Building design - waterproofing and drainage 

Root resistant waterproofing

It is vital that the waterproofing is robust and proven to deliver long-term root resistance, preventing plant roots and rhizomes from growing in to or through it.  Consideration must be given to the appropriate form of waterproofing so that it can meet the demands placed on it by the green roof, as such performance characteristics for durability and thickness of membrane (density), tensile strength (EN 12311-1) and elongation at break (EN 12311-1) need evaluation before appropriate selection can be made.

Bauder waterproofing membrane systems and liquid systems are manufactured in accordance with FLL guidelines for root resistance and carry BBA certification for green roof applications.

When designing  the detailing (e.g. upstands, pipe penetrations, rooflight upstands etc)  it is important that you take into account the increased build-up of the green roof construction. Building Regulations typically require the waterproofing detailing to finish 150mm above the finished roof surface i.e. the green roof surface not the surface of the waterproofing.  Our technical team can work with you to produce the correct detail drawings for your green roof project.

Bauder waterproofing for green roofs

Selecting the appropriate waterproofing system requires consideration so that its performance matches that required for the green roof.

Intensive green roofs would ideally use root resistant bituminous membranes or hot melt structural waterproofing systems as the nature of the access levels and maintenance regimes require a robust solution to ensure that the building is protected correctly. Extensive green roofs are generally possible on all forms of waterproofing that have root resistance including bitumen membrane systems, single ply, cold liquid applied and hot melt. The correct selection for waterproofing can often be determined because of the depth of substrate required to support the vegetation. The greater the depth of substrate, the more robust the waterproofing will need to be.

Creating falls on a green roof

The need for stability of the system is increased because the negative pressure forces that can develop during 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%. However, the UK’s National Annex to BS EN 12056 does not 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.

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