Building: Alterations and additions to a Class 1a dwelling: a lower level rumpus room cut into a slope
Location: Mornington Peninsula, Victoria
NCC edition: NCC 2022, Volume Two and Housing Provisions
Area of NCC requirements: H2P1, H2P2, H2P3 (drainage and weatherproofing); Housing Provisions 3.3.3(a) and (b) freeboard and fall away; 3.3.4 subsoil drainage
Raised by: The relevant building surveyor
Year: 2026
The challenge
The new rumpus room sits below ground, so an existing blockwork retaining wall becomes the external wall of a habitable room. The slab could not achieve the Deemed to Satisfy freeboard of 50 mm (impermeable surface) or 150 mm (permeable surface), and the ground could not fall 50 mm away over the first metre, because the retained soil sits against the wall. Midway through, the floor was confirmed as an existing 100 mm infill slab over an older polyethylene membrane, not the raft slab first documented.
The performance solution
Expert judgement and comparison with the Deemed to Satisfy Provisions, based on layered waterproofing and drainage rather than ground levels:
a positive side membrane on the soil face of the wall, running to 150 mm above finished ground and lapped behind a flashing
a two layer negative side membrane on the room face, protected by a fibre cement or drainage sheet
a waterstop at the wall to slab junction, with the membrane lapped 300 mm onto the slab, so wall and floor waterproofing form one continuous line that does not rely on the old, unseen under slab membrane
a 100 mm socked agricultural drain in granular backfill at the footing, and a spoon drain at the base of the cut, both connected to the legal point of discharge
a vapour permeable membrane at the base of the wall cavity, following the ABCB Condensation in Buildings Handbook
Conditions
Backfilling is a hold point until the membrane, backfill and drain are inspected. If the slab or footings turn out different to the drawings, work on the floor stops and the solution is reassessed. Waterproofing must be installed by a qualified applicator, with certification provided to the building surveyor.
Outcome
Final report issued August 2026 and revised October 2026 to match updated drawings and the confirmed slab. Successfully accepted by the relevant building surveyor.
Lesson for designers
Confirm what the existing slab really is before the waterproofing detail is drawn. Assumed construction is the most common reason a below ground solution has to be reissued.
Related: Performance solution report | Performance solutions Victoria
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