Falls and Drainage

Creating falls on a green roof

Intensive green roofs can be safely installed on horizontal decks, whereas with extensive green roofs minimum falls of 1:60 and above are preferred. The criteria is to have a depth of drainage layer deep enough to hold the landscape above any residual standing water that occurs on the surface of the deck, either by design or circumstance.


Roof drainage designs should comply with the requirements of BS EN 12056-3:2000 Gravity drainage systems inside buildings.

The soft landscaping on a green roof will retain a large percentage of the average annual rainfall, as for example an intensive green roof can retain up to 90%. The UK's National Annex to BS EN 12056 does permit the use of a coefficient to factor down the drainage infrastructure to take account for factors such as the retention performance of green roofs.

When specifying internal outlets, we would always recommend a minimum of two outlets per roof, regardless of roof size, as a precaution against accidental blockage.

All outlets should be protected by an inspection chamber with removable covers to allow access for maintenance, and be surrounded by a pebble vegetation barrier to prevent encroachment.