On roof and terrace gardens, most lost plants are not due to neglect but to a mistake in an invisible layer: drainage. A well-built drainage system is the most critical yet least noticed part of roof landscaping.
The layer system
A healthy roof garden is a layered system from bottom to top:
- Waterproofing and root-barrier membrane
- Drainage board (a hollow layer that evacuates excess water)
- Filter geotextile (prevents the soil from clogging the drainage)
- Lightweight, permeable growing medium
- Plant layer
Evacuating excess water
Standing water on the roof both drowns plant roots and poses a risk to waterproofing. Water collected by the drainage board should be directed with correct slope toward drains, which must remain accessible for maintenance.
Load and weight calculation
On a roof, the weight of soil, water and plants must match the structure's load capacity. That is why lightweight growing media and the correct depth are handled together with engineering at the start of the design.
Maintenance
Checking the drainage outlets and filter layer at the beginning of the season is the simplest way to prevent clogging. When this invisible layer is checked regularly, the roof garden lasts for years without problems.