Underground Drainage: 110mm and 160mm PVC Pipes, Fittings, and Installation to Building Regs Part H

Underground drainage is one of the most regulated and least forgiving parts of any construction or renovation project. Get the pipe sizing wrong, miss a rodding point, or lay to an insufficient gradient and you'll face expensive remediation — and potentially a Building Control rejection. This guide covers everything a plumber or groundworker needs to know about plastic underground drainage systems: pipe standards, sizing, gradients, inspection chambers, backfill, and compliance with Approved Document H.

Pipe Standards and Materials

All plastic underground drainage pipes for foul and surface water in the UK should comply with BS EN 1401-1 (unplasticised PVC — PVC-U) or BS EN 13476 (structured wall pipes). Ring-seal (push-fit) jointing is the norm for drainage; solvent weld is used for above-ground connections to the underground system.

The two dominant nominal diameters for domestic underground drainage are:

• 110mm (DN100) — used for all WC/toilet connections and combined foul runs from residential properties. This is the minimum bore for any run that receives WC waste.
• 160mm (DN150) — required for runs serving multiple dwellings, larger commercial connections, or where the calculated hydraulic load exceeds 110mm capacity.

PVC-U underground drainage pipes are available in standard lengths of 3m and 6m. They are typically supplied in brown or terracotta for foul drainage and black for surface water, though colour coding is not mandatory — labelling and documentation matter more on site.

Ring-Seal Jointing

Underground PVC drainage uses ring-seal (push-fit) rubber ring joints rather than solvent-weld jointing. The rubber ring sits in a socket at the end of each fitting or pipe, and the plain spigot end is pushed in with a slight twisting motion to compress the ring and form a watertight seal. This joint accommodates thermal movement and slight angular deflection — important in ground that can shift seasonally.

Key jointing points:

• Lubricate the spigot and ring with the manufacturer's joint lubricant only — never petroleum jelly or washing-up liquid.
• Mark the insertion depth on the spigot before pushing home so you can verify the joint is fully seated.
• Leave a 10mm expansion gap by pulling the pipe back slightly from the full-home position — this prevents buckling if the pipe expands in warm weather before it is covered.
• Check that the rubber ring has not been rolled or kinked during assembly — a twisted ring causes joint leakage under test.

Pipe Gradients

Gradient is critical to self-cleansing. Too shallow and solids settle; too steep and water runs ahead of solids, leaving them stranded. The Approved Document H guidance (and BS EN 752) sets the following for PVC-U foul drains:

The 1:40 gradient for 110mm is the standard domestic design gradient and is self-cleansing for typical residential flows. For surface water drainage, gradients can be shallower — but always check the approved drainage scheme for the site.

Practical tip: On long runs with a restricted fall, a stepped gradient using a 45° or 90° junction to drop level is preferable to running at 1:80 throughout, which risks surcharging at peak flow.

Fittings: What You Need

Underground drainage fittings are socketed at one or both ends to accept the plain-end pipe. Common fittings include:

• Bends: Available in 15°, 30°, 45°, and 90°. Long-radius 90° bends are preferred for foul drainage — sharp 90° bends should be avoided on runs that may need rodding. Use two 45° bends with a short straight between them where possible.
• Junctions (tees): Single 45° junctions are standard for connecting branch drains. Double-branch (Y-junctions) allow two branches on the same fitting. Always ensure flow is in the direction of the arrow marked on the fitting.
• Reducers: Allow 160mm pipework to reduce to 110mm, or connect 110mm soil pipe to 160mm drains.
• Couplers: Plain-ended pipe lengths can be joined with a slip coupler. Essential when cutting a pipe on site and the original socket is removed.
• Plugs and stoppers: For pressure testing individual branches and temporarily sealing unused connections.

Inspection Chambers and Rodding Points

Approved Document H requires that all underground drains are accessible for inspection and clearance of blockages. An inspection chamber or rodding point is required:

• At every change of direction greater than 45°
• At every change of gradient
• At junctions between drains
• At intervals not exceeding 45m on straight runs (22m for drains deeper than 600mm)
• At the head of every drain run

Inspection chambers are prefabricated polypropylene units available in 315mm, 400mm, and 450mm diameter. They consist of a base unit with pre-moulded channels (benching), riser sections in 100mm or 200mm height increments, and a cover and frame. The cover specification depends on use:

• A15 (green or light duty) — for pedestrian traffic only (gardens, paths)
• B125 (grey cast iron) — for vehicle access up to 12.5 tonnes (driveways, car parks)
• C250/D400 — highways/heavy vehicle use — rarely specified on domestic jobs

Rodding points are a simpler alternative where full inspection is not required. A 110mm rodding eye with a screw cap is fitted at the head of each branch. They take up less space than a full inspection chamber and are acceptable where access for cleaning rods (not a camera) is sufficient.

Depth and Cover

Approved Document H sets minimum cover depths to protect the pipe from damage and freezing:

• Under gardens/landscaped areas: minimum 600mm cover over the pipe crown
• Under driveways (light vehicle traffic): minimum 900mm, or 600mm with concrete surround or approved protective slabs
• Under roads (adoptable or public): minimum 1,200mm, subject to highway authority requirements

Where achieving minimum cover is impractical — for example at a shallow connection to a manhole — protect the pipe with a concrete slab, reinforced haunching, or a proprietary steel cover plate.

Bedding and Backfill

Plastic drainage pipe must be properly bedded and surrounded to prevent point loading and crushing. The standard domestic detail (Bedding Class D) uses:

1. Bed: 100mm of 6–10mm single-size aggregate (or sharp sand) below the pipe invert, graded and levelled
2. Haunch: Aggregate brought up to the pipe centreline, compacted by hand each side to avoid disturbing the pipe
3. Surround: Aggregate to 150mm above the pipe crown
4. Backfill: Selected granular fill or site-won material free of stones larger than 40mm, compacted in 150mm layers

Do not use heavy mechanical compaction directly above the pipe until at least 600mm of cover has been placed. On projects under or near trees, specify 10mm pea gravel throughout to allow root infiltration without point loading the pipe.

Hydraulic Testing

Approved Document H requires that every new drainage installation is hydraulically tested before backfilling. The standard test is a water test to BS EN 1610:

1. Seal the downstream end of the drain with an inflatable stopper or end cap
2. Fill the pipe to a head of 1.0m above the highest point of the section being tested
3. Allow 1 hour for pipe absorption
4. Top up to the 1m head mark, start timing
5. After 30 minutes, the drop in head must not exceed 6mm (equivalent to approximately 0.5 litres/m run for 110mm pipe)

Record and retain test results — Building Control will typically want to see evidence of a passing test. Where a water test is impractical (insufficient head, risk of saturating ground), a low-pressure air test is an acceptable alternative under BS EN 1610 Clause 9.

Connections to the Public Sewer

Connecting to an adopted public sewer requires a Section 106 consent from the sewerage undertaker (Thames Water, Severn Trent, etc.) under the Water Industry Act 1991. The application should be made well before groundworks start — consent can take 6–8 weeks. The sewerage undertaker specifies the connection point, pipe diameter, and invert level, and will inspect (and sometimes make) the actual connection into the sewer barrel.

For surface water drainage, the connection hierarchy under building regulations is:

1. Soakaway (preferred if ground conditions allow — BRE Digest 365 design)
2. Watercourse (if a suitable watercourse is within reach — requires Environment Agency or LLFA consent if culverting)
3. Surface water sewer (with sewerage undertaker consent)
4. Combined sewer (last resort — many sewerage undertakers will refuse new surface water connections to combined sewers)

Never connect surface water drainage to a foul sewer without consent — it is an offence under the Water Industry Act and causes combined sewer overflows (CSOs).

Soakaway Design to BRE Digest 365

Where ground conditions are suitable, a soakaway is the preferred disposal method for surface water from roofs and hard standings. BRE Digest 365 requires:

• A soil percolation test (VP test) to determine the soil infiltration rate
• Soakaway sized to contain a 1-in-10 year, 6-hour storm event without surcharging
• Minimum 5m from any building foundation and 2.5m from a site boundary
• Bottom of the soakaway at least 1m above the seasonal high groundwater table

Pre-formed plastic crate soakaways (e.g. Nidaplast Stormcrate or similar) are now preferred over rubble-filled pits — they offer predictable void ratio, are easier to size, and are less prone to silting.

Common Faults and How to Avoid Them

• Roots in joints: Ring-seal joints are not root-resistant. On sites with mature trees, specify root-resistant solvent-weld jointed systems in the affected zone, or install a root barrier membrane.
• Bellying (sag in pipe): Caused by inadequate bedding or differential settlement. Always bed on aggregate — never on clay or lumpy spoil. Check the string line gradient before backfilling.
• Cross-connections: Surface water accidentally discharged to foul sewer (or vice versa). Colour-code pipes on site, photograph before backfilling, and carry out a CCTV trace-dye test if there is any doubt.
• Surcharging: Drain running full and backing up. Usually caused by under-sizing, excessive bends, or blockage. Install correctly sized pipes and keep junction angles in the direction of flow.
• Unseated joints: Failure to fully push joints home or skipping the expansion gap mark. Always mark insertion depth on the spigot and check every joint before backfilling.

Key Regulations and Standards

• Building Regulations Approved Document H (2002, as amended) — Drainage and Waste Disposal
• BS EN 1401-1 — PVC-U pipes for non-pressure underground drainage
• BS EN 13476 — Structured wall thermoplastics pipes
• BS EN 1610 — Construction and testing of drains and sewers
• BS EN 752 — Drain and sewer systems outside buildings
• Water Industry Act 1991 — s.106 connection to public sewer; s.110 prohibition of trade effluent to foul sewer without consent
• BRE Digest 365 — Soakaway design

Summary

Underground drainage is straightforward when planned correctly and installed to Building Regs Part H. Use 110mm pipe at 1:40 for standard domestic foul runs, provide inspection chambers at every change of direction, bed in 10mm aggregate, test to BS EN 1610 before backfilling, and obtain Section 106 consent before connecting to the public sewer. Get these fundamentals right and you'll pass Building Control first time.