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Building Code Reference

Got a CMC 504.4.2 correction?

Dryer vent max length

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 504.4.2 limits a domestic 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 (CMC 504.4.2.1). 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 (CMC 504.4.2.3). Single-family and ADU permits are the most common types flagged under 504.4.2 when the laundry location is deep inside the floor plan.

How to fix it

  1. Locate all fittings in the proposed duct path

    Walk or sketch the full exhaust route from the dryer connection collar to the exterior wall cap. Count every 90-degree elbow: the 14-foot maximum already includes two of them, and each one beyond two takes 2 feet off the limit (CMC 504.4.2.1). Don't forget the elbow at the dryer transition connector and any elbow at the wall penetration — those are the two most commonly missed fittings.

  2. Calculate the allowed duct length

    Add up the combined horizontal and vertical duct length in feet. Then work out the allowed length: Allowed Length = 14 ft − (2 ft × number of 90° elbows beyond two). For example, three 90-degree elbows allow 12 feet and four allow 10 feet. If the run is longer than the allowed length, you have three options: shorten the physical run, eliminate or reposition elbows, or use one of the code's two routes to a longer run (step 5).

  3. Show the calculation on the mechanical plan sheet

    The duct length calculation must appear on the mechanical sheet, not just in a general note or spec section. A simple table works: 'Duct run: 11 ft. 90° elbows: 3 (one beyond two, minus 2 ft). Allowed: 12 ft. Complies.' Plan-checkers need to verify compliance at a glance — buried calculations get flagged as incomplete.

  4. Reroute if the run exceeds the allowed length

    The most reliable fix is shortening the physical path. Move the exterior termination closer to the dryer, eliminate an elbow, or relocate the laundry closet if you're still in design. Each 90-degree elbow you remove beyond the first two gives back 2 feet of allowed length. That's often enough to bring a borderline run into compliance without changing the floor plan.

  5. Use the manufacturer's instructions or a power ventilator if rerouting isn't feasible

    The code allows a longer run in two ways. First, where the dryer manufacturer's installation instructions permit a longer duct and the building department approves it (CMC 504.4.2.1): specify the dryer model on the plans and in the schedule, reference its installation instructions on the mechanical sheet, and note the maximum duct length they allow. Second, a listed dryer exhaust duct power ventilator installed per its instructions (CMC 504.4.2.3): note the ventilator model and listing on the plans. The plan-checker will verify either one.

  6. Use smooth-wall rigid metal duct for the full run

    CMC 504.4 (the parent section) requires dryer exhaust ducts to be smooth-wall, rigid metal — not flexible duct, not plastic, not vinyl. If any flexible duct appears in the path beyond the short transition connection at the dryer, that's a second correction on top of 504.4.2. Specify 4-inch diameter rigid metal duct, at least 0.016 inch thick, with a smooth interior throughout. The connection from the dryer to the rigid duct may be a listed transition duct up to 6 feet long, but it must not be concealed within construction (CMC 504.4.2.2).

  7. Call out the exterior wall cap or roof cap on the plans

    The termination fitting must be identified on the plans. Show the wall cap location on the floor plan or exterior elevation with a note confirming it meets CMC 504.4 requirements: a backdraft damper and no screen on the termination (screens trap lint and are prohibited). Missing this detail is a common oversight on remodels where the termination location changes.

  8. Annotate your resubmittal with a response letter

    When you resubmit, draw a revision cloud around the mechanical sheet showing the updated duct path, the duct length calculation table, and the dryer or power ventilator specification if you rely on one for a longer run. Write a response letter that directly addresses the 504.4.2 correction: state the duct length and elbow count, how it was brought into compliance, and which sheet shows the supporting calculation. This cuts review time significantly.

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Why you got flagged

Plan-checkers flag 504.4.2 most often when the mechanical plans show the dryer location and exterior termination point but no duct length calculation — or when the calculation is there but doesn't count the elbows. A run from laundry closet to exterior wall can look fine on paper, but once you count the elbows at the appliance connection, at a ceiling or chase, and at the wall penetration, the allowed length drops. With four 90-degree elbows the limit is 10 feet, not 14, so a 12-foot run with four elbows is 2 feet over and the plans get sent back.

The second common scenario is a stacked washer/dryer unit in an interior bathroom or hallway. The contractor routed the exhaust straight up through the floor above and then horizontally to an exterior wall, thinking the total physical distance was under 14 feet. The problem is that the limit is the combined horizontal and vertical length, and every 90-degree elbow beyond the first two takes 2 feet off it. A route up through the floor and over to a side wall can easily need three or four elbows, which drops the limit to 12 or 10 feet.

Interior laundry rooms on second floors create a third trigger. The duct must travel down through a soffit or chase, turn to exit through the side wall, and pick up a wall cap fitting. Even a tightly routed path often runs past the limit once the elbows are counted. Plan-checkers in Orange County cities are consistent about requiring the calculated duct length to appear directly on the mechanical sheet, not buried in a general note.

A fourth scenario comes up on remodel permits where the laundry was relocated. The new location may be further from an exterior wall than the original. The previous permit may have passed with a compliant run; the remodel plan doesn't re-examine the new duct path and the correction letter cites 504.4.2 on the resubmittal.

The official code

California Mechanical Code 2025, Part 4 — Dryer Exhaust Duct Systems (Section 504.4)

Read the source →

Common questions

What counts toward the 14-foot maximum under CMC 504.4.2?
The 14-foot cap is the combined horizontal and vertical length of the duct, and it already includes two 90-degree elbows. Each 90-degree elbow beyond two takes 2 feet off the cap, so three elbows allow 12 feet and four allow 10 feet (CMC 504.4.2.1). The CMC has no separate deduction for 45-degree elbows. A listed transition duct between the dryer and the rigid duct can be up to 6 feet long and must not be concealed. If you're close to the limit, map every elbow and calculate the total before submitting.
Can a dryer duct run be longer than 14 feet?
Yes, in two ways. CMC 504.4.2.1 allows a longer run where the dryer manufacturer's installation instructions permit it and the building department approves it. CMC 504.4.2.3 allows a longer run with a listed dryer exhaust duct power ventilator installed per its instructions. Either way, name the dryer or ventilator model on the mechanical plans and reference its instructions. The plan-checker will check them, so keep that documentation ready.
Does CMC 504.4.2 apply to ADUs and garage conversions?
Yes, it applies to any new or replaced dryer exhaust installation in a dwelling unit, including ADUs and garage conversions. ADU laundry setups are a common trigger because the laundry area is often tucked into an interior corner of the floor plan, far from an exterior wall. The duct run in those layouts regularly exceeds the limit once the elbows are counted. Plan for the duct path early in design rather than discovering the problem at plan check.
What duct material does CMC 504.4.2 require?
Section 504.4.2 covers duct length, but the parent section CMC 504.4 requires dryer exhaust ducts to be smooth-wall, rigid metal — galvanized steel or equivalent. Flexible plastic or foil duct is not permitted for the main exhaust run. Only the transition duct at the dryer connection may be flexible: it must be listed, no longer than 6 feet, and not concealed within construction (CMC 504.4.2.2). Plan-checkers often cite both the length violation and a material violation on the same corrections letter.
How do I show the duct length calculation on my plans?
Put a simple table directly on the mechanical sheet — not in the general notes or spec section. The table should show the combined horizontal and vertical duct length in feet, the number of 90-degree elbows, the allowed length (14 feet minus 2 feet for each 90-degree elbow beyond two), and whether the run complies. If you're relying on the manufacturer's instructions or a power ventilator for a longer run, add the model number and the length it allows. Plan-checkers need to verify compliance at the sheet — anything buried gets flagged.
Can I use 90-degree elbows at all in a dryer duct run?
Yes. The 14-foot limit already includes two 90-degree elbows, so the first two cost nothing extra. Each one beyond two takes 2 feet off the limit, so three elbows allow 12 feet and four allow 10 feet. Plans fail 504.4.2 when a long route needs several elbows and the run ends up longer than the reduced limit. Keep the route as straight as you can and show every elbow on the plans.
Is a roof termination allowed or does the duct have to exit through a wall?
Roof terminations are allowed under CMC 504.4, but they usually make the duct longer and add complexity. If you use a roof cap, it must still have a backdraft damper and must not include a screen. The longer vertical run to reach the roof, combined with the elbows needed to get there, often pushes past the limit unless the dryer manufacturer's instructions allow a longer run or a listed power ventilator is used.

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