Inspection Chamber Calculator — Depth, Risers & Cover Class

An inspection chamber is maintained from ground level using rods rather than by entry. A 450mm chamber is limited to 1.2 metres deep under Approved Document H before the cover opening must be restricted (350mm reducing ring), and to around 3.0 metres with that restriction fitted. A manhole is larger and designed for a person to enter (1000mm-plus, with step irons or a ladder). Building Regulations Part H groups both as ‘access points’. For domestic work, most chambers are 450mm diameter plastic inspection chambers. See our Manhole Calculator for deeper installations requiring step irons and larger covers.

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.

Yes. Benching (the sloped surface either side of the channel at the base) is required to direct flow and prevent debris accumulation. Approved Document H para 2.53 requires channels and branches to be benched up at least to the top of the outgoing pipe at a slope of 1 in 12, rounded at the channel with a radius of at least 25mm. Pre-formed bases with integral benching are available for 450mm and 315mm chambers.

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 pre-formed base unit height and the benching height, then divide by the riser height and round up to the next whole ring.

Approved Document H Table 11 requires a 450mm inspection chamber to have its cover opening restricted once the depth to invert exceeds 1.2 metres — a 350mm reducing ring (or a deep cover with a maximum 300×300mm clear opening) deters anyone, particularly children, from entering. With a 350mm restricting ring fitted, a 450mm chamber can extend to around 3.0 metres deep. At 1.2 metres or shallower, no restriction is required.