Weight loading

The load bearing capacity of a building must be known before planning a green roof, wall or facade. A structural engineer’s advice is essential to ensure comprehensive design development, based on the building’s construction, condition and weight loading capacity.

For retrofitting a green roof, wall or facade, it is important to establish early whether the installation will meet the existing structural capacity of the building, or whether this will be modified to support the installation. In some instances, it is possible to strengthen an existing roof in strategic areas (and not across the whole roof) in order to achieve the design outcome while also minimising costs.

For a green roof, wall or facade, the loads that the building structure must support include:

  • Dead load – the final constructed weight of all built elements and all components associated with the roof or wall assembly, including plants, growing substrate and any water held in the system
  • Live load – the weight of people who will use the space, and of any mobile equipment that will be used periodically on the site, for example, maintenance (live load generally applies to green roofs, not facades or walls, however it would be appropriate on a vertical surface if a trafficable maintenance platform was built into the system)
  • Transient load – moving, rolling or short-term loads, including wind and seismic activity

It is important to consider not just the weight of plants when planted but their weight at maturity, especially where shrubs and trees are proposed, as these are likely to be significantly heavier over time. The weight of saturated plants and substrate must also be included in the load assessment. Some example weight loadings of plants are provided below.

Damage to a wall can arise from wind forces, plant load, cable tension, and human access. This is particularly important where older walls are being used and where there is a large surface area of green facade (that is, wind uplift).

Weight loadings for some representative climbing species

Green facade species

Weight loading (kg/m2)

Jasminum (Jasmine), Rosa (Rose)

6-12

Clematis (Clematis), Tropaeolum (Flame Nasturtium)

3-12

Vitis (Ornamental Grape), Ampelopsis (Porcelain Vine)

12-26

Lonicera  (Honeysuckle), Actinidia (Kolomitka), Wisteria (Wisteria)

10-26

 

Source: Jakob Rope Systems. Note: these are figures from the Northern Hemisphere, where the effective growing season is shorter than Australia’s. Weight loadings are therefore likely to be an underestimate.

 

Green roof vegetation weight loadings

Green roof vegetation type

Weight loading (kg/m2)

Low herbaceous (succulents and grasses)

10.2

Perennials and low shrubs up to 1.5 m

10.2-20.4

Turf

5.1

Shrubs up to 3 m

30.6

Small trees up to 6 m

40.8

Medium trees up to 10 m

61.2

Large trees up to 15 m

150

 Source: FLL Guidelines

 

Some load-bearing capacities used in design of Melbourne green roofs and walls

The University of Melbourne’s demonstration green roof is designed for use by visitors and staff. It is built on a heritage-listed brick building in Burnley. It has areas of deep and shallow substrate and is designed for a dead load of 300kg/m2 and a live load of 300kg/m2 on the decking and walkway mesh areas, and 150kg/m2 on the planting area.

The Venny green roof has shallow substrates on two types of roof, the first roof is on shipping containers, and is designed for a dead load of 250kg /m2 and live load of 100kg/m2. The second trussed roof is designed for a dead load of 160kg/m2 and a live load of 40kg/m2.

Photo by Andrew Wutke

The Venny green roof, Melbourne. Photo by Andrew Wutke

The Triptych external green wall is designed for a dead load of 80kg/m2.

The Triptych green wall, Southbank