Where you'll find box gutters
Terrace houses are the classic case. Melbourne's terrace designers deliberately hid the roof behind a decorative parapet, so the water has to run along a box gutter behind the wall before it can get out.
They're also common where two roofs meet side by side, and on modern homes with parapets and hidden roofs.
Why box gutters leak
Blocked outlets. Leaves or a nest over the outlet, and the gutter fills like a bath until water rises over the edge into the roof space.
Rust where water sits. Box gutters are close to flat, so water and wet leaves linger, especially at low spots and around the outlet.
Failed joins and sumps. Old soldered or riveted joins crack, and the sump (the box around the outlet) rusts through.
No safe overflow. If there's nowhere for excess water to spill outside, a heavy downpour sends it inside instead.
If it's leaking through the ceiling right now
Stay out of any room where the ceiling is sagging, and keep people and pets away. If water is near light fittings or power points, turn the power off at the switchboard. Put a bucket under the drip. Don't try to clear the box gutter yourself: it can only be reached from the roof, which is a job for a roof plumber with safety gear, and never in the rain. For storm damage emergencies, call the SES on 132 500.
Repair or replace?
A roof plumber starts by clearing the outlet and water-testing the gutter to find where it's getting in. If the metal is sound, a blocked outlet, a cracked join or one rusted sump can be fixed on its own.
If rust has spread along the floor of the gutter, replacement is the honest answer. That means lifting the roof sheets or tiles along each side, fitting a new gutter and sump, then setting the roof back, ideally with an overflow and a rainwater head on the outside wall.
The NCC (the national building code) points to the plumbing standard AS/NZS 3500.3 for box gutter sizes, falls and overflow, so ask whether the new one is designed to it. Renovating near bush? NSW RFS guidance says box gutters should be avoided there, so ask whether the roof can do without one.
Questions to ask before you say yes
- Will you water-test the gutter to find the leak before quoting a fix?
- Is there an overflow, and where does it spill?
- Are you patching or replacing, and why?
- What's your plumbing licence or registration number?
What you can safely do
You can
- Watch for ceiling stains along the line of the box gutter after rain
- Check from the ground whether the overflow or rainwater head spills in heavy rain
- Note when leaks happen and take photos, which helps pin down the cause
Leave to a roofer
- Clearing or inspecting any box gutter, because that's done from the roof itself
- Water-testing to find the leak
- Repairing joins and sumps, or replacing the gutter
- Adding overflows and upsizing outlets
Where we'd begin
We'd get the box gutter cleared and water-tested before agreeing to any big job. If it's a blocked outlet, that's a small bill. If the floor of the gutter is rusting, we'd have it replaced, with a proper overflow and gutter guard over the top, because a box gutter that blocks again will leak again.
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.