What is your flat roof actually made of?
Few roofs are truly flat. E2/AS1 asks for at least 2 degrees of fall on butyl and EPDM roofs, and RANZ says membrane roofs often sit below 3 degrees, while trapezoidal and tray longrun can go down to 3 degrees.
So yours might be a rubber membrane such as Butynol, layered torch-on, low-pitch metal tray, or on an older house, a bitumen coating. Each is repaired differently.
Where flat roofs let water in
Ponding. Water lying for days keeps the surface wet and works dirt into laps. On coated steel, Dimond says ponding shortens the product's life and voids its warranty.
Outlets and overflows. E2/AS1 asks for a roof outlet of at least 75 mm plus an overflow or second outlet. Without one, the first leaf blockage can flood the roof.
Internal gutters. E2/AS1 wants a fall of at least 1 in 100 here, with no cross seams. A sagging internal gutter is a classic hidden leak.
Edges and penetrations. Joins at walls, parapets, vents and skylights open up long before the middle of the roof wears out.
How a roofer tackles a flat roof leak
Start underneath
Where the stain is, when it shows, and what the ceiling space reveals. Water can travel a long way before it drips.
Check the drainage
Outlets, overflows and internal gutters, plus dried silt tide marks that show where water sits.
Inspect the joins
Laps, upstands, parapet caps and every pipe or skylight through the roof, sometimes wetting one section to prove the leak.
Fix the spot and the reason
Patch or reseal the leak, then clear, enlarge or add outlets so water stops backing up.
Is it a leak, or condensation?
Not every stain under a flat roof is rain. Warm, damp air from inside can condense under a cold roof. E2/AS1 requires a gap of at least 20 mm between the underside of a membrane roof's substrate and any insulation. If stains show up in cold, dry spells, read condensation or a leak? before paying for a repair.
When a patch won't do
If the falls are wrong, the deck is soft or the roofing is worn out, patches keep failing. The fix may be re-falling the roof on a new deck, or replacing the roofing. For low-pitch metal, NZMRM points to guidance that an existing flat roof meeting the Building Code can be replaced with a comparable product without re-pitching.
Consent is where people get caught. MBIE says it's the homeowner's job to decide whether work is exempt, that the exemption doesn't cover replacing something that failed early by letting water in, and that if you're unsure you should ask the council or apply for consent. Non-compliant work can affect insurance and a future sale.
Flat doesn't mean safe to walk on
A flat roof feels safer than a pitched one, but the edge is just as far down. WorkSafe calls the belief that falls under 3 metres need no safety measures dangerous and incorrect, and lists skylights and clear sheets as brittle. Check from a window, not the roof.
Your part, and the roofer's
You can
- After heavy rain, look from upstairs for water still sitting the next day
- Note where the stain is and what kind of rain brings it on
- Tell the roofer the house's age and any past repairs
Leave to a roofer
- Anything on the roof, including clearing roof outlets
- Water testing and leak tracing
- Patching, resealing and rebuilding upstands
- Re-falling, new decking or full replacement
What we'd do about a flat roof that keeps leaking
Ask the roofer to explain why water is getting in, not just where. If it's the falls or outlets, a patch alone will fail again, so we'd pay to sort the drainage. Get it in writing whether the work needs a building consent, because as the owner, that call is yours.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.