Podium decks is a term I’m using here to describe a basement ceiling which also acts as a roof. This may be a subterranean roof/ green roof/ podium deck etc. I feel that this is an increasingly common design where a basement structure extends beyond the boundary of the building from ground level up.
This sort of structure requires special attention. Here are some reasons why:
- This sort of roof structure could become subject to hydrostatic pressure. Normal roofs should never have ponding; or certainly not ponds of several feet in depth.
- This sort of roof structure is incredibly difficult to access for repairs. Exposing this roof structure would require excavating soil/paths/patios etc. which is costly both in time and in reinstatement cost. Furthermore, there are difficulties around damaging the roof structure when exposing it
Waterproofing structures below ground largely has clear best practice laid out within BS8102 and other widely accepted documents. Podium decks; however, lacks this clearly articulated industry standard.
Here are some of the points I would raise as important features in good waterproofing design of subterranean roofs:
Primary waterproofing must be integrity tested
Standard practice on flat roofs with single ply waterproofing is to include integrity testing of the waterproofing prior to that layer being covered up. This testing is even more critical when the waterproofing layer will be covered up with earth/paving/insulation etc.
Electronic leak detection by IntegriTest
Reduce risk by preventing hydrostatic pressure
Wherever possible podium decks should be laid to fall, finishing in appropriate drainage such as a land drain; which of course must drain into a suitable outlet (eg. rainwater systems/ rainwater harvesting system/ attenuation tanks/ mains drainage etc.).
In order to promote drainage off the primary waterproofing layer a drainage layer should be included. Typical green roof products provide drainage as well as some surface water storage which helps reduce surface runoff rates. Critically important in this build up is the inclusion and detail around geofabrics to prevent silting of the drainage system. This drainage layer provides the added benefit of protection of the primary waterproofing from follow on trades.
Links to various sources of information on green roofs/ podium decks:
Platon products: This link is to a 7mm thick drainage membrane. The cuspated design stores some water and also provides a drainage layer.
MES Building Soutions: This link is to a consultant who provides calculations around water efficiency and rainwater harvesting
Oldroyd products: This link is to the green roof range of membranes supplied by safeguard.
IntegriTest: Third party independent integrity testing including electronic leak detection