lumber equal to or greater than 2 x 10 nominal dimension to be exempted from the application of the protective membrane to the underside of the floor framing members. The basis for this exception is that tests conducted on floor framing constructed of 2 x 10 lumber and loaded to 50 percent of full design load showed that the assemblies provided adequate time for occupants’ self-evacuation and safety for firefighters performing search and rescue. Floor assemblies using wood trusses may be approved for exemption if the floor assembly demonstrates equivalent fire performance.

Therefore, in order to apply this exemption to floor trusses, the trusses must be tested and evaluated for equivalency to the fire performance of nominal 2 x 10 lumber. This interpretation is consistent with the ruling by several states, such as the Ohio Board of Building Standards (BBS) and Massachusetts Board of Building Regulations and Standards (BBRS). Those rulings are available from the states' respective web sites.

What is the reasoning behind the 80-square-foot exception?

As reported by the American Wood Council (AWC), fire testing with an 80-square-foot unprotected area showed that, for a fire occurring under a protected area, fire blocking provides a minimal level of performance of the membrane system. The 80-square-foot allowance in the 2015 IRC Section R302.13 and 2012 IRC Section R501.3 is intended for passage of multiple HVAC or plumbing through floor penetrations, provided the aggregate area of the penetrations does not exceed 80 square feet per story and fire blocking is installed in accordance with Section R302.11.1 along the perimeter of the unprotected portion, separating the unprotected portion from the remainder of the floor assembly.

What is the fastener spacing required for the horizontally applied gypsum board listed in the code?

See Table R702.3.5 of the 2015 or 2012 IRC.

When 1x boards are used as strapping between the bottom of joist and the gypsum board, does it meet the requirements or must the gypsum be installed tight to the joists?

Based on an engineering analysis in accordance with the 2015 IRC Section 702.3.5, 1 x 4 (nominal) wood furring strips can be installed perpendicular to the bottom flange of I-joists at 16 inches on center, provided that the gypsum boards are directly attached to the furring strips using 1-1/4-inch (32 mm) Type W drywall screws at 12 inches (305 mm) on center. This provides the same number of fasteners with the same fastener penetration depth into the framing as gypsum directly attached to the I-joist flange, and is in compliance with the 2015 IRC Table R702.3.5.

In APA System Report SR-405, assembly FP-02 (gypsum on webs), does the gypsum have to continue all the way to the supporting girder (i.e., does it have to go behind the side of the joist hangers)?

Yes, but FP-02 requires only the I-joist web to be protected.

Do the recommended protections in the IRC and APA System Report SR-405 apply to I-joists spaced at 16-inch, 19.2-inch, and 24-inch centers?

Refer to APA System Report SR-405 for specific requirements and exceptions.

The Massachusetts Department of Public Safety recently put out guidance on this issue. Does Massachusetts recognize the APA guidelines?

Yes. In a March 2015 E-Memo released by the Massachusetts Department of Public Safety, the I-joist fire protective membrane methods published in APA System Report SR-405 are specifically recognized as compliant with the BBRS Official Interpretation (No. 2014_03) when used in conjunction with an APA Product Report or ICC-ES Evaluation Report to signify compliance with the Massachusetts code.

All of these solutions add cost. I question whether builders are going to incorporate them.

This comment gets to the intent of the IRC code requirement. These requirements are intended to increase the fire safety for occupants and firefighters in lightweight residential floors. Yes, the solutions add costs to varying degrees, but they also add benefits—the primary benefit being the increased fire performance of the floor assembly.

How does one handle locations where HVAC chases prevent the installation of gypsum direct to the bottom flanges of the I-joist?

If basement ductwork interferes with the installation of gypsum to the underside of the I-joists, the ductwork can be framed-in so that the gypsum protection is provided around the HVAC chase, similar to what is typically done when basements are finished.

How do intumescent coatings fit into the discussion?

A factory-applied intumescent coating by itself does not qualify an I-joist or plated truss to meet the membrane protection requirement for Exemption No. 4 in section R302.13 of the IRC. The I-joist would still need to be qualified under ICC Acceptance Criteria 14 (AC14). This Acceptance Criteria specifies testing according to the ASTM E119 time temperature curve, loading of the joists, consideration of holes, and durability requirements for the Intumescent coating. Field-applied intumescent coatings are outside of the scope of AC14.

Would a plated floor truss built with fire retardant lumber meet code requirements without further protection?

If fire retardant treated wood (FRTW) was used to manufacture a plated floor truss, the floor truss system would still need to be qualified to demonstrate equivalence to IRC Section R302.13, Exception 4. ICC-ES AC14 is specific to wood I-joists. APA is not aware of any similar ICC-ES Acceptance Criteria for metal plated floor trusses. Contact your plated truss manufacturer for information regarding the use FRT members in trusses.

What does “the perimeter of the unprotected portion” mean exception 3 of IRC Section R302.13 reference to?

“Portions of floor assemblies shall be permitted to be unprotected where complying with the following, 3.1 and 3.2,”

Exception 3.2 requires blocking be installed along the perimeter of the unprotected space in order to separate the unprotected portion from the remainder of the floor. Such blocking must be installed in accordance with Section R302.11.1, which lists several fire-blocking options, such as:

  1. Two-inch (51 mm) nominal lumber
  2. Two thicknesses of 1-inch (25.4 mm) nominal lumber with broken lap joints.
  3. One thickness of 23/32-inch (18.3 mm) wood structural panels with joints backed by 23/32-inch (18.3 mm) wood structural panels.
  4. One thickness of 3/4-inch (19.1 mm) particleboard with joints backed by 3/4-inch (19.1 mm) particleboard.
  5. One-half-inch (12.7 mm) gypsum board.
  6. One-quarter-inch (6.4 mm) cement-based millboard.
  7. Batts or blankets of mineral wool or glass fiber or other approved materials installed in such a manner as to be securely retained in place.
  8. Cellulose insulation installed as tested in accordance with ASTM E 119 or UL 263, for the specific application.