Box gutters and flat roofs: the strata roof risk inside the building

On a typical house, the gutter hangs off the edge of the roof. If it overflows in a storm, water spills down the outside of the wall and onto the ground. It is messy, but the building is mostly fine.

Many apartment buildings are different. Roofs drain into box gutters set inside the roof line, behind parapets or between roof sections. Flat and low-pitch roofs over top-floor units and plant areas are sealed with membranes and drain to outlets in the slab. Downpipes often run down inside the building. All of these hold water within the footprint of the building. When they overflow, block or fail, the water has only one place to go: inside, into ceilings, walls and the units below.

That is why box gutters and flat roofs deserve a place near the top of any strata committee's maintenance list.

What the code requires

For apartment buildings (Class 2) and other buildings covered by NCC Volume One, two clauses of Part F1 of NCC 2022 do most of the work:

• F1D3 Stormwater drainage requires stormwater drainage to be designed and constructed in accordance with AS/NZS 3500.3, the plumbing and drainage standard for stormwater. That includes roof drainage: gutters, box gutters, sumps, rainwater heads and downpipes. The Plumbing Code of Australia (NCC Volume Three) also references AS/NZS 3500.3 for this work.
• F1D5 External waterproofing membranes requires a roof, balcony, podium or similar horizontal surface to have a waterproofing membrane of materials complying with AS 4654.1, designed and installed in accordance with AS 4654.2.

In addition, Part F3 sets the performance requirement (F3P1) that a roof and external walls must prevent the penetration of water that could cause unhealthy conditions or damage to the building, and F3D2 lists acceptable roof coverings, including an external waterproofing membrane complying with F1D5.

For box gutters in particular, AS/NZS 3500.3 requires the gutter to discharge into either a rainwater head or a sump fitted with an overflow device, sized to handle a severe storm. Building regulators' guidance on box gutters, such as the Victorian Building and Plumbing Commission's practice note, summarises the overflow sizing as a 1% annual exceedance probability event, roughly a one-in-100-year storm. The edition of the standard that applies depends on when your building was approved, so older buildings may have been designed to earlier rules.

Why the rules are written this way

Because the water is held inside the building. An eaves gutter that overflows dumps water outside. A box gutter that overflows dumps water into the roof space and ceilings. The consequence of failure is completely different, so the design has to assume failure will happen and give the water a safe way out.

Because outlets block. Leaves, silt, dead birds, tennis balls, builders' rubbish and degraded membrane all end up at the outlet. A box gutter with a perfectly sized downpipe still fails if that downpipe is blocked during a storm. That is why the overflow is not optional. It is the path water takes when the downpipe cannot cope, and it has to discharge somewhere visible and harmless, usually through the face of the building, so that someone notices the blockage.

Because flat roofs do not shed water by gravity alone. A pitched tile roof sheds water quickly. A flat or low-pitch roof relies on the membrane being continuous, correctly turned up at walls and penetrations, and laid to falls so water reaches the outlets instead of sitting in ponds. Standing water stresses the membrane, finds every small defect and adds weight to the structure.

Because membranes have a life. A roof membrane is exposed to sun, heat, foot traffic and movement every day. It does not last forever, and it degrades long before it starts visibly leaking. AS 4654 exists to make sure membranes are suitable for the exposure and installed to a known standard. It cannot make them permanent.

Where it goes wrong

These are the problems we see most often on strata roofs:

• Blocked outlets and no working overflow. The most common cause of sudden roof leaks into top-floor units during storms. Sometimes the overflow was never installed. Sometimes it has been sealed over, or is itself blocked.
• Leaf and debris build-up. Trees that were small when the building was finished now overhang the roof. Box gutters fill silently between inspections.
• Ponding. Insufficient falls, sagging decks or blocked outlets leave standing water on flat roofs and in box gutters, accelerating corrosion and membrane breakdown.
• Membranes at the end of their life. Cracked, blistered, delaminating or patched membranes, particularly at upturns, joints and around outlets.
• New penetrations by owners and contractors. Air-conditioning units, antennas, satellite dishes, solar mounts and cabling are added after completion, with fixings and pipes punched through the membrane or the gutter and sealed with a bead of silicone. Each one is a leak waiting for the next storm. Work like this on common property generally needs owners corporation approval, as our post on renovating a strata apartment explains.
• Corroded box gutters. Metal box gutters that hold water, or that are in contact with incompatible metals or debris, corrode from the inside out.
• Undersized original design. Some older buildings were designed before current overflow rules. A gutter that "has always been fine" may simply not have met a big enough storm yet.

Who is responsible

In a strata scheme, the roof, box gutters, membranes and the stormwater system that serves them are usually common property. The owners corporation has a duty under the Strata Schemes Management Act 2015 to properly maintain and keep common property in good and serviceable repair. Check your strata plan and by-laws for exceptions, such as a roof terrace that belongs to a lot or is subject to an exclusive use by-law.

In practice, that means the committee and strata manager need a plan for the roof, not just a phone number to call when it leaks. Roof failures are rarely sudden. They are usually the end of a long period without maintenance.

Questions committees should ask

1. Where are the box gutters, internal gutters, sumps, rainwater heads and roof outlets on our building, and do we have a roof drainage plan?
2. Does every box gutter and flat roof outlet have a working overflow? Where does each overflow discharge, and would we notice if it was running?
3. When were the gutters and outlets last cleaned, and how often should they be, given the trees around the building?
4. How old are the roof membranes, what type are they, and what condition are they in?
5. What has been fixed through the roof since completion, and was each penetration approved and properly detailed?
6. Is roof membrane replacement in our capital works fund plan, and is the timing realistic?
7. If the building is still within the defects period, has the roof drainage been checked by an independent inspector?

How we approach it

When we look after or repair a strata roof, we treat roof drainage as a system, not a set of separate leaks.

• We map it first. Box gutters, sumps, rainheads, outlets, overflows and downpipes are located, photographed and recorded, so the committee knows what it has.
• We check the overflows, not just the gutters. A clean gutter with no working overflow is still a risk. We confirm where each overflow discharges and whether it would actually work in a storm.
• We recommend a cleaning and inspection cycle based on the building. As a starting point, we suggest checking and clearing gutters and outlets before the storm season and after major storms, and more often where trees overhang the roof. That is our practical recommendation, not a legal requirement.
• We assess membranes honestly. If a membrane is at the end of its life, we say so and help the committee plan replacement through the capital works fund, rather than patching the same spot every year.
• We detail penetrations properly. Any new pipe or fixing through a roof or gutter is flashed and sealed to suit the membrane system, not just siliconed.
• We inspect before we cover. On membrane repairs and replacements, falls, upturns and outlet connections are checked and photographed before the finish goes on, as described in our article on hold points and the handover record.

Roof drainage is closely linked to the rest of the building envelope. Our articles on facade and balcony waterproofing and where buildings leak cover the other common entry points, and if your building is new, our guide to strata defects reports explains what to do if the roof drainage is not right.

If you would like a roof drainage check on your building before the next storm season, get in touch.

Sources

• NCC 2022 Volume One, Part F1 Surface water management, rising damp and external waterproofing, Australian Building Codes Board (F1D3, F1D5)
• NCC 2022 Volume One, Part F3 Roof and wall cladding, Australian Building Codes Board (F3P1, F3D2)
• NCC 2022 Volume Three, Plumbing Code of Australia, Australian Building Codes Board
• Box gutters practice note (superseded), Building and Plumbing Commission (Victoria)
• Strata Schemes Management Act 2015 (NSW), NSW Legislation, section 106

This article is general information, not advice for your building. Roof drainage requirements depend on the building's approval date, design and the edition of the standards that applied, and responsibilities depend on your strata plan and by-laws, so get advice from a licensed plumber, building consultant or your strata manager. Full website disclaimer