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More about stiffened raft slabs

AS 2870 provides the required minimum dimensions and specifications for the depth, width and spacing of beams in a stiffened raft slab footing.

The excavation depth depends on:

  • depth of adequate bearing soil below the surface
  • depth of vegetation
  • slab height.

The height of the finished concrete slab floor level must be specified or calculated before construction. This ensures that the completed floor level finishes above the surrounding ground level. Raising the slab level above the ground level makes sure that floodwater or stormwater cannot readily flow into the house and cause water damage.

The minimum recommended distance from the top of the slab to the finished ground level (FGL) is 150 mm. Note NGL on the diagram which refers to natural ground level.

Diagram of a cross-section of a stiffened raft slab. The edge rebate is just above ground level and is shown with an FGL of 150 mm. The natural ground level NGL is also given as 150 mm. Reinforcement is in the top of the slab and the bottom of the footings. The total depth of the overall footings and slab is given as D. The depth of footings (DF) is given as 500 - 150 = 350 mm. The excavation depth is 350 mm from the finished ground level. Note that often the finished ground level is the natural ground level reinstated.

Turn to your copy of the BCA and refer to the following:

  • Figure 3.2.5.3 (a)
  • Figure 3.2.5.3 (b).