Structural Issues

One of the perceived barriers against green roofs is the structural implication to the building. A study in London contacted a number of professionals and asked them to agree with the statement: ‘the physical structure of the many buildings prevents the establishment of green roofs’.

These were the responses:

What is interesting here is that 92% of developers had concerns and yet only 27% of engineers had concerns? A number of structural engineers the author of this report has talked to have noted that it is not as much of a problem in theory but that many engineers are unfamiliar with green roofs and tend not to want to deal with things they are not familiar with.

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 table of saturated weights of various landscape elements including green roof systems. This have been obtained from Zinco International and calculated according to standards outlined in ‘German National Standard DIN 1055 – Design Loading for Buildings.

The load values are for saturated weights.

Gravel Surface: 90-150 Kg/m²

Paving slabs: 160–220 Kg/m²

Vehicle Surface: from 550 Kg/m²

Extensive Green roof: 60–150 Kg/m²

Intensive green Roof: 200–500 Kg/m²

It is interesting to note that many inverted roofs are covered in paving slabs [hard landscaping] in order to ballast out the insulation. The use of an extensive green roof system would have a negligible effect on the structural load in this instance.

Recently a structural report for a current commercial building in London allowed for an extensive green roof to be applied on top of the paving slabs of an inverted roof without any negative structural implications. The depth of substrate to be used is between 2-8cm.

Roofs designed as single ply are likely to be less structurally robust than inverted systems. Unless such systems are structurally strengthened, a simple extensive sedum mat may well be the only option.