How high should a slab be above the adjacent ground level?

We often get asked this question in our industry. The Building Code of Australia breaks down the requirements for Class 1a buildings as shown below. For a proposed concrete slab on ground the finished floor level (FFL) is required to be 100mm above the external finished ground level (FGL) for low rainfall intensity areas such as sandy, well-draining soil. Over the first 1m of the external finished ground level the fall is required to be 50mm over the 1m setback. This creates a gradient in which water will fall away from the building.

3.1.3.3 Surface water drainage

Surface water must be diverted away from Class 1 building as follows:

(a) Slab on ground – finished ground level adjacent to buildings:

the external finished surface surrounding the slab must be drained to move surface water away

from the building and graded to give a slope of not less than (see Figure 3.1.2.2) –

(i) 25mm over the first 1m from the building in low rainfall intensity areas for surfaces that are

reasonably impermeable (such as concrete or clay paving); or

(ii) 50mm over the first 1m from the building in any other case.

(b) Slab on ground – finished slab heights:

the height of the slab on ground above external finished surfaces must be not less than (see

Figure 3.1.3.2) –

(i) 100mm above the finished ground level in low rainfall intensity areas or sandy, well-drained

areas; or

(ii) 50mm above impermeable (paved or concreted areas) that slope away from the building in

accordance with (a); or

(iii) 150mm in any other case.