2021 IRC Chapter 32
Every plumbing fixture needs a trap, and every trap needs a vent close enough behind it to keep working. This lesson is about that pairing: what a trap seal actually does, which trap designs the code will not allow, how big a trap has to be for a given fixture, and how far the fixture drain between the trap and its vent, the trap arm, is allowed to run before the vent no longer does its job. These are some of the most frequently violated numbers in older houses and in rehab work done without a permit, because a trap arm that runs a few feet too far, or a trap that was never vented at all, still drains water just fine on the day it is installed. The problem shows up later, as a sewer gas smell that comes and goes.
A trap holds a standing column of water that blocks sewer gas, insects, and odors from the drainage system from entering the building through the fixture's drain opening. P3201.2 sets that water column, the trap seal, at not less than 2 inches and not more than 4 inches. Both limits matter for different reasons. Below 2 inches, ordinary evaporation, or a single strong siphon pulling the trap dry, breaks the seal and opens a direct path for sewer gas. Above 4 inches, a deep seal is harder to keep clear of solids, which is why the code caps the depth as well.
Several trap designs are prohibited outright by P3201.5 because they cannot reliably hold that seal. The S-trap, an S-shaped arrangement where the fixture drain drops straight down after the trap, is the one GCHS inspectors see most often in older houses: its own discharge can siphon the trap dry as the water falls past the weir, and the usual repair is to rebuild it as an ordinary P-trap with a horizontal, vented trap arm. A crown vent is a related defect: P3105.3 does not allow a vent connection within two pipe diameters of the trap weir, which rules out a vent taken off the crown of the trap itself, because a vent that close to the weir does not break the siphon. A bell trap, an inverted bell or dome that depends on a loose-fitting cover to hold its seal, fails the moment the cover is removed for cleaning or simply left off. A drum trap, a cylindrical trap body, is prohibited because its shape collects solids instead of carrying them through. A trap design with moving parts, such as a flapper or ball, is prohibited because those parts wear out and stick. A separate fixture trap with interior partitions is prohibited as well, with one exception: a lavatory trap made of plastic, stainless steel, or another corrosion-resistant material. Any of these found in an older house during a rehab inspection is flagged for correction, not left in place because it happens to be draining today.
P3201.6 requires each fixture to be separately trapped and prohibits double trapping, running one fixture's drain through a second trap downstream of the first. It also limits how far the trap can sit from the fixture: the weir may be no more than 24 inches below the fixture outlet, and the trap inlet no more than 30 inches from the outlet horizontally. Two traps in series create a sealed air pocket between them that neither trap can vent properly, and that pocket slows the flow enough to cause a chronic, low-grade stoppage. The code allows two exceptions in practice. A fixture with an integral trap, such as a water closet, needs no separate trap. And one trap may serve two or three like fixtures, limited to kitchen sinks, laundry tubs, and lavatories, when they are adjacent, in the same room, and connected by a continuous waste; with three fixtures the trap sits at the center one, and the outlets are no more than 30 inches apart. Table P3201.7 sizes the trap for a kitchen sink at 1-1/2 inches with or without a dishwasher and food waste disposer, and a bathtub with a shower head over it is one fixture with one trap.
Trap size follows the fixture it serves, and Table P3201.7 sets the minimums. A lavatory or bidet needs a trap of not less than 1-1/4 inch. A bathtub, a kitchen sink, and a laundry tub each need not less than 1-1/2 inch. A floor drain or a clothes washer standpipe needs 2 inches. A shower is sized by the total flow through its showerheads and body sprays: 1-1/2 inches at 5.7 gpm or less, 2 inches above that up to 12.3 gpm, 3 inches up to 25.8 gpm, and 4 inches up to 55.6 gpm. Where a fixture's tailpiece is larger than the table shows, the trap takes the tailpiece size, and a trap may never be larger than the drainage pipe it discharges into. A water closet has an integral trap built into the fixture itself, which is why it does not appear on the trap-size table the way other fixtures do. Undersizing shows up as a trap that clogs more often than the fixture load would suggest.
The trap arm is the section of fixture drain running from the trap weir to its vent connection, and P3105.1 limits how far that arm can run before the vent stops doing useful work. Beyond the table's maximum developed length, the column of water between the trap and the vent becomes long and heavy enough that a full flush can siphon past the vent connection and pull the trap's own seal down with it, exactly what venting is meant to prevent. The limits scale with pipe size: 5 feet for 1-1/4 inch pipe, 6 feet for 1-1/2 inch, 8 feet for 2 inch, 12 feet for 3 inch, and 16 feet for 4 inch. Self-siphoning fixtures such as water closets are not held to the length limit. The required slope also changes with size: 1/4 inch of fall per foot for the smaller trap arm sizes, easing to 1/8 inch per foot for 3-inch and 4-inch arms. Two more rules protect the arm. P3105.2 limits the total fall in a fixture drain to one pipe diameter, and the vent connection may not sit below the trap weir (water closets excepted). P3105.3 bars a vent within two pipe diameters of the weir. A trap arm has to hold its slope evenly along its whole run; a flat spot or a reversed pitch partway along is a defect even if the overall length and end points are within the table.
A floor drain in a laundry room, garage, or mechanical closet often goes weeks or months between uses, and a trap seal with no fresh water passing through it simply evaporates away on its own. Once that happens, the floor drain stops blocking sewer gas even though nothing about the piping has failed. P3201.2.1 addresses this: the seals of emergency floor drain traps and traps subject to evaporation must be protected by one of four methods, a potable water-supplied trap seal primer valve, a reclaimed or graywater-supplied primer valve, a wastewater-supplied trap primer device, or a barrier-type trap seal protection device. A primer valve bleeds a little water into the trap above the seal, and the barrier-type device blocks evaporation from the floor drain trap. An inspector who finds a dry floor drain with none of these in a rarely used space flags it, because the fixture will keep failing the same way on its own schedule no matter how it drains when tested.
In the field
A common GCHS rehab finding is a laundry room floor drain that smells faintly of sewer gas every few weeks, then clears up, then comes back. The homeowner assumes something is wrong with the sewer line. The actual cause is almost always evaporation: the floor drain only gets water when the washer discharges, which might be once or twice a week, and that is not enough to keep a standard 2-inch seal full between uses in a dry closet. Fixing it means adding a trap seal primer tied to a nearby cold supply line, or fitting a barrier-type seal protection device, not chasing a sewer line problem that does not exist.
A worked example on trap arm length: a lavatory has a 1-1/4 inch trap arm running from the trap weir to the vent connection. The table allows up to 5 feet of developed length at a slope of 1/4 inch per foot. The actual run measures 4 feet 6 inches, or 4.5 feet, which is within the 5-foot limit. At the required 1/4 inch per foot, the fall over that run works out to 4.5 times 0.25, or 1.125 inches, meaning the drain connection at the vent end sits about 1-1/8 inches lower than the trap weir, which is less than the 1-1/4 inch pipe diameter allowed as total fall. Both the length and the resulting drop are easy to check with a level and a tape measure before the wall is closed in, which is exactly when GCHS inspectors want to catch it.
1. Under P3201.2, a trap seal must be:
2. An inspector finds an S-trap with no vent connection above the trap weir. This is:
3. A drum trap is:
4. Under P3201, a fixture drain running through two traps in series (double trapping) is:
5. Under P3201.6, one trap is permitted to serve two or three fixtures only where they are:
6. The minimum trap size for a lavatory is:
7. Under P3105.1, the maximum developed length of a 2-inch trap arm is:
8. The required slope for a 1-1/2 inch trap arm under P3105.1 is:
9. A lavatory's 1-1/4 inch trap arm runs 4 feet 6 inches from the trap weir to the vent connection. Under P3105.1, this is:
10. A floor drain in a laundry closet develops an intermittent sewer gas odor that comes and goes over several weeks, with no change in the drain line itself. The most likely cause is:
11. A trap primer is installed to:
12. A vent connected at the crown of a trap, within two pipe diameters of the weir, is prohibited by P3105.3 because:
13. Which statement about prohibited trap designs under P3201.5 is correct?
14. A water closet's trap: