A listing that calls a building “brick” hasn't told you its ISO construction class. The brick may be veneer over wood studs, or it may form load-bearing walls beneath a wood roof. Those details affect the class recorded on a statement of values, with consequences for fire rating and catastrophe modeling.
ISO classification depends on the exterior walls, floors and roof, including their fire resistance. The six classes below provide a starting point; mixed materials and additions require a closer look. For context on the other underwriting fields, see what COPE data is.
The six classes at a glance
ISO sorts buildings into six classes, from most combustible to least. The definitions below follow ISO's published Guide for Determination of Needed Fire Flow (Edition 06-2014, Chapter 2). The same six classes and names are used in ISO's Commercial Lines Manual, which is a subscriber document.
| Class | Name | Exterior walls | Floors and roof | Fire-resistance threshold |
|---|---|---|---|---|
| 1 | Frame | Combustible, or non-combustible walls with combustible floors and roof | Combustible | None |
| 2 | Joisted masonry | Masonry, or fire-resistive rated at least 1 hour | Combustible | Walls only: masonry or ≥1 hr |
| 3 | Non-combustible | Non-combustible or slow-burning | Non-combustible or slow-burning, on non-combustible supports | None |
| 4 | Masonry non-combustible | Masonry at least 4 in. thick, or fire-resistive ≥1 hr | Non-combustible or slow-burning | Walls only: masonry or ≥1 hr |
| 5 | Modified fire resistive | Masonry or fire-resistive | Fire-resistive | 1 hour to under 2 hours |
| 6 | Fire resistive | Masonry or fire-resistive | Fire-resistive | 2 hours or more |
- Walls
- Wood or other combustible framing, including brick veneer or stucco over studs
- Floors
- Wood joists or trusses
- Roof
- Wood trusses, wood deck
- Walls
- Brick, block, poured or tilt-up concrete
- Floors
- Wood joists or heavy timber
- Roof
- Wood joists, glulam, panelized wood deck
- Walls
- Metal panels on steel girts
- Floors
- Steel framing, if any
- Roof
- Metal deck on steel purlins
- Walls
- Block, brick or tilt-up concrete
- Floors
- Concrete on metal deck, steel joists
- Roof
- Metal deck on steel bar joists
- Walls
- Masonry or rated assemblies, 1 to <2 hr
- Floors
- Protected steel or concrete, 1 to <2 hr
- Roof
- Protected deck and supports, 1 to <2 hr
- Walls
- Reinforced concrete or ≥2-hr assemblies
- Floors
- Concrete slabs ≥4 in. or ≥2-hr assemblies
- Roof
- Concrete or ≥2-hr assembly
Classify the walls separately from the floors and roof, then combine the results. The fire-resistance thresholds apply throughout the structure for Classes 5 and 6, but only to the walls for Classes 2 and 4. A block building with bare steel bar joists and a metal deck can therefore be Class 4 despite the roof's vulnerability to fire.
What "combustible" means to ISO
Several terms have specific meanings in ISO's fire flow guide (Ch. 2 §1):
- Masonry veneer is combustible. "Combustible wall materials with an exterior surface of brick, stone, or other masonry materials (commonly known as 'masonry veneer')" are listed as combustible. A brick-faced apartment building with wood studs behind the brick is Frame.
- Wood-iron clad is combustible. Metal siding or roofing on a wood skeleton (a pole barn, for example) is combustible construction.
- A metal roof can be combustible. Metal floors or roofs with combustible insulation or ceiling material attached to the underside, or within 18 inches of the supports, count as combustible. So do composite panels with a combustible or foam-plastic core, unless the foam qualifies as slow-burning behind an acceptable thermal barrier.
- Rated wood is still wood. "Combustible assemblies which are listed as having not less than a one-hour rating" remain combustible. A one-hour wood-stud wall with Type X gypsum is a rated combustible wall, not a fire-resistive one. ISO reserves "fire-resistive" for non-combustible materials or assemblies rated at least one hour.
- Masonry means at least 4 inches. Adobe, brick, concrete, block, stone, tile and similar materials qualify as masonry at a minimum thickness of 4 inches. Masonry walls thinner than that, standing on their own, are non-combustible but not masonry.
- Non-combustible includes unrated protection. Steel with fireproofing rated under one hour is just "non-combustible" for ISO purposes. Furring strips nailed to non-combustible supports don't change a wall's class.
Class by class
Class 1: Frame
Definition. Exterior walls, floors and roof of combustible construction, or non-combustible/slow-burning walls carrying combustible floors and roof.
Typical buildings. Garden-style and mid-rise wood apartments, wood-framed strip retail, small offices, houses converted to offices, pole barns, and most podium buildings (more on those below).
How to spot it. Look for a sloped roof on wood trusses, siding, vinyl or fiber-cement cladding, stucco on a building under four or five stories, and walls that sound hollow when tapped. Brick that ends at the window sill or has weep holes and a visible air gap is veneer. Permit records that mention "framing," "shear wall," or "trusses" settle it; the building permit history guide covers how to read them.
Common errors. Coding brick-veneer apartments as joisted masonry; coding a metal-sided building as Class 3 when the girts and purlins are wood; treating stucco as masonry.
Class 2: Joisted masonry
Definition. Exterior walls of masonry (at least 4 inches) or of fire-resistive construction rated at least one hour, with combustible floors and roof.
Typical buildings. Pre-war brick main-street buildings with wood joists, concrete-block strip centers with wood trusses, and tilt-up warehouses with panelized wood roofs (glulam beams, wood purlins, plywood or OSB deck). Heavy-timber mill buildings fall here too, because ISO treats timber as combustible no matter how thick it is.
How to spot it. Masonry or concrete walls you can see from both sides at doors and window returns, plus a wood ceiling structure when you look up inside. In a warehouse, a wood roof deck shows as plywood panels between purlins.
Common errors. Assuming every tilt-up is Class 4. The walls qualify as masonry, but the roof structure decides between 2 and 4, and panelized wood roofs are common on tilt-ups, especially in the West.
Class 3: Non-combustible
Definition. Exterior walls, floors and roof of non-combustible or slow-burning materials on non-combustible or slow-burning supports. No hourly rating is required.
Typical buildings. Pre-engineered metal buildings (steel frame, metal wall panels, metal roof), and steel-stud buildings with non-combustible sheathing.
How to spot it. Corrugated or ribbed metal walls with steel columns and girts visible inside, a metal roof on steel purlins, and no masonry wainscot taller than a few feet.
Common errors. Treating every "metal building" as Class 3. Foam-core insulated metal panels that don't meet the slow-burning criteria, foam insulation stuck to the underside of the deck, or wood girts and purlins all make the assembly combustible, which can drop it to Class 1.
Class 4: Masonry non-combustible
Definition. Exterior walls of masonry at least 4 inches thick or fire-resistive rated at least one hour, with non-combustible or slow-burning floors and roof. The roof deck stays non-combustible even with combustible roof insulation, felt or tar on top.
Typical buildings. Modern distribution warehouses (tilt-up or block walls, steel joists, metal deck), big-box retail, and many one-story schools and medical office buildings.
How to spot it. Masonry or concrete walls plus open-web steel bar joists and a fluted metal deck overhead. No spray fireproofing on the steel.
Common errors. Calling these "fire resistive" because they are "concrete buildings." Without rated floors and roof, concrete walls alone stop at Class 4.
Class 5: Modified fire resistive
Definition. Exterior walls, floors and roof of fire-resistive materials rated at least one hour but under two hours, or of masonry that is at least 4 inches thick but short of Class 6 thickness requirements.
Typical buildings. Mid-rise offices, hotels and medical buildings with one-hour protected steel; some older concrete-frame buildings with thin slabs.
How to spot it. Spray-applied fireproofing or gypsum-encased steel columns and beams, concrete-on-deck floors, and design documents listing one-hour (or 1½-hour) assemblies. This class rarely gets identified from the street; it usually comes from plans or a survey.
Class 6: Fire resistive
Definition. Exterior walls or structural frame of solid masonry or reinforced concrete at least 4 inches thick (or rated assemblies of at least two hours); floors and roof of monolithic reinforced concrete slabs at least 4 inches, concrete joist systems meeting ISO's spacing and slab rules, or two-hour assemblies; and load-bearing protected steel rated at least two hours.
Typical buildings. High-rise office towers, cast-in-place concrete residential towers, hospitals, and concrete parking structures.
How to spot it. Cast-in-place concrete columns and flat slabs, or heavily fireproofed steel in a high-rise. The building height alone is often a clue: tall buildings are usually built to Type I under the building code (see the IBC section below).
Which class is it?
When the materials are known, the class follows from five questions asked in order. Each question applies to the building as a whole; if no single answer covers at least two-thirds of the relevant area, go to the mixed construction rules in the next section.
- 1Is more than one-third of the exterior wall area combustible?Wood or steel studs sheathed in wood, brick or stone veneer over studs, metal siding on a wood frame, foam-core panels, rated wood assemblies.YesClass 1 · FrameNo↓ next question
- 2Are the floors and roof combustible?Wood joists or trusses, glulam, heavy timber, wood deck; also a metal deck with combustible insulation stuck to its underside.YesClass 2 if walls are masonry or ≥1-hr rated; otherwise Class 1No↓ next question
- 3Are the exterior walls masonry (≥4 in.) or rated ≥1 hour?Block, brick, tilt-up or poured concrete. Metal panels on girts are non-combustible but not masonry.Yes↓ next questionNoClass 3 · Non-combustible
- 4Are the floors and roof also rated ≥1 hour?Spray-fireproofed or encased steel, concrete slabs. Bare bar joists and metal deck are unrated.Yes↓ next questionNoClass 4 · Masonry non-combustible
- 5Do walls, floors and roof all reach ≥2 hours?Reinforced concrete slabs ≥4 in., concrete or 2-hr protected frame, solid masonry ≥4 in.YesClass 6 · Fire resistiveNoClass 5 · Modified fire resistive
Mixed construction and the 66⅔% rule
Real buildings mix materials: a concrete podium under wood floors, block walls on three sides and an EIFS-clad office front, a steel addition on a wood-roofed warehouse. ISO's mixed construction rule (fire flow guide, Ch. 2 §3) works from the most protective class down:
- Fire resistive if at least 66⅔% of the total wall area and at least 66⅔% of the total floor-and-roof area meet Class 6.
- Modified fire resistive if both shares meet Class 5, or Classes 5 and 6 together reach 66⅔% without either one doing it alone.
- Masonry non-combustible on the same test for Class 4, or Classes 4–6 combined.
- Non-combustible on the same test for Class 3, or Classes 3–6 combined.
- Joisted masonry if at least 66⅔% of the wall area is Class 2 construction, or Classes 2–6 combined reach 66⅔% of both walls and floor-and-roof area.
- Frame for anything left, and for any building with more than 33⅓% of its total wall area combustible, regardless of the rest of the building.
The combustible-wall threshold matters even in buildings with substantial concrete construction. Above one-third combustible wall area, the rule assigns Frame. Basement walls and the lowest floor are disregarded throughout.
The podium problem
Five-over-one and similar podium buildings are the most common place this rule changes a submission. Picture a hypothetical building with one story of cast-in-place concrete for parking and retail, and four stories of wood-framed apartments above. Ignoring the lowest floor, the floor-and-roof area is four wood floor plates plus a wood roof over a single concrete podium deck. The wall area is about four-fifths wood studs. With more than 33⅓% of the walls combustible, the building is Class 1 under ISO's rule, even though the ground floor would be Class 6 on its own. Some carriers rate the podium and the frame portion separately when a rated horizontal separation exists; that is a carrier-specific practice, so check the guidelines you're writing under.
ISO classes vs. IBC construction types
Architects, building departments and many listings describe buildings using the International Building Code's construction types (I-A through V-B). The IBC types are defined by Table 601, which sets minimum fire-resistance ratings in hours for each building element. The values below are from the 2015 IBC (ICC, Chapter 6); they were carried forward in later editions, which also added three mass timber types.
| IBC type | Structural frame | Exterior bearing walls | Floors | Roof | Usual ISO class |
|---|---|---|---|---|---|
| I-A | 3 hr | 3 hr | 2 hr | 1½ hr | 6 |
| I-B | 2 hr | 2 hr | 2 hr | 1 hr | 6 (sometimes 5) |
| II-A | 1 hr | 1 hr | 1 hr | 1 hr | 5 (often coded 4) |
| II-B | 0 | 0 | 0 | 0 | 3 with metal walls, 4 with masonry walls |
| III-A | 1 hr | 2 hr | 1 hr | 1 hr | 2 with masonry walls; 1 with treated-wood walls |
| III-B | 0 | 2 hr | 0 | 0 | 2 with masonry walls; 1 with treated-wood walls |
| IV-HT | Heavy timber | 2 hr | Heavy timber | Heavy timber | 2 |
| V-A | 1 hr | 1 hr | 1 hr | 1 hr | 1 |
| V-B | 0 | 0 | 0 | 0 | 1 |
| ISO class ↓ / IBC type → | I-A | I-B | II-A | II-B | III-A | III-B | IV-HT | V-A | V-B |
|---|---|---|---|---|---|---|---|---|---|
| 6 Fire resistive | |||||||||
| 5 Modified FR | |||||||||
| 4 Masonry NC | |||||||||
| 3 Non-combustible | |||||||||
| 2 Joisted masonry | |||||||||
| 1 Frame |
The rows that need care:
- I-B. Floors and walls are rated two hours, the roof one hour. Under the mixed construction rule, a building of three or more stories usually has enough two-hour floor area to clear 66⅔% of the floor-and-roof total, which lands it in Class 6. Low-rise I-B buildings can fall to Class 5.
- II-A. Every element is rated one hour and non-combustible, which matches ISO's Class 5 definition. In practice many submissions code these Class 4, often because the roof protection isn't documented. Ask for the code analysis on the drawings before giving Class 5.
- II-B. No hourly ratings at all, so the wall material decides: metal panels give Class 3; block or tilt-up walls give Class 4.
- III-A and III-B. The IBC allows fire-retardant-treated wood framing in Type III exterior walls. To ISO, a rated wood assembly is still combustible, so a Type III building with treated-wood exterior walls is Frame. Type III with true masonry walls is joisted masonry.
- Mass timber (IV-A, IV-B, IV-C). The 2021 IBC added these types for tall mass timber buildings, up to 18 stories for IV-A (STRUCTURE magazine). ISO's six-class definitions treat timber as combustible, so a literal reading puts them in Class 1 or 2 depending on the exterior walls. Carriers differ on how much credit they give for encapsulation and sprinklers.
Why construction class moves the rate
Construction class is a proxy for how much of the building is fuel and how long the structure stays up. In a Frame building the structure itself burns; in a fire-resistive building the fire is largely limited to contents and finishes on the floor of origin, and the frame is usually repairable. That difference shows up directly in ISO's own math. The fire flow guide calculates a construction factor C = 18 × F × √A, where A is the effective area and F is a coefficient set by construction class. ISO rounds C to the nearest 250 gpm, with a 500 gpm floor and caps of 8,000 gpm for Classes 1–2 and 6,000 gpm for Classes 3–6 (and for any one-story building).
- 1.5
- F for Frame
- 1.0
- F for joisted masonry
- 0.8
- F for non-combustible
- 0.6
- F for fire resistive
View data
| Category | Value (gpm) | Note |
|---|---|---|
| 1 Frame | 3,750 | F = 1.5 |
| 2 Joisted masonry | 2,500 | F = 1 |
| 3 Non-combustible | 2,000 | F = 0.8 |
| 4 Masonry NC | 2,000 | F = 0.8 |
| 5 Modified FR | 1,500 | F = 0.6 |
| 6 Fire resistive | 1,500 | F = 0.6 |
At a 20,000 sq ft effective area, a Frame building needs 3,750 gpm and a fire-resistive one 1,500 gpm: 2.5 times the water for the same footprint. Fire flow isn't premium, but the same logic runs through rating. Commercial property rating plans, ISO's included, vary fire loss costs by construction class, and catastrophe models use the class (or a finer version of it) as a primary input. A one-class error on a large schedule can move the modeled loss more than most other single fields.
What new construction is actually made of
Wood framing accounts for most new U.S. residential construction. The Census data below helps explain why an exterior finish alone is a poor basis for assigning a masonry class.
View data
| Single-family houses | Multifamily bldgs | |
|---|---|---|
| 2009 | 95% | 86% |
| 2010 | 95% | 84% |
| 2011 | 95% | 85% |
| 2012 | 94% | 87% |
| 2013 | 94% | 87% |
| 2014 | 93% | 86% |
| 2015 | 93% | 87% |
| 2016 | 91% | 84% |
| 2017 | 92% | 84% |
| 2018 | 93% | 84% |
| 2019 | 90% | 82% |
| 2020 | 91% | 85% |
| 2021 | 92% | 87% |
| 2022 | 94% | 88% |
| 2023 | 93% | 89% |
| 2024 | 94% | 86% |
| 2025 | 94% | 83% |
Between 2009 and 2025, 90% to 95% of new single-family houses and 82% to 89% of new multifamily buildings were wood-framed. Brick doesn't change that. In 2025, 164,000 new single-family houses had brick as the primary exterior wall material, but only 53,000 houses of any cladding had concrete framing and 5,000 had steel framing. Even if every concrete- and steel-framed house were brick-clad, at least 106,000 of the brick houses, or about 65%, had to be brick veneer over wood. In the South, where most brick is used, the same arithmetic gives at least 98,000 of 151,000.
View data
| Category | Value (thousand houses) |
|---|---|
| Brick primary exterior | 164 |
| Concrete framing (incl. ICF) | 53 |
| Steel framing | 5 |
Translating assessor codes and listing descriptions
At quote stage, the available records may be limited to an assessor entry, a broker's SOV and a sale or lease listing. Read their construction descriptions carefully: they may use a different classification system.
Assessor construction class
Many assessors classify buildings with a cost-manual system built around A, B, C, D and S construction types. California's State Board of Equalization publishes a representative set in Assessors' Handbook Section 531.10A (January 2025):
| Assessor type | What the handbook describes | Likely ISO class | Watch for |
|---|---|---|---|
| A | Structural steel frame fireproofed with concrete or spray, concrete floors and roof | 6 or 5 | Rating of the fireproofing |
| B | Reinforced concrete columns and beams, concrete floors and roof | 6 or 5 | Slab thickness, older thin slabs |
| C | Masonry exterior walls; floors of wood or poured concrete; wood roof structure | 2 (4 if the roof is steel) | Roof framing decides 2 vs. 4 |
| D | Wood frame; skin may be wood, stucco, masonry veneer or sheet metal | 1 | Veneer that looks like masonry |
| S | Specialized buildings that fit none of the above | Varies | In many other jurisdictions "S" means pre-engineered steel (often Class 3) |
Check whether a number refers to quality class before copying it. Assessor codes such as “D6A” or “C5” can combine a construction type with a quality grade and a shape letter. In “D6A,” D identifies the construction type and 6 the quality grade. These codes support cost estimating and do not map directly to ISO classes. They may also predate a remodel or addition.
Listing and SOV language
| Phrase | What it usually means | ISO class it supports |
|---|---|---|
| "Brick," "all brick," "brick exterior" | Brick veneer on most newer buildings; possibly solid multi-wythe masonry in older ones | 1 unless solid masonry is confirmed |
| "Concrete block," "CMU," "masonry" | Masonry walls | 2 or 4, depending on the roof |
| "Tilt-up," "tilt-wall" | Concrete wall panels | 2 with wood roof, 4 with steel joists and metal deck |
| "Steel building," "metal building," "PEMB" | Pre-engineered metal | 3, or 1 if wood girts or foam panels |
| "Steel frame" on an office or hotel | Could be protected or bare steel | 3 to 6; ask about fireproofing |
| "Concrete construction," "post-tensioned" | Concrete frame and slabs | 5 or 6 |
| "Five-over-one," "podium" | Wood over concrete | 1 under the 66⅔% rule |
A short field checklist
Before you accept a construction class on a schedule, confirm five things:
- What the exterior walls are made of behind the cladding. Veneer, stucco and EIFS describe the skin, not the wall.
- What holds up the roof. Wood trusses or a panelized wood deck make it 1 or 2; steel joists and metal deck make it 3 or 4.
- Whether anything is rated, and for how long. Classes 5 and 6 need documentation: plans, a code analysis, or a survey.
- The share of each construction type by area. If more than a third of the wall area is combustible, the building is Frame.
- What changed since it was built. Additions, re-roofs and façade replacements alter the class; permit records are the fastest way to find them (see building permit history).
The class you settle on then feeds the construction column of the SOV, alongside year built, stories and area. The statement of values guide covers how that column is used downstream.