Daily Code Talk #56: IMC 505 Part 2 (Makeup Air + Common Multistory Kitchen Exhaust 505.4–505.5)

🧠 Plain-English Highlights• 505.4 makeup air required: If a domestic kitchen hood system can exhaust more than 400 cfm, you must provide makeup air at approximately the same rate.– Makeup air must have a means of closure (damper)– It must automatically start and run whenever the hood exhaust runs (interlocked control)• 505.5 common multistory domestic kitchen exhaust: If multiple dwelling kitchen hoods connect to a shared vertical riser/shaft, the “common system” has strict rules:– The duct must be in an IBC-rated shaft.– Dampers are prohibited in the exhaust duct (except where 505.3 allows). Shaft/duct penetrations must be protected per IMC 607.5.5 Exception 2.– Use rigid steel duct inside the shaft: min 26-gauge and built per SMACNA.– No offsets in the shaft ductwork (keep it straight).– Fan/motor safety: motor per IMC 503.2 and located outside the airstream.– Fan runs continuously and must be on standby power.– Fan status must be monitored and alarm/indication provided if it’s not operating.– If any individual kitchen exceeds 400 cfm, it still needs makeup air per 505.4.– Provide a cleanout at the base of the shaft: min 12 in x 12 in.– No screens at the termination.– The shaft serves only kitchen exhaust and must be independent of other exhaust.🏗️ Why it mattersThe 400 cfm trigger is where “simple residential hood” becomes a pressure/control problem. And once you go to a common riser, it’s no longer “duct routing” — it’s a life-safety shaft system with standby power, monitoring, cleanouts, and strict construction rules.🗺️ Where to show it• M-001: “IMC 505.4 MAKEUP AIR” note: HOODS >400 CFM REQUIRE MUA APPROX. EQUAL TO EXHAUST, WITH AUTOMATIC START/STOP INTERLOCK AND MEANS OF CLOSURE.• M-101: Identify any kitchens on a common riser and label “COMMON DOMESTIC KITCHEN EXHAUST RISER PER IMC 505.5 (RATED SHAFT, NO OFFSETS).”• M-601: Riser/fan schedule fields for CONTINUOUS OPERATION, STANDBY POWER, FAN STATUS MONITORING, MOTOR OUT OF AIRSTREAM, CLEANOUT PROVIDED.✅ Do• Check hood submittals early for “capable of >400 cfm” and coordinate makeup air before ceiling layouts lock.• If using a common riser, coordinate shaft rating, cleanout access, standby power, and monitoring with Arch + Elec from day one.⛔ Don’t• Don’t treat a common kitchen riser like generic exhaust.• Don’t provide makeup air without an automatic interlock to the hood exhaust.🔧 Field tipOn multifamily jobs, add a small “HOOD >400 CFM?” checkbox next to each kitchen tag. If YES, add “MUA INTERLOCKED” right on the plan. It prevents the classic late surprise where the hood cut sheet triggers 505.4 after ceilings are already coordinated.Comment “IMC505” if you want a paste-ready M-001 note for 505.4 and a common riser callout for 505.5.