Drainage

Drainage in a building project is not about the soil; it is about the hydrostatic pressure exerted on the waterproofing. Every litre that is not removed in a controlled manner from the basement wall or the ground slab ends up putting pressure on the membrane or seeking out a crack in the concrete. This category comprises two technically distinct families: dimpled drainage membranes (geomembranes with conical profiles of 7.5–20 mm, made from HDPE or recycled HIPS, with or without a pre-bonded filter geotextile) and SN4 double-walled perforated drainage pipes, with or without an integrated geotextile sleeve. The former collect and convey water vertically or horizontally; the latter carry it away from the site. On a green roof or garden laid over a concrete slab, the requirements change: the drainage layer determines whether the roots will breathe or suffocate due to waterlogging. At the same time, it determines three factors of direct interest to the garden specifier — how much height it takes up within the build-up, how much weight it adds per square metre, and how much water it retains for irrigation. This is why the choice of dimple height (5 to 20 mm) is not a question of strength but of available thickness and water management. Both types fulfil three functions simultaneously: drainage, mechanical protection of the waterproofing layer during backfilling, and the creation of an air gap to allow moisture to escape.