Drain Testing: Air Test, Water Test & CCTV Survey Procedures

Quick Answer: New above-ground drainage must be air tested to 38mm water gauge pressure, held for 3 minutes without pressure loss exceeding 6mm water gauge (BS EN 1329 / Approved Document H). Below-ground drainage should be tested with water to 1.5m head (300mm above the invert of the top of the drain run), held for 2 hours — any visible loss of water level indicates a failed joint or damaged pipe.

Summary

Drain testing is required at the end of new drainage installation to confirm the system is watertight before it is covered over or connected to the building. It is a Building Control inspection stage item for drainage covered by Approved Document H — the inspector may wish to witness the test, or accept a signed test certificate from the installer.

Testing methods differ for above-ground and below-ground drainage. Air testing is faster and uses simpler equipment; water testing is the traditional method and gives a clear visible result. CCTV surveying is used for existing drain systems to identify blockages, root intrusion, joint failures, collapsed pipes, and incorrect gradients — it is not a pressure test but a visual inspection.

Understanding when and how to test drainage is important not just for compliance, but for your own protection — a drain system that passes a test before backfill or burial means you have documentary evidence of a sound installation if problems arise later.

Key Facts

• Approved Document H — Building Regulations for drainage; covers both surface and foul water drainage
• BS EN 1329 — Specification for PVC-U pipe systems for soil and waste drainage within buildings
• Air test (above-ground) — 38mm water gauge for 3 minutes; permissible drop ≤ 6mm wg; uses a manometer and rubber test plugs
• Water test (below-ground) — 1.5m head above invert of topmost pipe in test run; observe over 2 hours; fill, allow to stabilise for 30 minutes, mark level, observe for 2 hours
• Acceptance criteria (water test) — no visible level drop after initial absorption period; for HDPE, PVC and vitrified clay, total loss ≤ 0.05 litres per square metre of internal wetted area per 100m
• Smoke test — alternative to air test for above-ground drainage; non-toxic smoke introduced at low pressure; leak points show smoke
• CCTV survey — camera passed through drainage to assess condition; reports to MSCC4 standard (WRC); identifies cracks, root ingress, sags, joint failure, displaced joints, blockages
• Drain inspection chamber — must be provided at every change of direction, gradient, or pipe diameter; maximum 45m between access points on straight runs
• Ball test (gradient check) — 25mm ball inserted at top of drain; should roll freely to end without stopping; confirms minimum gradient has been maintained
• Gradient — 100mm foul drain minimum 1:80 (1.25%); preferred 1:40 (2.5%); maximum 1:25 (4%) without approval
• Test plugs — rubber expanding plugs with ratchet mechanism; for internal air test; available in 50mm, 75mm, 100mm, 150mm diameter
• Building Control inspection — notification required before backfill; inspector may attend to witness test

Quick Reference Table

Detailed Guidance

Above-Ground Air Test — Procedure

Equipment needed:

• Air pump (hand bicycle pump or test pump)
• U-tube manometer (measures water gauge pressure)
• Test plugs to fit every open end of the stack and branch pipes
• 3-way tap or valve at the manometer connection

Procedure:

1. Insert test plugs into all open ends of the drainage stack: both ends of each horizontal branch, the base of the stack (at the rodding point), and the top of the stack. Leave one connection accessible for the manometer.
2. Connect the manometer to the stack via the open connection.
3. Pump air into the system until the manometer reads 38mm water gauge. Close the pump valve.
4. Observe the manometer for 3 minutes.
5. Any loss must not exceed 6mm wg. If the pressure drops more than 6mm, the test fails.

Locating a failed joint:

• Apply soapy water (washing up liquid diluted) to every joint with a brush or sponge
• Bubbles form at the leak point
• For fittings behind walls, smoke testing is more effective as smoke exits at visible points

Common failures:

• Push-fit trap connections not fully engaged
• Solvent weld joints with missed surfaces or contamination
• Rubber ring in-ground connectors not seated correctly
• Test plug not expanding fully (especially on smooth-bore plastic pipe)

Below-Ground Water Test — Procedure

Equipment needed:

• Inflatable plugs for the downstream end of the test run
• A calibrated staff gauge or tape measure
• Water supply

Procedure:

1. Close off the downstream end with an inflatable plug seated in the drain at the lowest inspection chamber.
2. Fill the drain run with water from above, working upstream until the run is full to 1.5m head above the invert at the highest point.
3. Allow 30 minutes for initial absorption into the pipe joints and walls (vitrified clay and concrete absorb some water initially; HDPE and PVC less so).
4. Top up to the required level after the absorption period.
5. Mark the water level precisely (tape to the inspection chamber wall, or use a staff gauge).
6. Observe for 2 hours. The level should not drop visibly.
7. Record start and finish levels and temperature.

Acceptance: For PVC-U, HDPE, and vitrified clay, a small level drop may be acceptable within the BS EN 1610 tolerance: ≤ 0.05 litres per m² of internal wetted surface per 100m of pipe length — this is very small and usually imperceptible. Any visible drop at the scale of a normal domestic installation should prompt investigation.

Locating a failed joint below ground:

• Expose the length of pipe between the two nearest inspection chambers
• Visually inspect all joints for water seeping out
• Introduce dye at the upstream end if the escape path is not obvious

CCTV Survey — When and How

A CCTV drain survey is not a pressure test — it is a visual inspection of the inside of an existing drain run using a wheeled camera head passed through from an access point.

When to use CCTV:

• Property purchase: check condition of existing drainage before buying
• Repeated blockages: identify root intrusion, joint failure, sag (belly), or blockage build-up
• After ground movement: check for displaced joints, pipe fractures
• Before extensive landscaping or building work that could affect drains
• Post-flood: assess silt, damage, root intrusion
• Insurance claim assessment: document condition

CCTV survey output:

• Video recording of the entire run
• Report graded to WRC MSCC4 standard (Marsh McCormick & Co manual)
• Defect codes: O (crack), B (broken), F (fracture), J (displaced joint), R (roots), D (deposition/blockage), G (ingress)
• Grade 1 (minor) to Grade 5 (structural failure)

A Grade 3–5 finding on a CCTV survey typically indicates the drain needs repair or lining. Drain lining (CIPP — cured-in-place pipe) can repair structural defects without excavation.

Smoke Testing for Leak Detection

Smoke testing is an alternative to air testing for above-ground drainage, or can be used in conjunction with CCTV for below-ground drainage venting checks. Non-toxic smoke is introduced at low pressure through a specialist machine.

Applications:

• Finding concealed joint failures in above-ground drainage that are hard to soap test (behind walls, above ceilings)
• Checking that drainage venting is working correctly (no negative pressure in the system)
• Finding where drainage connections enter a building (useful for older properties with unknown drainage layouts)

Smoke testing above-ground: Connect smoke machine at cleanout access point; plug all fixtures (block all traps or fill with water); apply smoke at low pressure; walk through building looking for smoke at light fittings, floor boards, around pipe penetrations.

Frequently Asked Questions

Does Building Control need to witness drain tests?

Yes, typically. Drainage is an inspection stage item under Part H. The BCO (Building Control Officer) or Approved Inspector should be notified before you backfill or cover drainage. In practice, many inspectors accept a signed test record rather than attending in person — confirm this in advance. If the inspector doesn't attend and problems arise later, a signed test certificate from the installer is your evidence of compliance.

Can I use air testing for below-ground drainage?

Yes, as an alternative to water testing. For below-ground drainage, the test pressure is the same (38mm wg for PVC-U and similar). Air testing is faster and easier to set up than filling a drain run with water, but it is harder to identify where a leak is occurring. For short, simple drain runs, it's a practical choice; for complex networks, water testing gives a more definitive result.

What is the gradient for a 100mm foul drain?

Approved Document H Table 1: the recommended gradient for a 100mm foul drain is 1:40 (2.5%), which is self-cleansing and provides adequate flow velocity to prevent solids settling. The minimum is 1:80 for runs up to 3m where the gradient cannot be achieved. Steeper than 1:25 (4%) can cause the liquid to outrun the solids and should not be used without approval.

Regulations & Standards

• Approved Document H (2002, 2015 amendment) — Building Regulations Part H: Drainage and Waste Disposal

• BS EN 1329-1 — PVC-U pipes for soil and waste drainage within buildings

• BS EN 1610 — Installation and testing of drains and sewers

• BS 8000-13 — Workmanship on site: below-ground drainage

• WRC MSCC4 — Sewerage Rehabilitation Manual / Manual of Sewer Condition Classification 4th Edition (drain survey standard)

• Approved Document H — GOV.UK free download

• BS EN 1610:2015 — Installation and testing of drains and sewers (BSI)

• WRCPLC CCTV Drainage Survey Standard — MSCC4 guidance

• underground drainage — Pipe sizes, gradients, and below-ground drainage design

• blocked drains — Diagnosing blockages and drain repair options

• leak detection — Finding concealed leaks in supply pipework

• building regs overview — When Approved Document H applies