Drain Testing Explained: Air Test vs Water Test and What Each Proves

Drain testing means sealing a length of pipe and checking whether it holds air or water under a known pressure. It is the standard way to prove a new drain is watertight before building control signs it off, and a blunt but useful way to show that an existing drain is leaking. What it cannot do is tell you where the leak is. That is the job of a camera.

What is a drain test?

A drain test is a pass-or-fail check on the watertightness of a pipe run between two access points. The run is sealed at each end with inflatable stoppers, pressurised with air or filled with water, and watched for a set period to see whether the pressure or water level drops.

The result is a number, not a picture. A pressure gauge or manometer is read at the start and end of the test period, and the drop is compared with an acceptance limit. A good test record states which run was tested (chamber to chamber), the pipe diameter, the method, the starting pressure or head, the duration, and the final reading. Without those details a "passed" certificate is not much use to anyone checking it later.

How an air test works

An air test is the quicker and more common method. The sealed run is pumped up with air to a low pressure, measured in millimetres of water gauge on a U-tube manometer, left to stabilise, and then timed. If the reading falls more than the permitted amount in the test period, the run fails.

For most gravity drains on domestic work, the figure building control officers work to comes from Approved Document H of the Building Regulations: a starting pressure of 100 mm water gauge, with a drop of no more than 25 mm over five minutes. Runs that include a trapped gully are tested at a lower pressure so the water seal in the trap is not blown out. Your building control body will confirm the criteria it wants for a particular job, and on larger or adoptable work the design specification can set its own.

Air testing is clean, needs no water supply and gives an answer in minutes. Its weakness is sensitivity. Temperature changes inside the pipe, a stopper that is not seated properly, or a damp joint can move the gauge, so a marginal fail should be re-run before anyone starts digging.

How a water test works

A water test fills the run with water to a set head above the pipe at its upper end, usually by fitting a vertical standpipe, and measures how much water has to be added over the test period to keep that level. The loss is compared with an allowance based on pipe length and diameter.

Water testing is slower and messier, but it loads the pipe the way a surcharged drain is loaded in real life, so some engineers and loss adjusters prefer it where the question is whether a drain has been leaking into the ground. It also has limits. Filling a run on a steep gradient puts a high head on the lower end, which can over-stress old joints, and on some sites there is simply nowhere suitable to discharge the test water afterwards.

When building control expects a drain test

New below-ground drainage is normally tested before it is signed off. The building control officer, or an approved inspector, will usually want to see a test after the pipe is laid and bedded, and often again after backfilling, because backfill is when pipes get knocked out of line.

Extensions are the usual London case: a new run connected into an existing chamber, or an existing run diverted around new foundations. If the drain that is being connected into is old clay pipework, it is worth agreeing with building control in advance exactly which lengths are being tested. Testing only the new pipe is normal. Testing the new pipe together with a Victorian run it joins will almost always fail, and not because of your builder's work.

Soakaways and surface water systems are treated differently, because a soakaway is designed to lose water. What gets checked there is the pipe feeding it and, before installation, the ground's ability to absorb water. We cover that side on our soakaway installation page.

Why old drains fail pressure tests

Most pre-war London drains were laid in short lengths of salt-glazed clay with mortar or rope-and-mortar joints. After a century of ground movement, very few of those joints are airtight, so an air test on an old run tells you something you could have guessed: it leaks.

That does not mean the drain is failing in any practical sense. A run can lose pressure through slightly open joints and still carry flow perfectly well for decades. This is the most common misuse of drain testing we see: a failed air test on an old drain presented as proof that it needs replacing. A fail is a starting point for investigation, not a diagnosis.

The opposite mistake happens too. A drain that passes a test on the day can still have a deformed section, an intruding connection or a root mass that will block it next month, because none of those necessarily lets air out.

Drain test or CCTV survey: which do you need?

Use a pressure test when the question is "is this pipe watertight?" and a CCTV survey when the question is "what is wrong with this pipe, and where?". For most problems on existing drains, the survey is the more useful purchase, and the two together give the strongest evidence.

• New drain being signed off: a pressure test, because that is what building control asks for. A CCTV check after backfilling is a sensible extra on long or deep runs.
• Suspected leak under a floor or near a crack in the wall: a test to show the run leaks, then a CCTV drain survey to find the defect, record its position by chainage and grade it.
• Insurance claim for subsidence or water damage: the insurer or loss adjuster will usually specify the evidence. Both a test result and coded survey footage are commonly requested. Our drainage insurance claims page explains what they tend to look for.
• Unsure where the pipe runs or where it connects: neither a pressure test nor a standard survey on its own. That is drain tracing, with a sonde or dye.

If a test fails and a survey follows, ask for the survey report to reference the same chamber numbers as the test record. It sounds like admin, but it is what lets a surveyor, a builder and an insurer all talk about the same length of pipe.