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Building Code Reference

Got a CMC 504.4 correction?

Dryer exhaust duct

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CMC 504.4 is the California Mechanical Code section that governs how clothes dryer exhaust ducts must be built and routed. The duct must be rigid metal (no flexible plastic or foil), at least 4 inches in diameter, with a smooth interior and a maximum total run of 14 feet from the dryer connection to the exterior termination. That 14 feet already includes two 90-degree elbows, and each 90-degree elbow beyond two takes 2 feet off it, and screws are prohibited inside duct joints because they catch lint and create a fire hazard. Residential and multi-unit laundry rooms both hit this one.

How to fix it

  1. Add a dryer exhaust duct route to the mechanical plan sheet

    Draw a single-line diagram showing the complete path from the dryer connection to the exterior termination cap. Label the duct as 4-inch diameter rigid metal. Note the material spec (galvanized steel or aluminum, smooth interior). If you are submitting a set without a dedicated mechanical sheet, add a notation on the floor plan or laundry room detail. The duct route must be explicitly shown, not implied from the appliance location.

  2. Calculate the allowed duct length and show your math on the plans

    Add up the combined horizontal and vertical length of the duct and count the 90-degree elbows. The limit is 14 feet, and that already includes two 90-degree elbows. Take 2 feet off the 14 feet for each 90-degree elbow beyond two (CMC 504.4.2.1). If the run is still too long, shorten it or remove elbows. A longer run is allowed only where the dryer manufacturer's installation instructions permit it and the building department approves, or with a listed dryer exhaust duct power ventilator installed per its instructions (CMC 504.4.2.3). Show the calculation in a notes block on the plan sheet so the checker does not have to do it themselves.

  3. Specify rigid metal duct throughout the concealed run

    The concealed portion of the exhaust duct (inside walls, floors, or ceilings) must be rigid metal. Acceptable materials are galvanized steel or aluminum with a smooth interior surface. Corrugated metal flex, flexible foil, and plastic ducts are not allowed in concealed spaces. A listed transition duct up to 6 feet long may connect the dryer to the rigid duct, but it must not be concealed within construction (CMC 504.4.2.2). Call out the duct material on the plan sheet.

  4. Remove all screws from duct joint details

    Duct joints in a dryer exhaust run must not be fastened with sheet metal screws or similar fasteners that protrude into the duct interior. Screws catch lint, restrict airflow, and create fire hazards over time. If your plan details show screw connections, revise them to show approved slip joints with any sheet metal screws on the exterior only, or use foil tape rated for high-temperature dryer exhaust applications. Plan checkers look for screw symbols or notes in duct details, so make the detail explicit about fastener placement.

  5. Show the exterior termination cap location and clearances

    The dryer exhaust must terminate at an exterior wall cap with a backdraft damper. Mark the cap location on the exterior elevation and note the 3-foot minimum clearance from openings into the building (windows, doors) and from the property line. The cap must not have a screen, so a screened louvered vent will not pass. Some cities require notation of the cap model number or listing. If the duct exits through the roof, it needs a listed roof cap, but terminating through a sidewall is preferred and easier to maintain.

  6. Address any fire-rated assembly penetrations

    Where the rigid metal duct passes through a fire-rated wall or floor assembly, show a listed fire-stop collar or intumescent wrap at the penetration. Mark these penetrations on the plan and show the clearance or collar detail. This is a common secondary correction that comes back with the dryer duct response, so address it in the initial resubmittal rather than waiting for another round.

  7. Revision-cloud every sheet changed for CMC 504.4

    When you respond to the correction letter, draw a revision cloud with a delta triangle around every sheet where you made changes. Checkers in Orange County and Anaheim review corrections against the prior submission, not a fresh set. If they cannot quickly identify what changed, they may re-flag the same issue. A clean revision cloud on each affected sheet speeds turnaround and reduces the chance of a third correction on the same section.

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Why you got flagged

Plan-checkers flag CMC 504.4 for a handful of specific reasons, and most corrections letters cite more than one at the same time.

The most common issue is a missing or incomplete duct route on the plans. If the mechanical plan sheet does not show a single-line diagram of the dryer exhaust path from the appliance to the exterior termination point, the checker will not approve it. Showing only the dryer location is not enough. The route, the duct diameter, and the duct material all need to be called out. First-time submissions skip this because the architect or designer focuses on the equipment schedule and forgets the duct routing diagram entirely.

The second common trigger is exceeding the 14-foot maximum. Many laundry rooms in tract homes and ADUs are located away from an exterior wall, and the duct has to travel through a wall cavity or ceiling before it terminates outside. Once you add a 90-degree turn at the appliance connection, another at the wall penetration, and account for the horizontal run, you are often at 12 to 16 feet. The 14 feet already includes two 90-degree elbows, and each 90-degree elbow beyond two takes 2 feet off the limit, so a run with three elbows is capped at 12 feet and a run with four at 10 feet (CMC 504.4.2.1). Checkers measure the combined horizontal and vertical length from your plans and count the elbows, not the straight-line distance.

A third failure pattern shows up in renovation permits where the existing duct material is flexible foil or corrugated plastic. CMC 504.4 requires rigid metal for the concealed portion of the run. If your plans show or imply a flex duct connection inside the wall cavity, expect a correction. The flexible transition section from the dryer itself may be short and allowable, but the moment it enters the wall, it must be rigid.

The official code

2025 California Mechanical Code Part 4 — Section 504.4 Clothes Dryers

Read the source →

Common questions

What is the maximum dryer vent length allowed by the California Mechanical Code?
CMC 504.4.2.1 limits the dryer exhaust duct to 14 feet of combined horizontal and vertical length, and that 14 feet already includes two 90-degree elbows. Each 90-degree elbow beyond two takes 2 feet off the limit, so a run with three 90-degree elbows can be at most 12 feet and a run with four at most 10 feet. A longer run is allowed where the dryer manufacturer's installation instructions permit it and the building department approves, or with a listed dryer exhaust duct power ventilator installed per its instructions. Show the length and the elbow count on the plans so the checker can verify it without guessing.
Can I use flexible foil duct for a dryer exhaust in California?
Not in concealed spaces. CMC 504.4 requires rigid metal duct for any portion of the exhaust run inside a wall, ceiling, or floor. Flexible foil duct and corrugated metal flex are not acceptable in the concealed run because they sag, accumulate lint, and are difficult to clean. A listed transition duct up to 6 feet long is allowed between the dryer and the rigid duct, but it cannot be concealed within a wall, floor or ceiling (CMC 504.4.2.2). All the concealed footage must be rigid smooth-interior metal, either galvanized steel or aluminum.
Why does my dryer duct correction say no screws in the joints?
CMC 504.4 prohibits screws or fasteners that protrude into the interior of the duct. Lint is highly flammable, and screw tips catch lint over time. A lint buildup at a screw head restricts airflow and creates a fire risk inside the wall. Duct sections should be slip-jointed and sealed with foil tape or mastic rated for dryer exhaust temperatures. If your plans showed sheet metal screws penetrating into the duct interior, revise the detail to show exterior-only fasteners or tape-sealed slip joints. The prohibition applies to the entire concealed run, not just certain sections.
Does CMC 504.4 apply to ADU laundry rooms?
Yes. CMC 504.4 applies to any residential dryer installation in California, including ADUs, garage conversions, and standalone accessory structures. ADU permits are among the most common to get flagged on this section because the laundry is often tucked into a closet or utility alcove far from an exterior wall. The duct has to travel further to reach outside, and the 14-foot length limit does not expand for ADUs. Plan the laundry location and duct route early in the design so you are not redesigning the mechanical layout after the first corrections letter.
What type of exterior cap does a dryer exhaust duct need?
The duct must terminate outside the building with a backdraft damper, and no screen is allowed at the termination, so a screened vent cover will not pass. The damper prevents cold air and pests from entering when the dryer is not running. The cap must be installed on an exterior wall with at least 3 feet of clearance from openings into the building and from the property line, and not over a public way. Sidewall termination is preferred over roof termination because it is easier to inspect and clean, and most plan checkers will question a roof cap if a wall location is feasible.
Can a dryer exhaust duct be combined with a bathroom fan or other exhaust?
No. CMC 504.2 requires clothes dryer exhaust to be independent of all other exhaust systems. It cannot be combined with or connected to a kitchen exhaust hood, bathroom fan, or any other building exhaust system. The only exception is a common exhaust for multiple dryers where the dryer manufacturer permits it, built to CMC 504.4.4 (rigid metal, rated enclosure, continuous fan). If your plans connect the dryer into another exhaust system, you will get a correction regardless of duct length. This is one of the fastest ways to get a second corrections letter on the same project.

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