Drain Test Kit: Pre-Test Your Drainage Before the Building Inspector Arrives
What an air test kit is, the BS EN 1610 / Approved Document H test method, and why a £40 kit pays for itself by catching leaks before the BCO visit.
A homeowner's groundworker finishes the drainage run from the new extension to the existing soil stack. The trench is open, the pipe is laid, the bedding is clean. The Building Control Officer is booked for the drainage inspection on Friday. The BCO arrives, runs the prescribed air test, the pressure drops faster than allowed, and the test fails. The whole drainage run has to be exposed, every joint checked, the leak found and resealed, and a re-inspection booked. Two days lost, a re-inspection fee paid, the groundworker's payment milestone delayed.
A 40 pound drain test kit, used the day before the BCO visit, would have caught the leak when the trench was already open and the groundworker was still on site. The kit is the single highest-value tool a homeowner can buy for an extension build, and almost no homeowner project plans include it.
What a drain test kit is
A drain test kit is a small bag of equipment that lets you perform a pressure test on a section of drainage pipe before it's signed off by Building Control. The components are simple: a 110mm drain bung (the standard UK soil pipe diameter), a flexible hose, a small hand pump, and a pressure gauge or U-tube manometer.
You insert the bung into the inspection point upstream of the test section, seal it, then use the pump to raise the air pressure inside the pipe to a specified level. The gauge reads the pressure. You then watch the gauge for the specified test period (typically three minutes). If the pressure drops more than the allowed amount, the pipe has a leak somewhere along the test length.
The kit is a one-off purchase under 50 pounds from Screwfix, Toolstation, or any drainage supplier. Once you own it, you can pre-test every section of drainage in the build before the inspector arrives, and any leaks become trench-side fixes rather than re-inspection failures.
What Building Control actually tests
The drainage inspection in England and Wales is governed by Approved Document H of the Building Regulations and references BS EN 1610 (Construction and testing of drains and sewers). The Approved Document specifies that drainage be tested before backfilling and again after backfilling and concrete is placed.
The standard air test is a low-pressure test:
| Test stage | Pressure | Test duration | Allowable drop |
|---|---|---|---|
| Pre-backfill air test | 100mm water gauge (~10mbar) | 3 minutes | Drop not exceeding 25mm water gauge |
| Post-backfill air test | 100mm water gauge | 3 minutes | Drop not exceeding 25mm water gauge |
| Water test (alternative) | 1.5m water column above invert | 30 minutes | Drop not exceeding 1L per metre length per hour |
The air test is the standard for short residential runs because it's quick to perform and a single test covers a long length of pipe in one operation. The water test is the alternative; a sealed pipe filled with water to a standpipe height of 1.5m is allowed to lose water at a defined maximum rate.
For homeowner extension work, you'll almost certainly be tested with the air test. Knowing the pass criterion (less than 25mm drop in 3 minutes from a 100mm starting pressure) tells you what your own pre-test needs to demonstrate.
Tip
Buy a kit with a U-tube manometer rather than a digital gauge. The U-tube reads pressure in millimetres of water column directly, which is the unit the Building Regulations specify. Digital gauges in mbar require conversion (10mbar = 100mm water column) and the conversion adds a step where homeowners get confused.
How to run an air test before the inspector arrives
The procedure is the same one the BCO will run, just done a day earlier with the trench still accessible.
Identify the test section
A drainage run typically runs from the soil stack at the house to the connection to the existing soil pipe or new manhole. The test section is usually one continuous run between two access points (rodding eyes, inspection chambers, or open ends).
Seal the lower end with the drain bung
Insert the inflatable or expanding 110mm drain bung into the lower end of the test section. Inflate or expand it to seal against the pipe wall. The bung must hold without slipping under test pressure.
Connect the pump and manometer to the upper end
Attach the flexible hose from the pump to the upper end of the section. The U-tube manometer connects to the same line so it reads the pressure inside the pipe.
Pump up to 100mm water column
Pump air into the section until the manometer reads 100mm water column (or 10mbar on a digital gauge). Stop pumping immediately at 100mm.
Wait one minute for stabilisation
Pressure inside the pipe rises slightly when air is pumped in (the air heats up under compression) and then stabilises as the air cools to ambient temperature. Wait one minute and re-check the gauge reading; top up to 100mm if needed.
Watch the gauge for three minutes
Start a timer. Watch the manometer for three minutes without further pumping. The reading should not drop more than 25mm water column over the three-minute period.
If the test passes, you're ready for the BCO
A drop of less than 25mm in 3 minutes is a pass. Take a photo of the manometer reading at the start and end of the test as evidence. Pack the kit; you've already done the inspection.
If the test fails, find and fix the leak
A drop greater than 25mm means a leak. Common causes: a poorly seated rubber gasket, an unsupported pipe joint that's pulled slightly apart, a hairline crack from a pipe being dropped before laying, or the bung itself not sealing. Pour a soapy water solution along visible joints; bubbles indicate the leak. Re-seal the joint and retest.
For the post-backfill test (after the trench is filled and the concrete oversite is poured), the kit setup is the same. The leak is harder to find because the joints are buried, which is exactly why the pre-backfill test matters: catching leaks while the trench is open is twenty minutes of work; catching them after concrete is poured is half a day of breaking out and re-pouring.
Pre-test versus the BCO visit: the value calculation
A re-inspection visit from a Local Authority Building Control team costs typically 100 to 200 pounds in 2026, plus the time delay (usually one to two weeks for a re-booking). Approved Inspector private BCO services charge similar re-inspection fees.
A failed inspection also affects the groundworker's payment milestone. Most homeowner-managed builds pay groundworks at completion of drainage sign-off; a failure delays that payment by a week or more, which annoys the groundworker and can sour the working relationship.
Compare those costs against 40 pounds for the test kit. Even if the kit is used once and never again, it pays for itself the first time it catches a leak before the BCO visit. The Pareto reality is that most drainage runs pass on first inspection, so the kit catches a problem in maybe one in three uses; even at that rate the cost-benefit is overwhelmingly positive.
The same kit also lets you test the existing house drainage if you're tying into it, which catches age-related leaks in the existing run that would otherwise become your problem after the new work is signed off.
What to buy
The market for homeowner-grade drain test kits is small and the brands are interchangeable.
| Tier | Approx price | Models | What you get |
|---|---|---|---|
| Budget | £25-40 | Drain Master Air Test Kit (Screwfix), Floplast 110mm test bung + manual pump separately, generic eBay air test kits | 110mm bung, hand pump, basic gauge. Functional but the manometer may be a single rigid plastic tube rather than U-tube |
| Mid-range | £45-70 | Polypipe Air Test Kit (Toolstation), Drainage Sales Direct kits | Inflatable bung with screw clamp, U-tube manometer, robust hand pump, carry bag |
| Trade | £90-150 | Lanes Group test kit, Camlin Air Test Kit, Pearpoint MK20 | Calibrated digital gauge, multiple bung sizes (100mm, 110mm, 150mm), professional carry case |
| Hire | £25-35/week | Brandon Hire, HSS, Speedy Hire | Trade-grade kit with calibration; no purchase commitment |
For a single homeowner kit purchase: any of the budget or mid-range options work. The Drain Master kit at Screwfix (around 40 pounds) is the most accessible and the one most homeowners on UK forums report buying. The Polypipe kit at Toolstation is the next step up at around 55 pounds and includes the U-tube manometer, which is easier to read.
The hire option is worth considering if you only need the kit for a single drainage run and don't expect to use it again. 30 pounds for a week from any major hire shop, with a properly calibrated trade-grade kit included.
Warning
Drain bungs under pressure are stored energy. A bung that comes loose under air pressure can fly out of the pipe at significant force. Stand to one side of the pipe end during the test, never directly in line with the bung. The pressures involved (100mm water column is approximately 0.01 bar) are low, but a bung released suddenly can still cause injury at close range. The risk is greater on water tests where the pressures and energies are higher.
Where you'll use it
The drain test kit comes out at the drainage stage of the build:
- Drainage for pre-testing the new run before BCO inspection
- Building control inspection: drainage and oversite for the actual inspection
- Foundations and footings where drainage is being laid alongside or under foundation work
- Build-over agreement for any drainage that runs under the new extension and is being tied into water authority infrastructure
- Snagging checklist for any post-completion drainage testing (some warranty bodies require a final test)
Once the drainage is signed off, the kit goes back in the cupboard. The next time you'll need it is if you ever extend or alter drainage on this or any other property.
When to use other tests
The air test is the standard. Two adjacent tests have specific use cases.
Water test: When a drain run includes back-falls, dips, or sections where air can escape via vent pipes, the air test is unreliable. A water test seals the lower end with the bung, fills the pipe with water from the upper end, and measures water level drop over 30 minutes. Slower to perform but unambiguous on long or complex runs.
Smoke test: Specialist test using non-toxic smoke pumped into the drainage system to find leaks visually. Used by drainage surveyors when an air test fails but the leak location isn't obvious. Not a homeowner-grade test; commission a specialist if the air test repeatedly fails and the leak can't be found.
CCTV survey: A camera on a flexible rod inspects the inside of the drain run. Used for condition surveys of existing drainage, particularly when buying a property with old clay pipework. Around 150 to 300 pounds for a residential CCTV survey from a local drainage company.
Common mistakes
Skipping the pre-test entirely. The single biggest mistake. The 40 pound kit pays for itself the first time it catches a leak. Almost every homeowner-managed build that fails drainage inspection would have passed if a pre-test had been done.
Pumping above 100mm water column. The test pressure is specified at 100mm. Pumping higher means you can falsely pass the test (more pressure margin to fall through) and risks blowing the bung out. Stop at 100mm.
Not waiting for stabilisation before the test starts. The first minute after pumping, the air inside the pipe cools and contracts. Pressure drops naturally and looks like a leak. Wait one minute, top up to 100mm, then start the three-minute test clock.
Testing immediately after laying without time for joints to settle. Rubber gasket joints sometimes need a few hours to fully seat against the pipe wall. Test the day after laying rather than the same hour, particularly if joints have been disturbed during backfilling.
Reusing a damaged bung. The bung's seal must be perfect. A bung that has been dragged across rough concrete or stored in a damp toolbox can develop small surface defects that leak under test pressure. Inspect the bung surface before each use; replace if any cuts or compression damage are visible.
Forgetting the post-backfill retest. Approved Document H specifies testing both before and after backfilling. The pre-test is for your benefit; the post-test is the official record. Both matter.