Radon Vent Stack Sizing for Multifamily Buildings

Radon vent stack sizing for a multifamily building is decided by airflow, not by unit count or storey count. The multifamily mitigation standard makes the cross-sectional area of a 3-inch pipe the default floor, steps the system up to 4-inch once analysis shows it has to move more than 80 cubic feet per minute, and allows 2-inch only where analysis verifies the whole system needs 40 cfm or less. Every other sizing question, how many stacks, where they run, which fan goes on top, follows from those thresholds and the geometry of your slab.

The mistake that shows up in plan review is sizing a radon stack the way you would size a plumbing vent, off a fixture-unit style table keyed to building size. Radon stacks are sized off measured or modelled soil gas flow, because the thing the pipe has to carry is a volume of air pulled out from under a slab, and that volume depends on sub-slab material, slab area, and how well the barrier and the perimeter are sealed. Two buildings with identical unit counts can land on different pipe diameters.

Sizing is a two-input problem. The first input is the airflow the system has to move to hold a pressure field across the whole slab. The second is the resistance the pipe run adds, which climbs with length, with every elbow, and sharply as diameter drops.

Know which document you are sizing to. For existing multifamily buildings the current standard is ANSI/AARST SGM-MFLB-2023, which per the EPA consolidates the earlier ANSI/AARST RMS-MF and RMS-LB mitigation standards into a single publication covering existing multifamily, school, commercial, and mixed-use buildings. The sizing clauses below are quoted from the RMS-MF text that consolidation absorbed, and they are explicit: active soil depressurization duct piping from the exhaust point to the soil gas collection plenums shall be equivalent to or greater than the cross-sectional area of a 3-inch (75 mm) ID pipe, or as determined by pressure field extension analysis. The 3-inch area is what you owe absent analysis, and the analysis can move you in either direction, so the diameter on your riser diagram is a conclusion you have to earn rather than a number you inherit.

New construction sits under a different document. ANSI/AARST CC-1000-2018 gives prescriptive minimum requirements for the construction of any building intended for human occupancy, except for 1 and 2 family dwellings, which is what puts apartments, condominiums, congregate housing, schools, and commercial occupancies in scope. Both documents sit in the consensus set EPA recognizes as the radon standards of practice, so a reviewer expects the design to name the one that governs your project and size to it. Get that wrong and the sizing argument never gets read, because the standard it is argued against is the wrong one.

The standard resolves diameter into three bands keyed to the airflow the entire system needs, not the airflow at any one riser. Where analysis indicates more than 80 cubic feet per minute, duct piping from the exhaust point to the collection plenums must be equivalent to or greater than the cross-sectional area of a 4-inch (100 mm) ID pipe. Where analysis verifies the entire system needs 40 cfm or less, 2-inch (50 mm) ID duct piping is permitted.

Translating that into what goes on the drawing:

• 40 cfm or less, verified by analysis: 2-inch is permitted. Roughly 3.1 square inches of cross-section. The permission is conditional, and it tightens further where the design leans on unusually strong fan pressures.
• Between 40 and 80 cfm: the 3-inch default governs. Roughly 7.1 square inches.
• Over 80 cfm: 4-inch is the minimum. Roughly 12.6 square inches, a 78 percent jump in area over 3-inch.
• Equivalence is by area, not by count. Two 3-inch risers give about 14.1 square inches, so they satisfy a 4-inch requirement. One 3-inch riser plus one 2-inch does not.

Because the bands are cliffs rather than slopes, projects that model out near a boundary deserve a second look. Sub-slab conditions that turn out looser than assumed, or a barrier that seals less completely than drawn, both push actual airflow up. A building estimated at 75 cfm is one field surprise away from needing 4-inch, and buying that headroom costs a few hundred dollars in pipe at design stage against opening finished shafts later.

The 2-inch band deserves particular caution on multifamily work. It is the exception, granted only when analysis verifies the low flow, and it leaves a system with no capacity to absorb a slab that reads leakier than the design assumed.

Pressure field extension analysis, usually shortened to PFE, is the diagnostic that answers that question. A test suction point is cut, a vacuum is applied, and the induced pressure differential is measured at test holes at increasing distance from that point. The distance at which the differential decays below the target tells you the radius of influence for one suction point on that specific sub-slab material.

From there the design falls out arithmetically. Slab area divided by the area one suction point can hold gives suction point count. Suction point count multiplied by the airflow each point draws gives system airflow, which is the number the 40 and 80 cfm thresholds test. On new construction, where there is no slab to test yet, the same logic runs off the specified gas-permeable layer instead, which is one of the reasons CC-1000 is prescriptive about that layer in the first place.

Routing changes the effective size of the pipe you specified. Every fitting adds equivalent length, and equivalent length eats the static pressure the fan has available. A run with six tight 90 degree elbows can behave like a pipe one size smaller than what is drawn on the riser diagram.

Three routing decisions carry the most weight in a multifamily design. Run the stack vertically inside conditioned space wherever possible, which keeps the column of air warm and preserves the thermal draw the system depends on. Slope every horizontal segment back toward a suction point so condensate drains to the soil rather than pooling in a low spot and choking the run. Terminate above the roofline, clear of operable windows, air intakes, and any occupied roof deck, so exhausted soil gas is not pulled straight back into the building.

A passive stack has no fan. It moves air on the stack effect alone, and the driving pressure that produces is small, on the order of a few pascals in a mid-rise. Resistance that a fan would shrug off will stall a passive column.

The practical consequence is that passive designs are sized up, not down. Where a fan-assisted riser might meet its target at the 3-inch default, the passive version of the same building is usually drawn at 4-inch with long-sweep fittings and the straightest vertical run the architecture allows. Designing the passive layout at the diameter the active system would eventually need is what makes conversion a field change, a fan and a circuit, instead of a redesign that reopens finished shafts.

What lands in a permit set is a stack schedule: each riser identified, its diameter, its route, its fittings, its termination height, and the fan provisions and electrical rough-in that let it go active later. That schedule is what a plans examiner checks and what a mechanical contractor builds from, and it is the reason the sizing work belongs in the design package rather than in an installer's field judgment. If the project is HUD financed, the expectation is firmer still, since HUD requires the CC-1000 2018 standard for all multifamily buildings except those consisting of one- and two-family dwellings.

If you are scoping risers for a new apartment, condominium, or mixed-use building, start with the multifamily design page for what the package covers, then send your plans and we return a fixed price, with the stack sizing and the standard it answers to specified before any work begins.