110mm Underground Drainage
Complete guide to laying 110mm PVC-U underground drainage including trench preparation, bedding, gradient calculation, and connecting to inspection chambers.
Safety Warning — Excavation Work
Trench work carries serious risks including collapse, service strikes, and manual handling injuries. Always check for buried services using a CAT scanner and plans before digging. Shore or batter trenches deeper than 1.2m. Wear appropriate PPE including steel-toe boots, hard hat, and high-vis. Never enter an unsupported trench.
What You'll Need
Tools
- Mini digger or hand-digging tools
- CAT scanner (cable avoidance tool)
- Laser level or spirit level and boning rods
- String line and line pins
- Hacksaw or pipe cutter
- File for deburring
- Tape measure and builder's line
- Whacker plate (for backfill compaction)
Materials
- 110mm PVC-U drainage pipe (3m / 6m lengths)
- Ring seal couplers
- Bends (15°, 30°, 45°, 87.5° as required)
- Inspection chambers / access points
- Pea gravel or 10mm granular bedding material
- Pipe lubricant (silicone-based)
- Marker tape (drainage warning tape)
- Selected backfill material
Technical Specifications
Step-by-Step Installation
1 Plan the Drainage Route
Plot the route from the building discharge point to the main sewer or soakaway. The route should be as straight as possible — use bends only where necessary, and never exceed 87.5° without an access point. Install an inspection chamber at every change of direction, change of gradient, and junction. Check Building Regulations Part H for minimum cover depths and access requirements for your specific application.
2 Locate Existing Services
Before excavating, scan the proposed trench route with a CAT (cable avoidance tool) and Genny signal generator. Request utility plans from the local authority or use the LSBUD service. Mark any detected services on the ground surface. Hand-dig trial holes to confirm service positions — never machine-dig near known services. This step is critical and non-negotiable.
3 Excavate the Trench
Dig the trench to the required depth, maintaining a minimum width of 410mm (pipe OD + 300mm). The trench bottom must be excavated to allow for 100mm of bedding material beneath the pipe invert. For foul drainage, calculate depth using the 1:40 minimum gradient — that's 25mm of fall per metre of run. For surface water, the minimum is 1:80 (12.5mm per metre). Keep excavated material away from the trench edge.
4 Prepare the Bedding
Lay a minimum 100mm bed of granular material (10mm pea gravel or granular sub-base) along the full trench length. Level and compact the bedding to establish the required gradient. For Class N bedding (standard domestic), the granular bed should extend 100mm below the pipe invert and be levelled to the correct fall. Use a laser level or boning rods to check the gradient at regular intervals along the trench.
5 Lay the Pipe
Start laying pipe from the lowest point (the discharge end) and work uphill. Clean each socket and spigot end. Apply silicone pipe lubricant to the spigot end and ring seal. Push the spigot firmly into the socket until it reaches the insertion mark — do not force past this point, as the gap allows for thermal movement. Check alignment and gradient after each pipe is laid.
6 Install Inspection Chambers
Position inspection chambers at every change of direction, junction, and at maximum 45m intervals on straight runs. Set the chamber base on a concrete bed (minimum 100mm C20 concrete). Connect pipes using the appropriate chamber adaptors. Ensure the benching (internal channel) is smooth and directs flow correctly. The chamber cover and frame should be set to finished ground level.
7 Surround and Initial Backfill
Once the pipe is laid and inspected, carefully place granular material around and over the pipe to a depth of 100mm above the pipe crown. Hand-compact this surround — do not use a whacker plate directly over the pipe. This protective surround prevents point loading from stones in the backfill. Place drainage marker tape 300mm above the pipe to warn future excavators.
8 Test the System
Before completing the backfill, test the system. For a water test, plug the downstream end and fill the system with water to a head of 1.5m above the invert at the upstream end. Allow 2 hours for absorption, then top up and monitor for 30 minutes — any loss greater than 0.05 litres per metre of run indicates a leak. Alternatively, perform an air test using a manometer. Building Control may require witnessing the test.
9 Complete Backfill
Backfill the trench in 150mm layers, compacting each layer. Use selected backfill material free from large stones, debris, or frozen material. Where the trench passes under driveways or areas subject to vehicle loading, use concrete surround protection (Class S bedding) or duct the pipe through a larger sleeve. Reinstate the surface to match surrounding levels.
Common Mistakes to Avoid
- Insufficient gradient — A foul drain at less than 1:40 will not self-cleanse, leading to blockages. Always verify the gradient before backfilling.
- No bedding material — Laying pipe directly on the trench bottom risks point loading and cracking. Always use 100mm of granular bedding.
- Pushing past the insertion mark — Over-insertion prevents thermal expansion and stresses the joint. Insert to the mark only.
- Skipping the service scan — Striking gas, electric, or water mains can be fatal. Always scan before digging.
- No access points — Every change of direction and every junction needs an inspection chamber or access fitting. Blocked drains without access are a nightmare.
- Using SN4 pipe under driveways without protection — Standard SN4 pipe needs concrete surround or Class S bedding under trafficked areas. Consider SN8 for additional strength.
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