What is the difference between a manhole and an inspection chamber?

An inspection chamber is typically up to 1 metre deep and can be maintained from ground level using rods. A manhole is deeper and large enough for a person to enter. Building Regulations Part H uses the term 'access point' for both. For domestic work, most chambers are 450mm diameter plastic inspection chambers.

What cover class do I need?

A15 (1.5 tonne) for gardens and pedestrian areas. B125 (12.5 tonne) for domestic driveways where cars will park or drive over. D400 (40 tonne) for roads and areas used by heavy vehicles. Using the wrong class risks collapse under load, so always check the expected traffic.

Do I always need benching in an inspection chamber?

Yes. Benching (the sloped surface either side of the channel at the base) is required to direct flow and prevent debris accumulation. The benching should slope at 1:6 towards the channel. Pre-formed bases with integral benching are available for 450mm and 315mm chambers.

How many risers do I need?

The number of risers depends on the chamber depth (cover level minus invert level) and the riser height (typically 150mm for 450mm chambers). Subtract the base unit height and cover frame depth, then divide by riser height and round up.

When do I need a reducing piece?

A reducing adaptor is needed when the chamber diameter is larger than the cover frame. For example, a 450mm chamber often uses a 450-to-350mm reducer to fit a standard 350mm cover and frame. This is standard practice for keeping the chamber accessible whilst using a standard cover size.

TradeCalculator

Inspection Chamber Calculator — Depth, Risers & Cover Class

Calculate inspection chamber depth from invert level, riser count, benching requirements and cover class for domestic drainage to Part H.

Cover Level

Ground / finished level at chamber

Invert Level

Bottom of pipe at chamber

Chamber Diameter

Cover Class

Number of Inlets

Pipes entering the chamber

Price Per Chamber (optional)

Base price before cover class adjustment

How We Calculate This

This calculator determines the components needed for a plastic inspection chamber based on cover and invert levels.

The formula

Chamber Depth = Cover Level - Invert Level

Risers Needed = ceil((Depth - Base Height - Frame Depth) / Riser Height)

Components

Base unit: Pre-formed with benching and inlets
Risers: 150mm height increments (450mm dia)
Reducer: If needed to match cover size
Cover & frame: Rated to loading class

Frequently Asked Questions

Related Calculators

Manhole Calculator

Calculate manhole rings, benching, cover and frame for inspection chambers.

Land Drain Calculator

Calculate perforated pipe, gravel and geotextile for land drainage systems.

ACO Drain Calculator

Calculate channel drain lengths, end caps, outlets and gratings.

Sewage Treatment Plant Calculator

Size a sewage treatment plant by population equivalent per Building Regulations.

Septic Tank Sizing Calculator

Calculate septic tank capacity based on occupants and General Binding Rules.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.