The short answer
Box gutters sit inside the roof, so when they block, overflow or rust through, the water goes into the building instead of over the edge. Debris, too little fall, missing overflows, failed joints and corrosion are the usual causes. Clearing fixes a blockage. It won’t fix a gutter that has rusted through or was built too small or too flat. That needs a new gutter, designed to the standard.
This is general information, not legal or insurance advice. Check your own strata plan, by-laws and policy, and ask a qualified adviser about your situation.
Why box gutters are a weak point
An eaves gutter that overflows spills outside. A box gutter is built into the roof line, so an overflow can end up inside. Master Builders ACT calls box gutters one of the highest-risk details in residential construction. It says box gutters fill with debris, are often undersized with too little fall, and can lack a compliant overflow. Water then overflows into the building cavity instead of discharging outside.
The usual causes of a box gutter leak
- Debris. Lysaght’s gutter inspection guide says to check for dust and debris building up at the high end of box gutters, and around overflows and downpipe entries.
- Too little fall. BlueScope’s bulletin on steel gutters says gutters need enough fall to drain, because ponding water can speed up corrosion.
- No overflow, or a blocked one. The Australian Building Codes Board (ABCB) says allowing for overflow is critical to reduce the risk of damage to buildings.
- Failed joints. BlueScope says unsealed joints and laps may leak, causing corrosion and staining.
- Rust. Master Builders ACT says box gutters rust from the inside out.
What makes a steel box gutter rust
Ponding water and trapped debris are two causes. Other things that speed up corrosion of Colorbond and Zincalume steel, according to BlueScope:
- Lead and copper. Lead, copper, stainless steel and copper alloys must not be used with Colorbond or Zincalume steel (BlueScope TB-15). BlueScope says lead flashings that drain onto the steel should be painted, and water from copper pipes shouldn’t be directed onto it (BlueScope). Stramit also says copper or lead drainage shouldn’t discharge onto Colorbond components.
- Air conditioner water. BlueScope says condensate from air conditioners or solar heaters must go to the ground, not into the gutter and downpipe system.
- Metal offcuts and filings. Swarf and screws left behind by trades cause surface corrosion and staining, BlueScope says, so they need to be cleaned out of gutters.
Repair, clear or replace?
It depends on what the roofer finds:
- Debris and blockages. Lysaght’s maintenance manual says blockages should be removed straight away. It recommends maintenance at least every four months, ideally at the end of autumn and the end of spring, plus an inspection after extreme weather.
- Staining or surface rust. Lysaght says to work out whether it’s a metal deposit or a breakdown of the coating. A deposit should be removed straight away. Coating breakdown is usually from poor maintenance or scratching, and Lysaght refers to BlueScope’s bulletin on restoring it.
- Holes, a gutter too small or too flat, or nowhere for overflow to go. Clearing won’t fix these. A new gutter is the chance to get the size, fall and overflow right.
New box gutters have to be designed properly. The ABCB says the National Construction Code’s Housing Provisions no longer cover box gutters. A box gutter must be designed and installed in accordance with AS/NZS 3500.3, or a Performance Solution must be developed. Master Builders ACT lists what compliance covers: sizing for rainfall intensity, falls, overflow provisions, joints, corrosion protection and access for maintenance.
Overflows are part of the design. Victoria’s building regulator, in its practice note on box gutters, says each sump needs either a side overflow or a high-capacity overflow, and overflows must discharge to the open air. Its note is written for Victoria, but the standard it applies, AS/NZS 3500.3, is the same one the ABCB names.
Box gutters in strata buildings
In NSW strata schemes, the roof and gutters are usually common property. NSW Fair Trading lists them, and water damage coming from common property, as the owners corporation’s responsibility. Check the strata plan and by-laws, because schemes differ. See our guide to roof leaks in strata buildings.
Box gutters are often out of sight from the ground. Photos of the whole gutter, the outlets and the overflows help the committee see the cause and approve the right fix.
How JSJ can help
Inspections are free for home owners, with no obligation. Inspections and reports for strata buildings are charged. We confirm the fee when you send the job. We’ll contact you by the next business day to arrange an inspection. After the inspection you get a written quote with photos within 24 hours.
Strata managers can forward the work order to info@jsjroofing.com.au. See box gutter replacement and roof leak repairs.
Common questions
Why do box gutters leak?
Because they sit inside the roof, an overflow goes into the building. Master Builders ACT says box gutters fill with debris, are often undersized with too little fall, and can lack a compliant overflow, so water overflows into the building cavity. Failed joints and rust are other causes.
Why has my box gutter rusted?
Common causes include ponding water and debris. BlueScope says lead, copper and stainless steel must not be used with Colorbond or Zincalume steel, air conditioner water must not drain into gutters, and metal filings left by trades cause corrosion.
How often should box gutters be checked?
Lysaght’s maintenance manual recommends maintenance at least every four months, ideally at the end of autumn and the end of spring, and an inspection after extreme weather. It says blockages should be removed straight away.
Who pays for box gutter repairs in a strata building?
Usually the owners corporation. NSW Fair Trading lists the roof and gutters, and water damage coming from common property, as the owners corporation’s responsibility. Check the strata plan and by-laws, because schemes differ.
Is the inspection free?
Inspections are free for home owners, with no obligation. Inspections and reports for strata buildings are charged. We confirm the fee when you send the job.
How do I send JSJ a strata job?
Forward the owner’s email or work order to info@jsjroofing.com.au. We’ll contact you by the next business day to arrange an inspection. After the inspection you get a written quote with photos within 24 hours.
Sources
Checked 1 November 2026. Rules and policies change, so check the source for the current position.
- Box gutters: one of the highest risk details in residential construction (4 February 2026) (Master Builders ACT)
- Understanding the NCC: gutter and downpipe provisions for housing (2023) (Australian Building Codes Board (ABCB))
- Gutter inspection and maintenance (Lysaght (BlueScope))
- Roofing and rainwater maintenance manual (2024) (Lysaght (BlueScope))
- Technical Bulletin TB-15: Selection and use of steel gutter, downpipe and fascia products (Rev 10, 2013) (BlueScope Steel)
- Avoid incompatible metals (BlueScope Steel)
- Rainwater products NSW: product technical manual (2021) (Stramit)
- Plumbing practice note RP 02: box gutters (version 4.0, 2023) (Victorian Building Authority)
- Strata repairs and maintenance (NSW Government (NSW Fair Trading))