What about their weight?

Any landscape feature on a roof will have loading implications and the saturated weight of any such features must be used to calculate the structural load. Below is a list of weights of various landscape elements, including green roof systems. This has been calculated according to standards outlined in ‘German National Standard DIN 1055 – Design Loading for Buildings’.

  • Gravel surface: 90-150 Kg/m2
  • Paving slabs: 160–220 Kg/m2
  • Vehicle surface: +550 Kg/m2
  • Extensive green roof: 60–150 Kg/m2
  • Intensive green roof: 200–500 Kg/m2

Some roof systems are covered in paving slabs to ballast the insulation. An extensive green roof system would have a negligible effect on the structural load in these instances, while providing the range of other benefits. The structural report for a commercial building in London allowed for an extensive green roof with a substrate depth of up to 8cm to be applied on top of the paving slabs without any negative structural implications. However, unless other roof systems are structurally strengthened, a simple sedum mat may well be the only feasible option.