Roofing, rainwater and stormwater systems
Roofing, rainwater and stormwater systems
This domain tests roof cladding, gutters, downpipes, stormwater drainage and disposal under NZBC E1 and E2 and AS/NZS 3500.3. Expect catchment sizing, falls, overflows, flashings and soakage/detention.
Core rules
- Roof cladding must meet minimum pitch, lap and fixing requirements for the profile and wind zone; flashings must be lapped so water sheds, not ponds.
- Gutters need fall to outlets, and outlets need strainers. Provide overflow paths so a blocked outlet does not back water into the building.
- Size gutters and downpipes from catchment area, rainfall intensity and the standard’s tables, not from the old pipe size.
- Stormwater drains need self-cleansing grades, inspection openings, sumps and trapped gullies where required. Discharge to an approved point: kerb, council main, soakage or detention.
- Keep rainwater harvesting separate from potable supply unless a compliant backflow arrangement is installed.
Worked example
A roof catchment is 120 m² and the design rainfall intensity is 100 mm/h. Flow = area × intensity = 120 × 0.100 m/h = 12 m³/h = 3.33 L/s. Select a gutter and downpipe combination from AS/NZS 3500.3 that carries at least 3.33 L/s at the proposed fall, then check the outlet and overflow can handle the design flow.
Traps
No overflow, downpipes undersized after a roof extension, gutter fall the wrong way, flashing laps reversed, and soakage used where the ground cannot accept the flow. Also watch blocked outlets hidden by strainers and sumps without access.