Box gutter replacement and repair

Published 5 November 2026

What is a box gutter?

An ordinary eaves gutter hangs off the edge of the roof, so if it overflows the water falls outside. A box gutter is a flat-bottomed channel set inside the roof line: between two roof sections, behind a parapet wall, along the join between a terrace and its neighbour, or across a unit block. It collects the water and drains it to a sump, an outlet and a downpipe.

Because a box gutter that overflows sends water into the roof space or the walls, it’s designed for heavier rain than an eaves gutter. Lysaght, the BlueScope roofing brand, writes that “Drainage systems for larger roofs use box gutters at the perimeter and internally”. Where overflowing into the building is likely, which it calls “typical of box gutters”, it recommends designing for a 100-year storm, against a 20-year minimum for eaves gutters where overflowing into the building is unlikely.

Why box gutters overflow

• Leaves in the outlet or sump. On the Beaches, gum leaves wash down to the outlet and block it, and the gutter fills faster than it drains.
• Ponding and rust. BlueScope’s eave gutter bulletin (TB-15) says gutters must be installed “to avoid ponding, or pooling, of water which can cause accelerated corrosion”. By our reading, the same applies to water that sits in a box gutter after rain.
• No overflow, or a blocked one. If the outlet blocks and the water has nowhere else to go, it backs up under the roof sheets or flashings.
• Undersized for the roof. An old box gutter may be too small, or have too few outlets, for the roof area that drains into it.
• Water from incompatible metals. Run-off from copper, or from old lead flashings, eats into a Zincalume or Colorbond gutter (see the metals section below).

Tracing a drip to its cause, and telling a box gutter leak from a skylight or flashing leak, is in our guide to skylight and box gutter leaks.

Box gutter repair, relining or replacement

Depending on its condition, a box gutter can be patched, relined with a waterproof membrane, or rebuilt in new metal with new sumps and overflows. A leaking join or one corroded section may be repairable if the rest of the gutter is sound. Where the gutter is rusting along its length, holds water or has no overflow, ask about replacement, which can mean lifting the roof sheets or tiles along each side.

Ask the roofer which of the three they’re quoting and why. Relining is a different trade from metal work: NSW’s roof plumbing licence “does not include” coverings of “malthoid, bituminous or similar membrane material”, and waterproofing with membranes is a licence category of its own. Ask which licence covers the work in the quote.

Box gutter sumps, overflows and sizing: AS/NZS 3500.3

The building code (NCC 2022, clause H2D6) lets eaves gutters on houses be designed from the simple tables in Part 7.4 of the Housing Provisions, but it says Part 7.4 “excludes box gutters”. The Australian Building Codes Board explains: “The Housing Provisions no longer provide requirements for box gutters. Instead, a box gutter must be designed and installed in accordance with AS/NZS 3500.3 or a Performance Solution be developed.”

AS/NZS 3500.3 is the plumbing standard for stormwater. It covers sizing the gutter for the roof area and the local rainfall, its fall to the outlet, the sump and outlet, and an overflow that discharges outside the building if the outlet blocks. As of November 2026, NSW uses NCC 2022, which references the 2021 edition of the standard; NCC 2025, which NSW adopts on 1 May 2027, references the 2025 edition.

The standard isn’t free to read, so we don’t reproduce its figures. Ask the roofer to show how the new gutter and outlets are sized, and where the overflow goes.

Colorbond, Zincalume and compatible metals

Many box gutters are folded from Colorbond or Zincalume steel. Lysaght’s compatibility bulletin says: “Drainage from copper or copper alloy roofs, pipes, etc, must not be discharged onto or into aluminium alloy, zinc, zinc-coated or zinc/aluminium alloy-coated steel gutters, downpipes and the like.” BlueScope’s eave gutter bulletin (TB-15) adds that “Stainless steel, lead, copper, and copper-containing alloys must not be used in conjunction with COLORBOND® steel, ZINCALUME® steel or zinc-coated steel”.

By our reading, that means a box gutter that takes water from a copper roof, copper pipes or old lead flashings shouldn’t be Zincalume or Colorbond, so mention any of those when you ask for a quote. BlueScope also recommends sealing joins with neutral-cure sealant, and says air conditioner condensate should be piped to the ground, “not along the roofing profile or into the gutter and downpipe system”.

For how close to the surf each grade of steel suits, see Colorbond, Ultra or Zincalume near the coast.

Box gutter replacement cost

We found no Northern Beaches or Sydney roofer that publishes a price for a residential box gutter. The nearest is a national cost guide:

Box gutter replacement cost per metre

A labour-hire company’s national cost guide (published 1 October 2026), checked 22 and 23 October 2026; it says its figures include GST. It’s a per-metre rate for the gutter, custom folded and installed. A quote depends on the gutter, the roof around it and access.

On top of the gutter itself, the price moves with how much roof has to be lifted to get it out, new sumps, rainheads or overflows, an internal downpipe, two-storey access or scaffold, and any rotten timber underneath. Eaves gutter, fascia and downpipe prices are in our gutter and fascia replacement cost guide.

Who replaces a box gutter: licences and strata

Box gutters are guttering, so the metal work is roof plumbing. The NSW Government defines roof plumbing as “any work involved in fixing, installing, renovating, altering, repairing and maintaining guttering, downpipes, roof flashing and roof coverings”, and you need a contractor licence for work valued at more than $5,000 in labour and materials, including GST. Check a licence on the Service NSW licence register.

In a strata building, a box gutter in the roof is usually common property. The Strata Schemes Management Act says an owners corporation “must properly maintain and keep in a state of good and serviceable repair the common property”, and an owner “must not carry out work on the common property unless the owner is authorised”. Report it to the strata manager or committee; strata roof leaks in NSW covers what happens next.

Common questions