Fall and drainage on paving

Fall is the slope of the surface. It is expressed in per mille (‰ = mm per m), percent (%) or cm per m. The relationship is simple: 10 ‰ = 1 % = 1 cm/m. Too little fall gives puddles; too much gives discomfort and run-off.

Suppliers typically recommend more conservatively than the standard's minimum: terrace 15 ‰ (1.5 cm/m) and driveway 20 ‰ (2 cm/m). Note both — the standard's minimum and the industry recommendation — and choose according to the project.

Fall directs water somewhere — and there must be something to direct it to. The common drainage components in green systems:

• 01Linear drain (slot drain): a long grate in the surface that collects water along a line — e.g. in front of a garage or at a terrace edge.
• 02Point drainage / catch basin: a single outlet with grate where falls from several directions meet.
• 03Grit trap: a pit with a sedimentation chamber that retains sand and gravel before water flows on — must be able to be emptied.
• 04Fascine: an underground cavity (cassettes or stones) where rainwater is stored and seeps into the ground.
• 05Drain pipe: perforated pipe that collects and drains water from the ground (minimum 3 ‰ fall in straight line, NfA 1992).
• 06Permeable surfacing: joints or stones that let water seep through instead of leading it away.

How much water must the system be able to drain? The runoff from an area is calculated from the size of the catchment, its runoff coefficient and the design rainfall intensity. On private land the intensity is set according to DS 432 or the Danish Wastewater Committee's publications (no. 27 on functional practice, no. 30 on climate factors). DS 432:2020 gives 150 l/s per ha for a 2-year rain (before climate factor); the older 140 l/s per ha is the DS 432:2009 level and still appears in some municipalities' rules. The municipality sets the binding value.

The runoff coefficient φ expresses what proportion of the rain runs off the surface (the rest evaporates or permeates). Indicative values from municipal design bases (Wastewater Committee practice):

Example — drainage from a terrace

A 100 m² tiled terrace: A = 100 / 10,000 = 0.01 ha, φ = 0.90 (dense paving), i = 150 l/s per ha (DS 432:2020, 2-year rain). Q = 0.90 · 0.01 · 150 = 1.35 l/s. That is the amount of water the channel drain or the gully must handle in the chosen rain. If the water is to soak away instead, the soakaway is sized by the soil's infiltration capacity and a climate factor (publication 30).