Landscape and surface drainage is the system that collects and removes rainwater standing on gardens, car parks, roads and paved areas. Its purpose is to prevent surface ponding and to keep water away from the building and its foundation.
When it is needed
Water standing for days in the garden or car park after rain. Surface water collecting at the base of the building and running towards the foundation. Cracks in paving caused by freeze and thaw. Water pooling at basement entrances, ramps and garage doors. Plant roots rotting from sitting permanently in water.
System components
Linear channel gratings
Used at ramp entrances, in front of garages and across large paved areas. The load class is chosen for the location. A pedestrian class grating where vehicles pass will break quickly.
Point gullies
Placed at low points where falls converge. Types with a leaf and silt basket make maintenance easier.
Underground collection line
Carries water from gratings and gullies to the discharge point. Fall and diameter are set according to the catchment area.
Soakaway or sewer connection
Collected water is either connected to the municipal line or, where the ground allows, released to the soil through a soakaway. Soakaways do not work in clay soils.
Garden drainage
In planted areas a perforated pipe and filter layer are placed below the root zone. This prevents root rot and preserves the bearing capacity of the ground.
What governs the design
Surface fall. Water must reach the grating by itself. A grating set into a level surface is only a decorative line.
Catchment area. How many square metres drain to that line? Pipe diameter and grating capacity follow from this.
Ground permeability. In clay, water does not soak away and must be carried entirely by the line. In sandy ground, infiltration solutions become viable.
Frost depth. The line must sit below the local frost depth. Otherwise it freezes in winter and water stays on the surface.
Maintenance access. Without inspection chambers and removable gratings the line cannot be cleaned. Leaves and silt accumulate in every system.
Common mistakes
Setting a grating into a surface with no fall. Connecting rainwater downpipes to the foundation drain. Building a soakaway in clay. Choosing the wrong load class. Providing no inspection chamber. Laying the line before resolving the discharge point.
Frequently asked
Can it be added to an existing garden
Yes. Channels are cut in paved areas and trenches excavated in planted areas. Some landscaping may need to be reinstated.
Can it share a line with foundation drainage
No. Surface water flow is high and would flood the foundation drain. The two systems must run separately.
If water stands in your garden or car park, an on site assessment of falls and discharge options will determine the right system.