Because it decides whether the box reaches at all. The valve sits at whatever depth the main was laid to, and the cover has to finish flush with a surface that may not exist yet when the box is ordered.

Get it short and the box is buried below the finished level, which means excavating to find it every time the valve is needed. Get it long and the cover stands proud, where traffic will find it.

This is why adjustable and extendable designs dominate: they absorb the difference between the design level and the level that was actually built.

The base straddles the valve so that soil load and traffic load go around the valve rather than onto it — a valve carrying the weight of a road above it will not operate.

The shaft keeps a clear route from the surface to the operating nut, so a key can reach it without excavation.

The cover closes the opening at ground level and takes the surface loading. The three are sized together, which is why a set is bought as a set rather than assembled from loose parts.

The box, and that is its structural purpose. A properly installed set transfers wheel loads through the base and into the surrounding bedding, bridging over the valve.

Where the base is missing, undersized or badly bedded, that load reaches the valve body and the pipe joint beneath it. The failure shows up as a valve that will not turn, or a leak at the joint, long after the box was installed.

It is also why the cover class has to match the road it sits in rather than the pipe it serves.

How the height is adjusted. A screw type threads the upper section into the lower and is adjusted by turning, giving fine control and holding its setting.

A slide type telescopes and is set by position, which is quicker on site and more tolerant of debris in the thread, but can settle if it is not bedded properly.

Screw types suit finished surfaces where the level is known and precision matters; slide types suit rough ground and situations where the box may need moving again.

Cast or ductile iron where traffic loading is real, because it takes repeated wheel loads without deforming and can be recovered and reset.

Polymer and composite sets are lighter, do not corrode and are much easier to handle, which matters in a verge, a footway or a landscaped area.

The deciding factor is usually the surface rather than the pipe: the same valve under a carriageway and under a grass verge wants different boxes.

Often, and it is normal practice. Resurfacing raises the finished level, so the box is either extended or the upper section is reset to the new height.

That is what the adjustment range exists for, and it is the most common reason a box is disturbed at all.

Where the new level exceeds the adjustment available, an extension piece is added rather than replacing the whole set — see the extensions and risers alongside these.

By a base that seals reasonably against the valve body and a cover that seats properly, so the shaft stays a clear void rather than a hole full of earth.

A box packed with soil is functionally the same as no box at all: the key cannot reach the nut and the valve has to be dug out.

Marker posts, correct bedding and prompt replacement of missing covers do more for this than any feature of the box itself.