Through purpose-built access points rather than through the slot. Removable sections, inspection boxes or catch pits are designed into the run at intervals, and jetting or rodding is carried out from those.

The spacing between them is the critical design decision, because it sets the maximum distance a jetting hose has to travel and therefore whether the run can be cleared at all.

Getting it wrong is expensive to correct: adding an access point later means lifting the surface the slot drain was chosen to keep unbroken.

It restricts intake rather than capacity. The channel below can be generously sized, but water has to get into it through the slot, and at high intensity the opening becomes the bottleneck.

Intake is a function of slot width and the length of slot available, so the usual response to a heavy design storm is a longer run rather than a deeper channel.

Surface fall matters more than with a grated channel too. Water has to be delivered to the slot, and a flat surface either side leaves it standing where the opening cannot reach it.

Where the surface finish is the point. Stone paving, resin-bound surfaces, polished concrete and architectural landscaping all lose something visually to a grating, and a slot reads as a line rather than a feature.

They also suit surfaces crossed by narrow-wheeled traffic and by pedestrians in heels, where a grating with wide apertures is a trip and catch hazard.

They suit leaf-laden environments least. A slot blinds quickly under organic debris, and a courtyard under trees will need more attention than a grated channel in the same position.

The same classification as any other surface channel, set by the traffic crossing it rather than by the drain type: pedestrian areas at the light end, through car parks and roads, up to heavy industrial and dock duty.

The slot edge takes concentrated load because it is a narrow interruption in the surface, so the edge detail and the material of the top section carry more significance than they do on a wide grating.

The surround counts toward that rating. A channel rated for road traffic bedded in unsuitable concrete performs to the bedding, not to the label.

Either, and the choice affects the depth. A channel laid to a built-in fall keeps velocity up and is self-cleansing, but it gets progressively deeper along the run, which the excavation has to accommodate.

A constant-depth run laid level is shallower and simpler to build, and relies on the surface fall and on more frequent outlets to move water away.

Long runs generally favour a built-in fall because velocity is what keeps silt moving, and a slot drain that silts up is harder to clean than one that does not.