Green roof drainage

What the board is doing up there

The board has one job on a planted roof: keep a void open under the growing medium so water can travel to an outlet. Everything else in the build-up depends on that void surviving.

That is why the compressive figure matters as much as the flow. Soil is heavy, and it is heaviest when it is saturated, which is exactly when the board is carrying the most load. A board that flattens has not failed in any visible way. It has simply stopped draining, and the water it was meant to move is now sitting on the waterproofing.

Where the water goes

Drainage only works if the water has somewhere to go. Three details decide that.

The outlet. An outlet sized for the drained area, and kept clear of the growing medium, is what turns a wet board into a working one. A board that drains well into a blocked outlet behaves like no board at all.

The edge. At a parapet or an upstand the board has to turn up without leaving a gap that the growing medium can wash into. The detail at the edge is where most planted roofs leak.

The filter. A separation layer above the board keeps fine particles out of the void. Without it the void fills over time and the drainage path closes from the top down.

What to settle before ordering

The board profile follows from the build-up rather than from a catalog page. Decide what sits above the board and what that layer weighs when saturated. Decide where the water leaves and whether that outlet is sized for the area. Decide how panels overlap and what holds them before the next layer goes down. Those three answers fix the board.