Because the step was doing the work. A raised threshold physically holds back surface water and wind-driven rain, and removing it for accessibility removes that barrier.

Accessibility requirements have made flush thresholds the norm at principal entrances, so the water has to be intercepted rather than blocked. A channel directly outside the opening does that.

It is also a weatherproofing junction, not just a drainage one. The channel works with the damp proof course and the door threshold detail, and a channel installed without regard to those simply moves water into the build-up instead of over it.

It arrives horizontally and it arrives at the door rather than where the paving falls lowest. Paving falls that cope with rainfall perfectly well in still conditions do nothing about water thrown against a face.

That water runs down the door and the surrounding wall to the base of the opening, which is why the channel is placed hard against the threshold rather than a comfortable distance away.

Exposure decides how much of it there is. An entrance on the windward face of an exposed building collects far more than a sheltered one, and the channel is sized for that rather than for the plan area in front of the door.

Shallower than a standard channel, which is the reason a specific product exists. There is rarely depth available at a doorway: the slab, the damp proof membrane and the threshold detail are all in the way.

Shallow channels recover capacity by being wider and by outletting more frequently, since they cannot achieve it with depth. That means more connections to the drainage below.

Where depth is genuinely absent, the alternative is a channel discharging sideways into a deeper run beyond the door rather than downward beneath it.

More than anywhere else, because everyone entering the building crosses it. Apertures have to be small enough that a stick tip, a heel or a small castor cannot drop in, and the surface has to remain slip-resistant when wet.

It is also the most looked-at drain in the building, positioned at the entrance, so appearance and the tidiness of the junction with the paving carry real weight.

Accessibility guidance on aperture size and orientation applies directly here. A grating with slots running across the direction of travel is the usual requirement so wheels and sticks cross rather than enter.

Where it interrupts a thermal envelope at the door, yes. A metal channel bridging from outside to the heated floor build-up is a straightforward cold bridge, and it sits exactly where condensation is least welcome.

Insulated versions carry a thermal break within the channel body or are bedded into an insulated surround, keeping the cold outer face separate from anything connected to the internal slab.

In an unheated threshold, an external porch or a service entrance, this is not a concern and a standard channel is appropriate.