Can plastic-free materials replace drainage boards in green roof systems?

Eoin Sullivan

Test Methods
Practical experiments and
Technical research

For copy of full dissertation, contact:
Eoinsullivan1@gmail.com

Supervisors:
Jim Roche
David Knight


Using the characteristics outlined in the green roof design codes this study explores if  lightweight expanded clay aggregate is a viable replacement for standard plastic drainage boards.

The aim for this research project is to investigate if a plastic free material can replace the standard plastic drainage boards in green roof systems. This report’s proposed replacement material is LECA pellets. This report aims to show that the addition of the plastic free LECA pellets does not impact the efficiency of the green roof system in any of the areas outlined in the German FLL guides. The characteristics for determining the suitability of a drainage layer are as follows; “Water storage capacity, Filling volume, Flow rate, Weight (dry & saturated), Compressive strength.” (Green Roof Code of Best Practice for the UK, 2014) This report proposes if the above characteristics are met, and the performance of the system is not affected by the change of drainage layer then the plastic free environmentally friendlier option should be used.

This report will compare the proposed plastic free system to an existing extensive green roof system with a standard plastic drainage layer. This was achieved by using recognized design guidelines such as the German FLL guide 1, American FM guide 2 and the British GRO guidelines 3. The two system were compared using the five important characteristics that have been outlined in the design codes mentioned before. These characteristics are water storage capacity, flow rate, filling volume, dry/wet weight, compressive strength. The results of these tests showed that the proposed plastic free system functioned correctly under the criteria outlined in the design codes. This report also conducted an embodied carbon comparison of the existing and the proposed plastic drainage layer. The results showed that the proposed lightweight expanded clay aggregate (LECA) drainage layer produced significantly less embodied carbon than virgin high impact polystyrene (HIPS) used in the standard plastic drainage boards but slightly more than the recycled HIPS. 

The results of this report show that light weight expanded clay aggregate is a technically viable replacement for standard plastic drainage layer. This report also showed the proposed lightweight expanded clay aggregate drainage layer produced less embodied carbon than the virgin plastic drainage board but slightly more than the recycled plastic drainage board.