Building: Class 1a dwelling, new first floor addition to a single storey house
Location: Inner north Melbourne
NCC edition: NCC 2025, Volume Two and Housing Provisions
Area of NCC requirements: H2P1 rainwater management (also H2P2 at the roof edge flashing); Housing Provisions Part 7.4 gutters and downpipes
Raised by: The architect, at design stage
Year: 2026
The challenge
The architect wanted a clean first floor roof edge with no gutter. The metal roof ends in a flashing and sheds water onto the existing ground floor roof below. Part 7.4 of the Housing Provisions does not cover this arrangement, and one small section of the upper roof drops straight onto a garden bed instead of a lower roof.
The performance solution
Expert judgement and comparison with the Deemed to Satisfy Provisions showed the roof drainage works as a system:
Water from the upper roof lands on the lower roof, whose extended eaves gutter and downpipes carry it to the legal point of discharge.
The lower gutter and downpipes are to be sized for the combined catchment of both roofs, using the 5 minute, 5% AEP rainfall intensity in Housing Provisions Table 7.4.3d or the ABCB gutter, downpipe and overflow calculator.
Overflow measures under 7.4.6 and 7.4.7 are to be sized for that combined catchment, so a 1% AEP storm is directed away from the building.
The roof to wall flashing is to be installed to AS 1562.1 with enough upstand and lap to stop wind driven rain, which meets H2P2 whether or not a gutter is there.
Where the upper roof discharges to the garden, a stormwater pit or agricultural drain under that edge is to be connected to the site stormwater system.
Outcome
Report prepared at design stage in September 2026 and successfully accepted by the relevant building surveyor.
Lesson for designers
Gutterless roof edges can work, but only when the water has a clear, sized path somewhere else. Bring the hydraulic designer in early, because the lower gutter now carries more than its own roof.
Related: Performance solution report | Performance Solution vs Deemed to Satisfy
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