One Hour or Two? How Long a Shaft Must Resist Fire

Your plans for the four-story building come back with one comment circled: the duct riser and the trash chute both need a rated enclosure, and the reviewer wants to know how many hours. Your contractor says one hour. Your architect says two. They are arguing about a rule that turns on a single question — how many floors does that vertical opening connect?

A shaft enclosure must have a fire-resistance rating of not less than 2 hours where it connects four stories or more, and not less than 1 hour where it connects fewer than four stories. It must also be rated at least as high as the floor assembly it passes through, but never needs to exceed 2 hours (§ 713.4).

A shaft enclosure is the fire-rated box built around a vertical opening that runs between floors — an elevator hoistway, a stairway, a duct riser, a trash or laundry chute. Its job is to stop a fire on one floor from using that opening as a chimney into every floor above.

Key numbers

Counting the stories

The count is narrower than most people assume, and the code says so directly:

The number of stories connected by the shaft enclosure shall include any basements but not any mezzanines (§ 713.4).

So a building with three above-grade floors plus a basement served by the same shaft is connecting four stories, and lands on the 2-hour requirement. A mezzanine in the middle of that shaft does not add to the count. Getting this wrong in either direction is expensive — an under-rated shaft fails inspection, and an over-rated one costs money for nothing.

There is one exception to the rating table: shafts in high-rise buildings may use a reduced rating under a separate high-rise provision (§ 713.4). That provision's own text is not covered here.

The rating is a floor, not a ceiling

Two rules stack. First the story count sets a minimum. Then the shaft must be rated at least as high as the floor assembly it penetrates. If your floors are rated higher than the story count would require, the shaft rises with them — up to the 2-hour cap, and no further (§ 713.4).

Where an exterior wall forms part of the shaft

If an exterior wall serves as part of a required shaft enclosure, that wall follows the exterior-wall rules instead, and the fire-resistance-rated enclosure requirements do not apply to it (§ 713.6). The exception is narrow: exterior walls that are separately required to be rated for exterior egress balconies, interior exit stairways and ramps, or exterior exit stairways and ramps keep those requirements.

Closing the top and the bottom

A shaft that stops short of the bottom of the building cannot simply end in open space. It has to do one of three things (§ 713.11):

1. Be closed off at the lowest level with construction rated the same as the lowest floor it passes through, and never less than the shaft's own rating.
2. Terminate in a room whose use relates to the shaft's purpose, separated from the rest of the building by fire barriers or horizontal assemblies with at least the shaft's protection.
3. Be protected by approved fire dampers installed per their listing at the lowest floor level inside the enclosure.

Exceptions relax the room separation where the only openings into the building occur at the bottom and the shaft is closed off around penetrating items or sprinklered, and where the shaft contains no combustibles and no openings into the building interior at all (§ 713.11).

The top has a parallel set of choices (§ 713.12): extend to the underside of the roof sheathing, deck or slab; terminate below the roof and be enclosed at the top with construction rated the same as the topmost floor penetrated, never less than the shaft rating; or extend past the roof assembly and comply with the rooftop-structure rules.

Trash, recycling and laundry chutes

Chutes are the strictest version of the same idea. A shaft containing a waste, recycling or linen chute may not be used for any other purpose and is enclosed to the same rating rule above (§ 713.13.1). Waste and recycling chutes may share one enclosure.

The details that trip up remodels of existing buildings:

• Openings into a chute may not be located in corridors. Access openings sit in rooms or compartments enclosed by 1-hour fire barriers, with doors rated at three-quarters of an hour that are self-closing or automatic-closing on smoke detection (§ 713.13.3).
• Doors into the chute itself must be self-closing (§ 713.13.1).
• The chute must discharge into an enclosed room separated by fire barriers rated at least as high as the shaft enclosure (§ 713.13.4).
• A chute that serves and is contained within a single dwelling unit is excepted from these chute provisions entirely (§ 713.13).

The same number shows up elsewhere

When a fire barrier separates building areas from a shaft, its rating is set by the shaft rule, not by a separate table — and the same is true for enclosures around exit access stairways (§ 707.3.1, § 707.3.3). If a reviewer cites the fire-barrier section at you, it is pointing back to the two-hour and one-hour rule above.

What this does not tell you

These sections set the rating of an enclosure. They do not decide which vertical openings must be enclosed in the first place, what rating the doors and dampers in the shaft need, or how the high-rise reduction works — those live in other sections not covered here. Bring the story count, the floor assembly rating and the chute's route to your plan reviewer early; the difference between one hour and two changes wall assemblies, not just paperwork.

The codes cited are the 2025 editions, in effect since January 1, 2026. More explainers are on our blog.

Sources

• 2025 California Building Code § 713.4 — Shaft enclosure fire-resistance rating
• 2025 California Building Code § 713.6 — Exterior walls as part of a shaft
• 2025 California Building Code § 713.11 — Enclosure at the bottom
• 2025 California Building Code § 713.12 — Enclosure at top
• 2025 California Building Code § 713.13 — Waste, recycling and linen chutes
• 2025 California Building Code § 707.3 — Fire barrier fire-resistance ratings