If you have designed a building of Type II, III, IV or V construction, you probably considered the allowable building area, which depends on the construction type, occupancy classifications, presence of fire sprinkler systems, and a few other criteria.
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One common way to increase the allowable area for a building is to use a frontage increase. The frontage increase provision in the IBC, found in Section 506.3, recognizes the safety benefits of having open space directly adjacent to a building. When a fire department or other emergency responders arrive to a site, there is an increased level of effectiveness when those personnel can access a large portion of the building exterior.

There is no requirement for a building to use the frontage increase provisions of IBC Section 506.3, but when used, the following requirements apply:

• A minimum of 25% of the building perimeter must be adjacent to a public way or open space
• To count as frontage, the open space must be either on the same lot or a dedicated public space. Open space on an adjacent lot does not count.
• The width of the open space must be at least 20 feet, measured at right angles from the building exterior wall to: (1) the closest interior lot line, (2) the entire width of the public way, or (3) the exterior face of a building on the same property.
• The open space must be accessed from a street or fire lane

The frontage calculation can be performed by hand, but I recommend the building frontage calculator. If you want to do the calculation on your own, here are the steps. Note that this method is valid through the 2018 IBC. Starting in 2021, the method has changed, which I will cover in a future update.

1. Determine the weighted average of the width of open space (W). 

This value, W, is determined by the following equation:

2. Determine the total building perimeter, P.
3. Solve IBC Equation 5-5 to determine the frontage increase factor. 

Link: Separated Mixed Occupancy Area Calculator
Link: Nonseparated Mixed Occupancy Area Calculator

Using the frontage increase provisions of IBC 506.3 is a common way to increase the allowable area of a building. There is no requirement to use these provisions, but when you do, the requirement described above do apply.
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When working with architects, I frequently see code summary sheets with a frontage calculation for Type I buildings that have an unlimited allowable area. Since there is no requirement to take a frontage increase, I typically remind architects they don’t need to perform the frontage calculations if the building already has an unlimited allowable area. This saves time and also prevents possible permit review comments on a code provision that wasn’t needed in the first place.

​NFPA 285 is the “Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components.” How’s that for a title? Despite the lengthy name, the goal of the test is quite simple – to understand how fire will spread on an exterior wall. In this article, we’ll briefly review the history of NFPA 285, discuss the criteria for the rest, and then review when compliance with NFPA 285 is required in the IBC.

The NFPA 285 standard was first published in 1998 and is currently on it’s 4th edition, published in 2019. But the origins of NFPA 285 go all the way back to the 1970s, when foam plastic insulation became a popular choice for energy conservation in (or on) exterior walls. Based on work by the Society of Plastics Industry, the first test for flammability characteristics of an exterior wall was developed. This test was named UBC 17-6 and was first adopted by the Uniform Building Code (UBC) in 1988. This same test was later renamed as UBC 26-4 [1].

UBC 17-6/UBC 26-4 was a massive, full-scale test, consisting of a two-story exterior wall, totaling 24 feet in height (12 feet for each story). The test evaluated both vertical and lateral flame spread on nonload-bearing exterior walls containing foam plastics. Several wall assemblies containing foam plastics passed the test and, starting with the 1988 UBC, could be used as exterior wall assemblies in Type I, II, III and IV construction. A few years later, a modified test, using a smaller scale was adopted as UBC 26-9.
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10 years after the first adoption UBC 26-4, NFPA released the first edition of NFPA 285, entitled “Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components.” This version of NFPA 285, published in 1998, is the same as the UBC 26-9 test.
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Notice the difference in the name between the 1998 version of NFPA 285 and the 2019 version. The “nonload-bearing” restriction has been removed, now allowing load-bearing walls to be tested (although the standard itself does not require the application of a live load during testing).

The NFPA 285 testing criteria is nuanced, with variations based on the materials used in the wall assembly and the thickness of certain components in the wall assembly. For a full explanation of the acceptance criteria, review Section 10.2 of the 2019 edition of NFPA 285.

Generally, the acceptance criteria involves the following:

Flame propagation on the exterior face of the wall assembly

1. ­Vertical flame spread of less than 10 feet above the window opening in the test
2. Lateral flame spread of less than 5 feet from the centerline of the window opening
3. Temperature less than 1000 F at a point 10 feet above the window opening

Flame propagation through combustible components and insulation
Depending on the materials used in the exterior wall assembly, there are various temperature requirements that must be met. These include:

1. Limitations on temperature within exterior wall panels containing combustible components
2. Limitations on temperature within the air cavity between exterior wall panels and the wall itself

These temperature requirements apply both vertically and laterally and are measured by thermocouples placed above and to the side of the first story window opening.

Temperatures in the second story

1. The temperature in the second story room cannot rise more than 500 F above the temperature at the start of the test, measured 1-inch from the interior side of the wall assembly

Flame Propagation to second story

1. Flames cannot spread to the inside of the second story room

Flame propagation to adjacent horizontal spaces

1. Flames cannot spread horizontally beyond the side walls of the test

In the 2021 IBC, there are a number of triggers for NFPA 285 compliance. If you are using an earlier version of the code, the requirements are fairly similar, but refer to the corresponding sections in the applicable code edition for the specific details.
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Water Resistive Barriers (IBC 1402.5)
Exterior walls in buildings of Type I, II, III or IV construction that are greater than 40 feet above grade plane must comply with NFPA 285 if they contain a water-resistive barrier. There are two key exceptions:

1. If the water-resistive-barrier is the only combustible component in the exterior wall and the wall has a brick, concrete, stone, terra cotta, stucco or steel covering meeting the thickness requirements in IBC 1404.2.
2. If the water-resistive-barrier is the only combustible component and it:
   1. Has a peak heat release of less than 150 kW per square meter, a total heat release of less than 20 MJ per square meter and an effective heat of combustion of less than 18MJ/kg when tested per ASTM E1354 with an incident heat flux of 50 kW per square meter.
   2. Has a flame spread index of 25 or less and a smoke-developed index of 450 or less as determined by ASTM E84.

For this section, fenestration products, flashing of fenestration and water-resistive-barrier flashing are not considered to be part of the weather-resistive-barrier. If any of these materials are combustible, the exceptions above do not apply.

Metal Composite Materials (MCMs) (IBC 1406.10.3)
Exterior walls containing MCMs in buildings of Type I, II, III or IV construction that are greater than 40 feet above grade plane must comply with NFPA 285. Note that the 2021 IBC has removed several exceptions that existed in previous editions regarding the height of the MCM panels and the area of wall covered by the panels.

High-Pressure Laminates (HPLs) (IBC 1408.10.4)
Exterior walls containing HPLs in buildings of Type I, II, III or IV construction must comply with NFPA 285. There are two key exception for when HPLs are installed 40 feet or less above grade plane:

1. When the fire separation distance is 5 feet or less and the area of HPLs does not exceed 10% of the exterior wall surface area.
2. When the fire separation distance is greater than 5 feet. In this case, the area of HPLs is not limited.

Mechanical Equipment Screens (IBC 1511.6.2)
Compliance with NFPA 285 is one of three possible code paths for providing combustible mechanical equipment screens on the roof decks of buildings of Type I, II, III or IV construction.

Foam Plastic Insulation (IBC 2603.5.5)
Exterior walls containing foam plastic insulation in buildings of Type I, II, III or IV construction of any height must comply with NFPA 285. There are several exceptions:

1. One story buildings where the foam plastic has a flame spread index of 25 or less, smoke-developed index of not more than 450 and is covered by at least 0.032” of aluminum or 0.015” of corrosion-resistant steel and the building is fully sprinkler protected per NFPA 13. This exception only applies to foam plastic insulation with a thickness of 4” or less.
2. When the foam plastic insulation is covered on each face by minimum 1” of masonry or concrete and one of the following conditions is met:
   • No airspace between the insulation and concrete/masonry cover.
   • The insulation has a flame spread index of 25 or less per ASTM E84 and the maximum airspace between the insulation and concrete/masonry cover is 1”.

​Fireblocking (IBC 718.2.6, Exception 3)
Fireblocking is not required in exterior walls when the exterior wall covering complies with NFPA 285.

​Wall Assembly vs. Wall Components
NFPA 285 tests wall assemblies as a whole. Individual components of the wall are not considered, rather the entire exterior wall assembly is evaluated for compliance with the standard.

Deviation from NFPA 285 Assemblies
The use of Engineering Judgements (Ejs) for NFPA 285 compliance is a well-debated topic in the design industry. On one hand, it can be impractical to test an exterior wall assembly with every possible variation of material and component (remember that NFPA 285 is a full-scale test of a specific wall assembly). But on the other hand, it can be difficult to judge how variation from a tested assembly will impact performance.

While there are many exterior wall assemblies that have passed the NFPA 285 test, in my experience, many designers choose an assembly that has not been specifically tested. Many manufacturers of exterior wall products will obtain a testing report from ICC Engineering Services (an ICC ES Report, or something similar from another testing agency) that claims the product would pass NFPA 285 if tested. These reports, which essentially amount to a generalized EJ, are typically based on substituting one or more products into a wall assembly that has passed NFPA 285.

If the EJ route is chosen, the EJ should be provided by a qualified design professional, such as a licensed fire protection engineer with experience in the evaluation of exterior wall assemblies.

NFPA 285 vs. ASTM E119
NFPA 285 is different from ASTM E119, the test most commonly used to establish fire-resistance ratings for wall assemblies. This means that if your exterior wall is required to have a fire resistance rating, it also needs to have been tested per ASTM E119. Unfortunately, there are many walls that have a fire-resistance rating per ASTM E119 but do not pass NFPA 285. Similarly, there are walls that comply with NFPA 285 but may not be tested to ASTM E119.

This means that if your exterior wall is required to have a fire resistance rating and comply with NFPA 285, there are at least four potential code paths available:

1. Provide an exterior wall assembly that has been tested to ASTM E119 for the required fire-resistance rating and has also passed NFPA 285
2. Provide an exterior wall assembly that achieves the required fire-resistance rating using the prescriptive fire resistance method (IBC 721) or calculated fire resistance method (IBC 722) and has also passed NFPA 285.
3. Provide a wall that uses an IBC ESR report (or similar) to achieve compliance with one or both of the testing requirements.
4. Obtain an Engineering Judgement (EJ) to achieve compliance with one or both of the testing requirements.

Note that the acceptance of the last two items will vary by jurisdiction. In my experience, most AHJs are satisfied by an ICC ESR report, but the acceptance of EJs can vary greatly. See the section below for more on Ejs.

Finding NFPA 285 Wall Assemblies
To my knowledge, the best place to find wall assemblies that have been tested to NFPA 285 is the UL database. You can search “FWFO” in the UL directory to find the list, or checkout the database below of all UL-listed assemblies.