What is foul drainage?

Foul drainage carries wastewater from toilets, baths, showers, basins, sinks and appliances to the public sewer or private treatment system. It must be kept separate from surface water drainage in areas with separate sewer systems. All foul drainage must comply with Building Regulations Part H.

What minimum cover depth do I need?

Part H specifies: 600mm under gardens and footpaths, 900mm under driveways and parking areas where cars may drive, and 1200mm under roads used by heavy vehicles. If minimum cover cannot be achieved, additional protection (concrete surround or ductile iron pipe) is required.

Where do I need inspection chambers?

An inspection chamber is required at every change of direction, change of gradient, junction with a branch drain, and at intervals not exceeding 45m on straight runs. You also need a chamber within 12m of the connection to the public sewer for rodding access.

What gradient should a foul drain have?

For 100mm pipe serving a single dwelling: minimum 1:40 (25mm fall per metre). For 150mm pipe or multiple dwellings: minimum 1:80 (12.5mm per metre). The maximum gradient is 1:10 to prevent water running ahead of solids.

Can I use a 45-degree bend instead of an IC?

Two 45-degree bends used to create a gentle sweep may not require an inspection chamber, provided a rodding access point is available nearby. However, any change of direction greater than 45 degrees requires an IC. Always confirm with your building control officer.

TradeCalculator

Foul Drainage Calculator — Pipe Routing, Cover Depth & IC Positions

Plan foul drainage pipe routing with inspection chamber positions, minimum cover depth checks, gradient fall and material quantities.

Total Run Length

Total pipe run from building to sewer

Pipe Diameter

Bends (>45°)

Each needs an inspection chamber

Direction Changes

Gradient or horizontal direction

Branch Connections

Where other drains join

Location (Cover Depth)

Price Per Metre (optional)

Installed cost per metre

How We Calculate This

This calculator plans your foul drainage system by determining inspection chamber positions, checking cover depths and calculating material quantities.

The formula

IC Positions = Bends + Direction Changes + Branches + 2 (head & end)

Total Fall = Gradient × Run Length

Pipe Runs = ceil(Length / 3m per length)

Minimum cover depths

Garden/footpath: 600mm
Driveway: 900mm
Road/highway: 1200mm

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.