What the code expects, and what to check
The building code sizes eaves gutters for a five-minute burst of rain and asks for overflow measures for heavier ones. Acceptable overflow includes slots in the gutter front, a controlled gap at the back, or a front bead set below the fascia. Each downpipe can serve no more than 12 m of gutter, and downpipes should sit as close as possible to valleys.
Valley gutters need a roof pitched more than 12.5 degrees and at least 15 mm of freeboard. Box gutters, the flat gutters hidden inside a roof, are designed to AS/NZS 3500.3. When a box gutter overflows, it overflows inside, so they need extra care. Our guide to box gutters explains why.
Lysaght's Half Round gutter comes in sizes from 150 up to 400 and has high rainfall capacity, which can suit long roof runs. A roof plumber should work out sizes for your roof area, not just match what was there.
Licences for roof plumbing in Queensland
Plumbing and drainage need a QBCC licence whatever the job is worth, and anyone doing regulated plumbing must also hold an occupational plumbing licence. Plumbing contractors have to show their licence number in all advertising. Check any name on the QBCC register before they start. Stormwater work like repairing a downpipe or gutter falls under the Home Warranty Scheme once it's over $3,300.
Questions for a Cairns roof plumber
- Have you worked out gutter and downpipe sizes for my roof area, or matched the old ones?
- Where does the water go when a gutter overflows?
- Do my valleys and box gutters have enough depth and fall?
- How many downpipes do I need for my gutter lengths?
- What's your plumbing licence number?
Our view
We'd ask for an overflow plan before a bigger gutter. Even well-sized gutters get overwhelmed in a Cairns cyclone, so the important question is where the excess goes. Slotted fronts or a back gap that tips water outside the wall beat a gutter that fills up and pours into the eaves.
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.
Tell us your suburb and whether it's overflow, leaks or blocked downpipes. Where there's a suitable licence-checked roof plumber near you, we'll send it their way, and if there isn't, we'll say so. Our national roof plumbing page and the Cairns hub have more.
How much water a Cairns roof has to move
Monster months
Wikipedia notes wet-season monthly totals in Cairns can exceed 1,000 mm, with a record of more than 1,417 mm in January 1981. February averages 475.5 mm (1991 to 2020 normals). A system that handles an average month can still fail in a big one.
Jasper showed the upper end
Cyclone Jasper dropped 2,166 mm over five days at Black Mountain near Kuranda and 2,025 mm at Myola, and 21 stations recorded more than 1,000 mm. No roof plumbing is built for that, which is why overflow paths matter: when gutters can't keep up, water should spill outside the house, not into it.
Wet nearly half the year
The wet season runs from November to May. With 15 to 16 rain days a month from January to March, a blocked downpipe or a sagging gutter gets tested constantly rather than once in a while.