Construction Types and Fire Resistance: Tables 601/602, Hourly Ratings, and Material Requirements by Type

How the IBC classifies buildings into five construction types (I through V), the fire-resistance ratings required by Table 601 for structural elements, and the exterior wall ratings required by Table 602 based on fire separation distance. Covers material restrictions (combustible vs. noncombustible), hourly rating values for structural frames, bearing walls, floors, and roofs, and the 2021 IBC addition of mass timber subcategories (IV-A, IV-B, IV-C, IV-HT).

ARE · Project Development & Documentation

Overview

Before you can figure out how tall or how large a building can be, you need to know what it's made of and how long those materials can resist fire. That's the entire point of IBC Chapter 6 and Tables 601 and 602.

The IBC divides all buildings into five construction types, from Type I (the most fire-resistant, all noncombustible) down to Type V (the least restrictive, any code-permitted material). Each type has an A and B subclassification, with A providing higher fire-resistance ratings than B. The 2021 IBC added mass timber subcategories under Type IV: IV-A, IV-B, IV-C, and the traditional IV-HT (Heavy Timber).

Table 601 sets the minimum hourly fire-resistance ratings for structural frames, bearing walls (interior and exterior), floor construction, and roof construction across all construction types. Table 602 handles exterior wall ratings specifically, basing them on fire separation distance rather than construction type alone.

These two tables drive nearly every downstream code decision. Allowable height and area (Tables 504.3, 504.4, 506.2) depend on the construction type you select. Sprinkler trade-offs, occupancy separations, and means of egress requirements all link back to this classification. Getting construction type wrong cascades errors through the entire code compliance process.

For the ARE, you need to apply these tables to project scenarios, not just recall rating values. Expect questions that ask you to determine a required rating for a specific building element given the construction type, or to evaluate how a change in occupancy or fire separation distance alters your code obligations.

Essential

Every building in the United States must be classified under exactly one construction type per IBC Section 602.1. You pick the type based on the project's anticipated height, area, and occupancy, then verify that every building element meets the minimum fire-resistance ratings. The type determines material restrictions (combustible vs. noncombustible) and the hourly ratings each element must achieve. This classification is the single most consequential code decision you'll make on a project. ## The Five Construction Types Type I (IA and IB): All building elements must be noncombustible. This is the most fire-resistant classification. Type IA requires a 3-hour structural frame and 3-hour floor construction. Type IB drops to 2-hour structural frame and 2-hour floors. These are your high-rises, hospitals, and large assembly buildings. Bearing walls hit 4 hours (exterior) and 3 hours (interior) for IA; 3 hours exterior and 2 hours interior for IB.

Professional

Consider an architect designing a 4-story mixed-use building with ground-floor retail (Group M) and three floors of apartments (Group R-2) on a tight urban lot. The nearest lot line sits 8 feet from the proposed east wall. The construction type selection will drive every aspect of the building's documentation. With 8 feet of fire separation distance on the east side, Table 602 requires 1-hour rated exterior walls on that face for most construction types, and 2 hours for Types IA, IB, and IV. If the architect selects Type IIIA, the exterior walls must be noncombustible (per Section 602.3), and the east wall needs at minimum a 1-hour rating from Table 602. But the exterior bearing walls also must meet Table 601, which requires 1 hour for Type IIIA. Since both tables require 1 hour, the governing rating is 1 hour.

TLDR

The IBC defines five construction types (I through V), each with an A (higher rated) and B (lower rated) subcategory. Type IV now includes IV-A, IV-B, IV-C, and IV-HT for mass timber.
Types I and II require noncombustible materials throughout. Type III requires noncombustible exterior walls but allows combustible interiors. Type IV requires noncombustible exteriors with heavy timber or mass timber interiors. Type V allows any code-permitted material.
Table 601 sets minimum hourly fire-resistance ratings for structural frames, bearing walls, floor construction, and roof construction by type. Ratings range from 3 hours (Type IA structural frame) down to 0 hours (Types IIB, IIIB, VB).
Table 602 sets exterior wall ratings based on fire separation distance. Below 5 feet, walls need 2 or 3 hours. At 30 feet or more, all types drop to 0 hours.
Exterior bearing walls must meet the higher of Table 601 or Table 602.
Footnote a: Structural frame and bearing wall ratings can drop by 1 hour when supporting only one floor or roof.
Footnote h: Sprinklered Group A, B, E and parking garages can reduce structural frame, floor, and roof ratings by 1 hour (minimum 1 hour).
Fire-resistance ratings are established through ASTM E119 or UL 263 testing. The unexposed surface temperature rise cannot exceed 250 degrees F.
A building must be classified as a single construction type. The type drives allowable height, area, and downstream code requirements.
Type IIB is noncombustible with zero-hour ratings. Do not confuse "unprotected" with "combustible."

ELI5

Think of buildings like suits of armor. Some buildings wear thick, heavy armor that can take a lot of heat before anything goes wrong. Others wear lighter armor, or no armor at all. The building code sorts all buildings into five groups based on how much "armor" they're wearing. The five groups (Types I through V): Type I is the tank. Everything is made from materials that don't burn (noncombustible), and those materials are wrapped in fire protection that can last for hours. Skyscrapers and hospitals are Type I. Type II is still noncombustible, but with less armor. Type IIB has noncombustible materials with zero hours of fire protection. It's like wearing a metal breastplate without padding underneath. Type III has a brick or concrete exterior shell (noncombustible), but the inside can be built with wood. Picture a classic downtown building with masonry walls and wood floor joists.

AREProject Development & Documentation

Construction Types and Fire Resistance: Tables 601/602, Hourly Ratings, and Material Requirements by Type

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Why Construction Types and Fire-Resistance Ratings Shape Every Building You Design

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