A "subslab soil exhaust system" is a dedicated vent pipe/duct that pulls soil gases from beneath a slab (from a subslab aggregate layer, collection mat, or suction point) and discharges them above the roof so those gases don't build up under or inside the building. In practice, you'll see it on radon mitigation systems and other vapor-intrusion / soil-gas venting designs.
Plain English:
511.1 general: If you provide a subslab soil exhaust system, the duct must comply with IMC 511.
511.2 materials: Use UL 181 listed Class 0 air-duct material OR IPC-approved sanitary DWV/vent materials (cast iron, galvanized steel, copper/copper-alloy Type DWV or heavier, or plastic piping).
511.3 slope: Do not trap the duct. Provide minimum slope 1/8 in per 12 in (1%) so it can drain and not hold condensate.
511.4 termination: Extend through the roof. Terminate at least 6 in above the roof AND at least 10 ft from operable openings or outside air intakes.
511.5 identification: Permanently label the duct at each floor level (tag, stencil, or approved marking).
On Plans: Why it matters
Common misses: duct run gets "accidentally trapped," roof termination is too close to an OA intake or operable window, and no labeling-so the system gets cut, capped, or tied into the wrong thing during future work.
Code Path: Where to show it
M-001: "IMC 511 SUBSLAB SOIL EXHAUST" general note (materials, 1% slope/no traps, roof termination clearances, labeling each floor).
Plans/sections: Show routing with slope arrows and call out "NO TRAPS."
Roof plan: Dimension the termination to nearby operable openings and OA intakes.
Check: Do
Coordinate the roof termination early to stay 10 ft clear of intakes/openings.
Add a simple "IMC 511 TAG REQUIRED EACH FLOOR" callout near the riser.
Review Risk: Don't
Don't add low points or offsets that create a trap.
Don't terminate near intakes/openings and assume it will "pass later."
Field Tip:
Call it out clearly as "SUBSLAB SOIL EXHAUST (SSES)" on the riser and roof plan. If it's labeled like plumbing vent or generic exhaust, it's more likely to be misrouted or capped during a remodel.
Masterbuild QA Lens
A subslab soil exhaust system is judged on three field-verifiable details: no traps in the duct run (minimum 1/8 in per 12 in slope so it drains), roof termination at least 6 in above the roof and at least 10 ft from operable openings or outdoor air intakes, and a permanent label at each floor level identifying the duct. Confirm the duct material is UL 181 listed Class 0 air duct or IPC-approved sanitary DWV/vent material - not an arbitrary substitution - and that the roof plan actually dimensions the 10 ft clearance rather than just noting it.
An unlabeled subslab exhaust riser is the most common way this system gets capped or misrouted during a later remodel.
Drawing / Submittal Check
For subslab soil exhaust (IMC 511): the drawings must show the sub-slab depressurization point locations, the collection manifold routing, the exhaust fan location, and the discharge location. The discharge must be at least 6 inches above grade and away from occupied entries, windows, and air intakes. Confirm the duct material is appropriate for soil contact (PVC or listed material). Show the monitoring port location if required.
Common Review Risk
Subslab soil exhaust systems are often added to a project after permit issuance - when radon testing reveals a problem during construction or occupancy. Adding a subslab system after the slab is poured limits the options for pipe routing. If the project is in a high-radon area, design the subslab system into the permit set from the beginning. A passive subslab system (no fan, just piping with a future fan provision) is inexpensive to install before the slab and expensive to add after.
When To Escalate
Escalate for subslab exhaust in an existing occupied building where the sub-slab conditions are unknown - where the sub-slab gravel layer, the vapor barrier condition, and the soil permeability have not been confirmed. Installing a subslab exhaust system in an existing building without characterizing the sub-slab conditions produces a system that may not perform as designed.
Special Exhaust Coordination
For specialty exhaust, start with the contaminant and source. Then confirm capture method, duct material, routing, discharge, separation, controls, and whether another consultant or AHJ review is required.
Duct System Coordination
Subslab soil exhaust systems use dedicated PVC or equivalent duct embedded below the slab. Show duct routing, pitch direction, aggregate layer depth, and termination point above the roofline on the drawings.