A plumbing chase wall is a wall built extra deep, or with a hollow space inside, to hold drain, vent, and water pipes. It makes room for big pipes, back-to-back restrooms, and wall-hung toilet carriers. In commercial buildings a chase between floors may need a fire-rated enclosure, and pipes through rated walls must be firestopped.
Why do buildings use a chase wall?
Because pipes need room, and cutting framing to fit them has limits. A few common reasons:
- Large drain pipes. A 3- or 4-inch drain and its fittings don't fit comfortably inside a 2x4 wall. A deeper wall avoids overcutting the studs.
- Back-to-back restrooms. Two restrooms sharing one wall can share a chase, with fixtures on both sides tied into common drains and vents.
- Wall-hung toilets and urinals. These hang on steel carriers inside the wall. The plumbing code requires this in Section 405.4.3: "Wall-hung water closet bowls shall be supported by a concealed metal carrier that is attached to the building structural members so that strain is not transmitted to the fixture connector or any other part of the plumbing system." Jay R. Smith, a carrier maker, describes these supports as "installed behind the finished wall to support toilets and urinals."
- Vertical runs through floors. In multistory buildings, stacks and risers run from floor to floor in a stacked chase.
- Access and future work. A chase with an access panel lets a plumber reach valves and piping without tearing out finished walls.
Keeping pipes out of exterior walls matters too, though in Phoenix the concern is summer heat more than freezing. The Department of Energy's Building America guide on insulating plumbing pipes says "It is preferable to design the home so that plumbing fixtures align with interior walls."
How deep does a chase wall need to be?
It depends on the pipes and fixtures it holds. There isn't one code number for chase depth. The depth is set by the largest pipe and fitting in the wall, and for wall-hung fixtures, by the carrier. Carrier makers publish minimum chase dimensions for each model. Zurn, for example, publishes tables titled "Minimum Chase Requirements for Standard Wall Installation" and "Minimum Chase Requirements for Narrow Wall Installation." Back-to-back carriers need more depth than a single one.
In practice:
| Wall | What drives the depth |
|---|---|
| Home bathroom wet wall | The drain and vent size, often a 3-inch stack |
| Single commercial restroom, floor-mount toilets | Drain and vent sizes |
| Wall-hung toilets or urinals, one side | The carrier's minimum chase dimension |
| Back-to-back wall-hung fixtures | Two carriers plus the shared drain |
| Multistory stack | The stack, branches, and any fire-rated shaft enclosure |
The designer sizes the chase on the permit drawings. When a tenant improvement adds wall-hung fixtures to an existing wall, the wall often has to be rebuilt deeper.
What code rules apply to pipes in walls?
Three sets of rules: protecting the pipe, protecting the framing, and keeping the pipe reachable.
Protecting the pipe from nails and screws. IPC Section 305.6 requires steel shield plates where pipe (other than cast iron or galvanized steel) passes through a stud or joist less than 1-1/4 inches from its edge. "Such plates shall cover the area of the pipe where the member is notched or bored, and shall extend not less than 2 inches (51 mm) above sole plates and below top plates." The plates have to be at least No. 16 gage steel. A chase wall with room to center the pipes cuts down on how many plates are needed.
Protecting the framing. In homes, the residential code limits how much a stud can be cut. Section R602.6 says a stud in an exterior or bearing wall "shall not be cut or notched to a depth exceeding 25 percent of its depth," and "The diameter of bored holes in studs shall not exceed 60 percent of the stud depth." Commercial framing has its own limits. These limits are a big reason plumbers ask for a deeper wall.
Keeping cleanouts reachable. Section 708.1.11 says "Required cleanouts shall not be installed in concealed locations," and names "within walls" as a concealed location. Cleanouts in a chase wall have to be extended to the wall face or reached through an access panel, and Section 708.1.10 requires at least 18 inches of clearance in front of cleanouts on 6-inch and smaller pipe. See cleanout code requirements.
### What about back-to-back toilets?
They're a common reason for a chase, and the drain fittings matter. Section 706.3 says "Double sanitary tee patterns shall not receive the discharge of back-to-back water closets," but allows them where the horizontal developed length from each toilet outlet to the double tee is 18 inches or more.
What fire rules apply to a chase in a commercial building?
A chase that runs through floors is a shaft under the building code, and a pipe that passes through a fire-rated wall or floor is a penetration. Both have rules.
The building code defines a shaft as "An enclosed space extending through one or more stories of a building, connecting vertical openings in successive floors, or floors and roof." Section 713.4 says "Shaft enclosures shall have a fire-resistance rating of not less than 2 hours where connecting four stories or more, and not less than 1 hour where connecting less than four stories." There are exceptions, and the architect decides which apply.
Where pipes pass through fire-rated walls and floors, Section 714 applies. It says "Through penetrations shall be protected using systems installed as tested in the approved fire-resistance-rated assembly." In practice that means a listed firestop system matched to the pipe material and size, installed per its listing. The right product depends on whether the pipe is plastic or metal, and on its size.
An open chase also needs to be closed off. The DOE's Building America guide, writing about a chase for solar lines, says "the chase should be capped on both ends and sealed at all floor and ceiling penetrations to maintain air tightness and mandatory fire ratings." It also warns "It is not recommended to use a boxed-in open chase in the wall as other contractors may inadvertently install wires, plumbing, and ductwork through it."
What should you plan for when building or opening a chase wall?
For new construction and tenant improvements:
- 1Size the chase from the fixtures. Get the carrier model and its minimum chase dimension before the walls are framed.
- 2Plan access. Put access panels where the valves, cleanouts, and any flush valves or carrier adjustments are.
- 3Coordinate the trades. Electrical, low-voltage, and HVAC often want the same space. Decide who gets what before rough-in.
- 4Firestop as you go. Seal penetrations after the pipes are in and before the drywall, and document the firestop systems used.
- 5Pass the rough-in inspection. Pipes in the chase are tested and inspected before the wall closes. See which plumbing permits and inspections new construction needs in Phoenix.
For remodels, opening an existing chase is usually easier than opening a regular wall, because there's room to work. Our page on moving a toilet, sink, or shower to a new spot covers what changes when fixtures move.
HQ Plumbing & Air plans and installs restroom rough-ins, carriers for wall-hung fixtures, and drain and vent stacks for commercial build-outs and tenant improvements across the Phoenix area. We're licensed and insured, with free estimates and upfront pricing. See our commercial plumbing services.
Give the pipes a wall of their own. It's easier to build, inspect, and fix later.