Geotextile and drainage sheet often appear in the same waterproofing quote, but they serve different, complementary functions and are not interchangeable.
Geotextile: mechanical protection
Geotextile is a non-woven fabric laid over the waterproofing to protect it mechanically against puncture (stones, roots, the very weight of the soil on planted roofs) during backfilling or the laying of subsequent layers. It adds no waterproofing itself: its function is to prevent the waterproof layer from being damaged.
Drainage sheet: evacuating the water that arrives
The drainage sheet (with a studded or dimpled profile) creates a cavity that lets water flow and drain towards a drainage point, instead of accumulating against the wall or the waterproofing, reducing the sustained water pressure on that layer.
Why they are used together in many projects
On planted roofs or buried walls it is usual to combine waterproofing, protective geotextile and drainage sheet in the same system, each layer with its specific function. Omitting one of them to cut the budget usually shortens the life of the main waterproofing, the layer that is most expensive to repair if it fails.
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