A significant share of the deficiencies we photograph in multifamily properties are not lint — they are duct materials that were never code-compliant, or transitions installed in ways the code specifically prohibits. Here is the practical guide.
Concealed Duct — Rigid Metal Only
Dryer exhaust duct running through concealed spaces — inside walls, floors, ceilings, chases — must be rigid metal, smooth-walled, with joints running in the direction of airflow and no screws protruding into the airstream. Foil flex duct, vinyl duct, and semi-rigid flex have no place in a concealed run in any jurisdiction we work in.
Why it matters beyond the code: the corrugated interior of flex duct is a lint collector. Every ridge is a place fibre catches. A flex-duct run blocks in a fraction of the time a smooth metal run takes, and cannot be cleaned as effectively because the brush cannot reach the valleys.
Screws Into the Airstream
Duct joints fastened with sheet metal screws that protrude into the duct interior create lint snags at every joint. Foil tape or clamps outside the duct are the correct method. We photograph and log protruding screws as deficiencies because they guarantee accelerated re-blockage.
The Transition Duct — Different Rules
The transition duct is the flexible section between the dryer outlet and the concealed duct. Codes generally permit listed flexible transition duct here, but with strict limits: it must be listed for the purpose, it must not be concealed, and its length is capped (commonly eight feet, with reductions for bends). It must not be kinked, crushed or coiled behind the appliance.
In practice, the crushed and over-length transition behind a pushed-back dryer is one of the most common deficiencies in multifamily. It is cheap to fix and materially affects airflow.
Termination Caps — No Screens
Dryer exhaust terminations must not use screened or meshed caps. A screen at the termination catches lint on the inside, builds a mat, and blocks the vent. This is a common and entirely counterproductive retrofit — usually installed to keep birds out, which a functioning backdraft damper does properly. Purpose-made dryer vent caps with a free-swinging damper flap are the correct specification.
Length and Bends
Codes limit total developed duct length, with each elbow counting as an equivalent additional length. Long runs with multiple bends move less air, accumulate lint faster, and in some configurations require a listed booster fan. Where we measure runs materially exceeding permitted developed length, it is documented — because the vent will keep blocking regardless of how often it is cleaned.
Why This Belongs in the Deficiency Report
Cleaning a non-compliant duct restores airflow temporarily. It does not fix the reason the duct blocked in twelve months instead of thirty-six. A property that cleans annually and never addresses the foil flex duct in its concealed runs is paying for the same problem repeatedly.
Our reports photograph and locate every material deficiency we can access. Get a quote.