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Building Code Reference

Got a CMC 504.3 correction?

Kitchen exhaust hood

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 504.3 is the California Mechanical Code section that requires residential kitchen range hoods to exhaust cooking vapors directly to the outdoors rather than recirculate air back into the kitchen. The hood must deliver at least 100 CFM of airflow, use smooth-wall metal ductwork, and terminate outside the building with a backdraft damper. Single-family homes, ADUs, and any residential addition that includes a new cooking appliance will hit this section during plan check.

How to fix it

  1. Specify a ducted hood on the equipment schedule

    Replace any reference to a recirculating, ductless, or charcoal-filter hood with a ducted model. On the equipment schedule or kitchen elevation note, add the make, model, and rated CFM. The CMC minimum is 100 CFM, and for new dwellings the 2025 Energy Code (Table 150.0-G) requires 110 to 280 CFM or a capture-efficiency rating, by floor area and range fuel; most residential hoods run 300 to 600 CFM, which is fine as long as the duct is sized to match the manufacturer's published requirements. Note the duct diameter required by the manufacturer, typically 6 or 7 inches for residential hoods.

  2. Show the full duct path on a plan or section

    Draw the exhaust duct route on the floor plan (dashed line above the range) and on a building section if the path goes through an attic or concealed cavity. The plan-checker needs to trace the duct from the hood collar to the exterior termination. Label the duct material and size at each change in direction. Typical routes: up through the cabinet, into the attic, out a roof cap; or up and back through the exterior wall, out a wall cap. Show both on the same sheet if the route requires it.

  3. Call out smooth-wall metal duct material

    CMC 504.3 requires the exhaust duct to be smooth-wall metal. Acceptable materials include galvanized steel and stainless steel with smooth interior surfaces. Flexible aluminum duct, semi-rigid aluminum flex, and PVC pipe are all non-compliant for range hood exhaust. Add a note to the plans: 'Kitchen exhaust duct: smooth-wall galvanized steel, [diameter] inch, per CMC 504.3.' If a transition is needed at the hood collar, the transition piece must also be smooth metal.

  4. Show the exterior termination with a backdraft damper

    Detail the termination at the exterior. A wall cap or roof cap with a built-in backdraft damper is required. Most listed wall and roof caps include the damper; confirm it is listed and note it on the plans. The cap must terminate outdoors, not into an attic, crawlspace, garage, or any interior space. Minimum clearances: 3 feet from openings into the building and 3 feet from a property line. Show the termination location on the exterior elevation.

  5. Verify duct length against the manufacturer's published limit

    The duct run length from hood collar to termination affects performance. Most residential hood manufacturers publish a maximum equivalent duct length, typically 30 to 50 feet equivalent, accounting for elbows (each 90-degree elbow typically counts as 10 feet equivalent). If your run exceeds the manufacturer's limit, you have two options: upgrade to a higher-CFM blower, or shorten the run. Note the calculated equivalent length on the plans and confirm it is within the manufacturer's specification.

  6. Check makeup air for high-CFM hoods

    Neither the 2025 CMC nor the 2025 Energy Code sets a 400 CFM makeup-air trigger for residential hoods. CMC 701.3 requires makeup air where exhaust fans or kitchen ventilation interfere with appliance operation, and the Energy Code points to CMC Chapter 7. If your hood is large and the home has gas appliances, add a note addressing makeup air and check local amendments. Typical approaches include a passive makeup air duct (a gravity damper-equipped duct from the exterior) or an interlocked supply fan.

  7. Resubmit with a response letter noting each change

    When you resubmit, include a correction response letter that cites the CMC 504.3 correction item by number and describes what you changed on which sheet. Example: 'Item 12 (CMC 504.3): Revised A4.1 to show ducted range hood model [X], 350 CFM; added duct route on A2.1 floor plan and A5.1 building section; added wall cap with backdraft damper on west elevation A3.2; duct material noted as smooth-wall galvanized steel 6-inch.' This reduces back-and-forth because the checker can find every change immediately.

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Why you got flagged

Plan-checkers flag CMC 504.3 for a few recurring reasons, and the corrections letter usually lands before you notice the issue.

The most common trigger is showing a recirculating hood on the plans. Recirculating hoods, the kind with a charcoal filter that blows air back into the room, are not compliant under CMC 504.3 for new construction or permitted remodels. If the kitchen elevation or equipment schedule lists a ductless or recirculating model, the plans will come back with a correction. The fix is specifying a ducted hood and showing the exhaust path all the way to the exterior.

A second common flag is missing duct routing on the plans. The plan-checker needs to see where the duct goes: through the cabinet above the range, up through a soffit or wall cavity, across the attic, and out a roof cap or wall termination. If the floor plan and sections don't show that path, the correction cites CMC 504.3. This is especially common on kitchen remodels where the designer shows the hood on the elevation but forgets to coordinate the duct route on the reflected ceiling plan or building section.

The third scenario is incorrect duct material. CMC 504.3 requires smooth-wall metal duct, not flexible duct, not PVC. Plans that call out semi-rigid or fully flexible duct for the hood exhaust will get flagged. Even if flexible duct is specified only for the last few inches of the connection, the correction will cite this section and ask for smooth-wall metal the full run.

The official code

California Mechanical Code 2025 — Chapter 5 Exhaust Systems, California Building Standards Commission

Read the source →

Common questions

Does CMC 504.3 require a range hood on every kitchen remodel?
Not automatically, but once you pull a permit for a kitchen remodel that includes new or relocated cooking appliances, plan check will verify the exhaust hood meets CMC 504.3. If you already have a compliant ducted hood in place and you're not touching the cooking appliance or exhaust system, the checker may not flag it. Once you're adding a new range or changing the hood, you need to show a compliant ducted exhaust system on the plans.
Can I use an over-the-range microwave instead of a range hood?
An over-the-range microwave with a built-in exhaust fan can satisfy CMC 504.3 only if it's the ducted version that exhausts to the outside. Many over-the-range microwaves come in both recirculating and ducted configurations. If you specify the ducted model, show the duct routing to the exterior on the plans, and meet the 100 CFM minimum, it qualifies. If you specify the recirculating model, it does not comply regardless of CFM rating.
What duct size does CMC 504.3 require for a residential range hood?
CMC 504.3 defers duct sizing to the hood manufacturer's published specifications. There is no single code-mandated diameter, but most residential hoods require a 6-inch round duct minimum; higher-CFM hoods often require 7 or 8 inches. Use the manufacturer's installation manual to determine the correct size for the specific model you specify. Note that undersizing the duct from the manufacturer's spec voids the equipment listing and creates a compliance problem separate from CMC 504.3.
Does flexible duct work for range hood exhaust?
No. CMC 504.3 requires smooth-wall metal duct for range hood exhaust. Flexible aluminum duct, semi-rigid flex, and plastic duct are all non-compliant. The reason is grease accumulation: corrugated or ribbed interior surfaces trap grease, creating a fire hazard over time. Smooth galvanized steel is the standard choice; stainless steel is acceptable and required when the hood is rated for high-heat commercial-style cooking.
Where does the exhaust duct have to terminate?
The duct must terminate outdoors through a listed wall cap or roof cap equipped with a backdraft damper. It cannot discharge into an attic, soffit, garage, crawlspace, or any concealed interior space. Most plan checkers also want to see that the cap is at least 3 feet from any operable window, door, or air intake. Show the termination location on the exterior elevation in your plans.
Does CMC 504.3 apply to ADUs?
Yes. ADUs are residential buildings subject to the California Mechanical Code, including CMC 504.3. A new ADU with a kitchen needs a ducted range hood exhausting to the exterior. This is a common correction on ADU permits because the kitchen is sometimes an afterthought in the design, and the exhaust path through an existing structure's wall or roof needs careful coordination. Show the full duct route on the ADU floor plan and section.
What is the minimum CFM for a residential kitchen hood under CMC 504.3?
100 CFM is the minimum. In practice, most residential ducted hoods are rated well above this, commonly 300 to 600 CFM for standard residential cooking. The code minimum is 100 CFM, so any listed ducted hood on the market will meet it. The bigger concern is not being below the minimum but making sure makeup air is addressed if the hood exceeds 400 CFM, as that triggers a separate California Energy Code requirement.

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