The problem
A public forecourt in Copenhagen has roughly ninety frost days. Standing water in a stone channel through a freeze–thaw cycle does not crack the stone; it lifts the bedding, opens the joints, and two winters later the channel leaks into the basement car park below.
What we changed
The whole installation empties. A buried 30 m³ surge tank sits under the lower terrace, and the channel drains to it in six minutes flat — we sized the outlet for that figure, because anything slower means the controller cannot react to a forecast overnight frost within a single evening closing window. Below 3 °C on a rising forecast, the system runs down and stays down. It comes back up on its own when the ten-day forecast clears.
The channel invert has a trace heater in it, not to run the water in winter — that would be indefensible — but to stop the last 2 mm of residual film from freezing into the joints. It draws 340 W and runs perhaps 200 hours a year.
Five winters on
Joints were re-pointed in year three on two treads at the lower end, where the drainage fall was 0.4 per cent instead of the specified 0.8. That is a construction defect we should have caught on the survey. Nothing else has been touched. The library counts 190 days of operation a year and has never once asked us why it is off.
