ACO Drain

A threshold drain, sometimes called an ACO drain after a common manufacturer's name (used generically the way "Hoover" is for vacuum cleaners), sits just outside the door on modern buildings to stop surface water tracking in through the threshold.

Historically, door thresholds were raised above the external ground level to keep water out, but a raised threshold is an obstacle for wheelchair users. A modern, level threshold needs another way to keep water out, so the ground is given a slight fall away from the door instead, with a threshold drain to collect whatever surface water reaches it.

The channel itself is usually topped with a metal grating that takes foot traffic while still letting water through, channelling it into a connected drainage pipe that carries it away from the area.

Set into paving immediately outside external doors, and elsewhere surface water needs collecting from a hard-landscaped area before it reaches a building or a lower point in the site.

Channel width and grating load rating are chosen from the manufacturer's range to suit the paved area and the traffic it needs to carry (pedestrian only, or vehicular). The connected pipe run and fall (slope) to the drainage connection are designed as part of the site's overall surface water strategy, not decided at the channel alone.

The grating itself can be chosen for looks as well as performance: a fine "stiletto-safe" pattern, for instance, keeps narrow heels from catching in the slots.

A drainage channel is only useful if it's laid to a genuine fall towards its outlet and the pipe run beyond it actually has somewhere to discharge to. Getting the channel right while the downstream drainage is inadequate just moves the water problem a short distance away.

• Pavers / Paver slab: the drain sits within the paved area, flush with its surface
• Cill: the drain sits just outside the threshold the cill forms