How to Water or Air Test a DWV Rough-In

Why this matters

A drain joint that weeps under a slab or behind a finished wall is found by the stain, months later, after the concrete, tile or drywall has gone over it. The rough-in test is the one moment every joint in the drain, waste and vent system (DWV) is still visible and can be remade with a can of cement and a coupling. It is also what the inspector signs off on, and a rough that fails in front of the inspector costs a second trip and a rescheduled pour. Most failed tests are not bad joints; they are a test ball that was never seated or a gauge that cannot read the pressure it is supposed to show.

What the codes ask for

Both model codes give the same two choices for a new DWV rough:

• Water test: 10 ft of head held 15 minutes. IPC 2021 Section 312, as adopted, and UPC 2021 Section 712, as adopted, call for the system or the section under test to be filled with water so that no part of it sees less than a 10 ft column, and for the level to hold for 15 minutes with no leak. When a building is tested in sections, each section overlaps the one below it by enough that every joint has seen 10 ft of head; in substance, only the uppermost 10 ft of the finished system is allowed to test at less. Check the current edition for the exact overlap wording.
• Air test: 5 psi held 15 minutes. The same sections allow air at 5 psi (or a mercury-column equivalent) held 15 minutes without adding air. The IPC keeps it as an equal option; your jurisdiction's amendments may not.

Ten feet of water is about 4.33 psi at the bottom (0.433 psi per foot times 10), so the two tests load the bottom joints about the same. The difference is in what is stored inside the pipe, which is the next section.

Where the inspector or the local amendment names a different test, that wins. Some jurisdictions accept only water on plastic; a few accept a smoke test on an existing system. Ask when you book the inspection, not at the slab.

Why many plastic pipe makers forbid the air test

Water barely compresses. If a PVC or ABS fitting cracks under 10 ft of head, the stored energy is almost nothing and the water simply runs out. Air compresses, so 5 psi of air in a run of 3 in and 4 in pipe is a charged spring. If a cold, brittle or damaged fitting lets go, that energy releases at once and can throw fragments of pipe and the plug itself.

That is why many PVC and ABS DWV makers, and the plastic pipe industry's own trade association, tell installers not to test their product with compressed air or gas. The maker's instructions are part of the listing, so on plastic DWV the air test is out when the maker says so, even where the code text would allow it. Cast iron and copper DWV can be air tested where the code and inspector accept it. On a mixed system the plastic decides.

Start state

What you need. Inflatable test balls or mechanical test plugs sized to every opening, a test cap or plug for each closet flange, glued or mechanical caps on every fixture stub, a length of pipe and a coupling for the standpipe, a hose, a ladder for the fill, and a tape measure. For an air test on metal pipe, a test gauge with a 0 to 15 psi range: 5 psi on a 0 to 100 psi gauge sits inside the needle's width and no drop is readable.

Check before you begin:

1. Every solvent-weld joint has had its full cure time from the cement maker's chart for that pipe size and temperature. Testing early pushes a joint apart that would have been sound by morning. Cold weather stretches the cure.
2. Every joint is still exposed. The test proves nothing about a joint already backfilled, insulated or covered.
3. The pipe is strapped and supported to its final spacing. A water-filled 4 in line weighs several times what it did dry, and an unsupported run can sag and pull a joint during the test. Spacing is in Pipe Support Spacing by Material.
4. Weather. Water in exposed pipe at or below freezing splits fittings. Test, read and drain the same day, or use the air option on metal.

The test, step by step

This is the water test on a plastic rough, the common case. The air variant follows.

  UNDER-SLAB ROUGH, WATER TEST

   open top (fill here)
   |
   |  temporary standpipe on the
   |  2 in vent stub, top at least
   |  10 ft above the highest joint
   |
   |      cap      cap      test cap
   |      lav      shower   closet flange
  -+-------+--------+--------+---- finished slab
   |       |        |        |
  =+=======+========+========+===> building drain
                                 |
                       test ball in cleanout
                       (bypass hose capped)

1. Plug the outlet first. Seat the test ball in the cleanout or test tee downstream of everything you are testing, inflate it to the pressure printed on the ball, and cap its bypass hose. The ball holds back the whole column, so a ball inflated below its rating can slide under load. Skip this check and the "leak" you chase for twenty minutes is water running out past the ball. Hazard: never put your face or body in line with an inflated ball under head, and do not over-inflate past the printed rating; a ball can burst or blow out of the fitting.
2. Cap every other opening below the standpipe. Fixture stubs get glued or mechanical caps, the closet flange gets its test cap, and any vent or branch not part of this section gets a plug. Count the openings on the drawing and count the caps in place. One missed stub floods the slab bed.
3. Stand the standpipe so its top is at least 10 ft above the highest joint in the section. On a slab rough with no vents up yet, this is a temporary riser coupled onto a vent stub. On a full rough with the vent through the roof, the vent itself is the standpipe, and the overlap rule above covers its top.
4. Fill from the low end if you can, and bleed air at the high openings. Air trapped in a high branch compresses as the column rises, and a pocket that slowly dissolves or escapes reads as a falling level. Hazard: filling at the top of a 10 ft riser means a ladder; keep three points of contact and do not reach past the rails with a live hose.
5. Fill to the top of the standpipe, wait two minutes, top off, and mark the level. The short wait lets trapped air rise out and wet pipe settle. Mark with a pencil on the pipe or tape a line.
6. Hold 15 minutes and walk every joint while you wait. Look for drips, a dark line on the slab bed, or a wet fitting shoulder. Watching joints is how you find the leak; the level only tells you there is one.
7. Read the level at 15 minutes. No drop is the pass. Any drop is a fail, however small, because the code test is to hold, not to nearly hold.
8. Drain under control. Open the ball's bypass hose to a floor drain, a bucket or outside before deflating the ball. Deflating first releases the whole column in a rush that can blow the ball down the line and wash out your slab bed.

Air variant, metal DWV only where allowed. Plug the same openings, connect the 0 to 15 psi gauge and a hand pump or regulated compressor through a test plug, raise it to 5 psi, let it settle a few minutes as the air temperature equalizes, then re-set to 5 psi and hold 15 minutes with no air added. Find leaks with soapy water on the joints, never by ear alone. Hazard: never exceed the test pressure, stand to the side of plugs while pressurized, and bleed to zero before removing a plug.

Worked example: a basement bath on the slab

The rough. A 3 in building drain runs from an existing cleanout wye to a toilet, a shower and a lavatory, all PVC, glued the day before at about 60 F. The lav's tee is the highest under-slab joint, 14 in below the top of the 2 in vent stub. The building drain is plastic, so air is out.

Plugs. A 3 in test ball in the cleanout, a closet flange test cap, a glued cap on the shower riser, a glued cap on the lav stub. Four openings on the drawing, four closures in place.

Standpipe. The lav's tee sits 14 in below the stub top, so 10 ft above that tee is 120 minus 14, or 106 in above the stub. A coupling and a 9 ft length (108 in) of 2 in pipe puts the top 108 plus 14, or 122 in, above the tee: just over 10 ft.

How big a leak a level drop means. A 2 in Schedule 40 standpipe has an inside diameter of about 2.067 in, so its cross-section is about 3.36 square inches. A weep losing one cup of water (14.4 cubic inches) over the hold drops the column about 14.4 divided by 3.36, or 4.3 in. Going the other way, 4.3 in of drop times 3.36 square inches is 14.4 cubic inches, one cup. A drop you can see with a pencil line is a real leak, not noise.

First fill. Filled through the top, topped off after two minutes, marked. At 5 minutes the level was 2 in down. No joint was wet. At the cleanout, a thin stream was running from the downstream side of the ball. The ball had seated against the wye's branch, not in the straight run.

The fork. A drop with every joint dry points at a plug, so the plugs get checked before anyone cuts pipe. A drop with a wet joint is a joint: drain, cut it out and remake it with a coupling, then wait out the cement's cure again before retesting.

Second fill. Drained through the bypass, ball deflated, re-seated in the straight run, re-inflated to its printed pressure, standpipe refilled and marked. At 15 minutes, no drop and all four closures dry. The level was left up for the inspector, who saw it the next morning still at the mark and signed the card.

What would change it. If this had been a cast iron building drain with plastic only above the slab, the plastic still decides and the test is still water. If the vent had already been through the roof with its top 20 ft above the tee, no temporary standpipe is needed, because the vent itself carries more than 10 ft over every joint below its top 10 ft.

How to verify you got this right

• Holding at 15 minutes against a pencil mark, with every closure dry, is the pass. A level topped off during the hold is a failed test, rerun.
• The plug count matches the opening count on the drawing, written on the ticket.
• The inspector saw it under test, or the test was rerun for them. Some inspectors want it up when they arrive; ask when you book.
• The system was drained and the plugs pulled before backfill or cover, except the closet flange cap, which stays on to keep debris out.

Getting from a passed test to a signed inspection card, including what else the inspector looks at on a rough, is covered in Rough-In Inspection Readiness Walkthrough.

References

• IPC 2021 Section 312, as adopted by your jurisdiction: drainage and vent water and air tests.
• UPC 2021 Section 712, as adopted by your jurisdiction: testing of drainage and vent systems.
• The pipe and solvent cement makers' installation instructions: air-testing prohibitions on plastic DWV and cure-time charts.
• See related: Rough-In Inspection Readiness Walkthrough; Pipe Support Spacing by Material; How to Cut, Prime and Glue a PVC DWV Joint.