How to Calculate Gradient

Steep sites require careful consideration of the contours for an appropriate design response. This typology aims to minimise the amount of cut and fill needed to accommodate the slope. Building designs that follow the natural contours of the site and minimise the need for extensive foundations levels and retaining walls will be encouraged on sloping sites. This will ensure the minimisation of unusable space and contributes to the cost efficiency of the building, whilst minimising disturbance to the landform. Three environmental strategies often used on steep sites are to:

  • Balance cut and fill
  • Avoid excessive retaining walls
  • Build along contours

Some Development Plans respond to the inherent difficulties of building on sloping land by linking site area guidelines to the gradient of the land.
In calculating gradient the following techniques are typically applied:

Average Approach

The distance from the front boundary to the rear boundary is calculated and then divided by the difference in the ground levels experienced at corresponding mid points along the front and rear boundaries.

e.g. 45 m (distance calculation) / 1.5 m (ground level height difference) = 30 (expressed as a 1 in 30 fall)

Site Characteristic Approach

In some cases the land will not experience an even fall (i.e. a cross fall, or a valley, or existing bench levels that are different from natural ground levels). In these circumstances a more practical approach to defining the gradient can be applied. In the ERD Court decision of Simmons v City of Mitcham [2010] SAERDC 37 (starting at para. 43) the gradient was described in “patches or segments” to provide a useful method in determining the acceptability of slope for practical application and an assessment of functional needs (access, parking, private open space use, building siting and excavation-cut/fill required etc).