Gutters overflowing in heavy rain: what is normal, what is a blockage, and what to do in Cairns

Some overflow in a severe downpour is expected, and on a well-built roof it is planned for. The explanatory notes to Part 7.4 of the ABCB Housing Provisions (National Construction Code 2022) say roof drainage systems are not designed to remove all water during exceptionally heavy rain, particularly in tropical areas. Eaves gutters are sized for a storm with a 5 per cent chance of happening in any year, provided they are not blocked.

For rarer storms, up to a 1 per cent chance in any year, the provisions call for overflow measures that send the extra water away so it does not pond against, enter or damage the building, even if the drainage system is blocked. The Australian Building Codes Board describes two kinds:

• Continuous measures: these run along a length of gutter, for example slots at regular intervals along the front face of the gutter.
• Dedicated measures: specific points where overflow can occur, for example a rainhead at the top of a downpipe, whose overflow should be directed away from the building.

So water spilling out of the front of a slotted gutter in a cloudburst may be the system doing its job. Water getting into the eaves lining, the roof space or down an inside wall is not. Some gutters use a controlled back gap, a spacer that lets overflow spill down the outside of the fascia, so judge by where the water ends up. A licensed roof plumber can tell you whether your gutters have overflow measures.

Gutter design in Australia starts from local rainfall intensity, and Table 7.4.3d of the Housing Provisions lists it town by town. For Cairns, the five-minute design intensity is 230 mm an hour for the gutter itself and 279 mm an hour for its overflow measures. Melbourne's figures are 132 and 187. A Cairns gutter is built for much heavier bursts than a southern one, so advice written for a Melbourne roof does not always carry across.

The other difference is how often the gutters are tested. Bureau of Meteorology long-term statistics for Cairns Aero show 404.8 mm of rain in an average January, 437.5 mm in February and 422.1 mm in March, with about five days in each of those months bringing 25 mm or more. The highest daily totals on record at the station are 368.3 mm in January and 402.8 mm in March. A gutter half full of leaf litter meets those days with less room than it was built for.

Where the water comes over usually points to the cause. Watch from the ground or a window during a heavy shower, then match what you see:

• Overflow along a whole run, with leaves or seedlings visible at the gutter edge: a blocked gutter. Debris holds the water back and the gutter fills before it can drain.
• Overflow concentrated near a downpipe, with little water coming out at the bottom: a blocked downpipe. A gutter can be clear and still overflow when the outlet below it is packed.
• Overflow at the far end of a long run, away from any downpipe: possibly too few downpipes. The Housing Provisions say insufficient and poorly located downpipes are a frequent cause of poor roof drainage, and limit each downpipe to 12 m of gutter.
• Water sitting in the gutter days after rain: a fall problem or a sag. The Housing Provisions call for eaves gutters to fall at least 1:500 to the outlets (the ABCB notes an exception for gutters fixed to a metal fascia), and a flat or dropped section holds water instead.
• Water overshooting the gutter below a valley, where two roof slopes meet: concentrated flow. The provisions place downpipes near valley gutters so the concentrated water from those areas is removed.

Only the first two are cleaning jobs. The rest come down to how the gutters were sized, fixed or laid out, which is work for a licensed tradesperson.

A blocked downpipe usually gives itself away at the bottom before anyone needs to climb to the top. These checks need no ladder:

1. 1In steady rain, look at where the downpipe meets the ground or its stormwater pipe. A strong flow means it is moving water; a trickle while the gutter above is spilling over points to a blockage.
2. 2Follow the water to where it leaves the property. Cairns Regional Council says roof water pipes connect to the gutters and carry the water to a lawful point of discharge, typically the kerb and channel at the front. Roof water pipes on the verge belong to the homeowner, who maintains them up to and including the kerb adaptor, so clear any leaves or silt you can reach from the street.
3. 3Look for water bubbling up or pooling along the line of the underground pipe. Council says broken pipes are the property owner's responsibility to repair, and that roof water must not be directed so it pools and becomes stagnant.
4. 4Keep garden waste out of the system. Council warns that dumping grass and tree clippings into drains may cause blockages during heavy rain.

Clearing the top of a downpipe is a ladder job for a dry day. For standing water around the yard, see our guide to stopping mosquitoes breeding in a Cairns yard.

Do not climb up to fix an overflowing gutter during a storm. The Queensland Government's ladder safety advice says never to use ladders in thunderstorms, rain or wind, and that over 1,000 Queenslanders are hospitalised each year after falling off ladders, most of them around the home. The job during the storm is to record it:

• Note where the water comes over: the front, the back, near a downpipe, at a corner or below a valley.
• Take photos or a short video from the ground or a window.
• Once the rain stops, check the ceilings, walls and eaves lining under the overflow point for stains or damp.

In a rental, report it to the property manager, ideally in writing with photos. The Residential Tenancies Authority says that if the tenant has not done anything to block the gutters, the property manager or owner may be responsible for cleaning them, and that, for drains, a tenant who caused a blockage may have to pay for removal and any damage. This is general information, not legal advice; the RTA can help with a specific tenancy. Our guide to who cleans the gutters in a Queensland rental covers the detail. Before anyone goes up in dry weather, read cleaning your own gutters: ladder safety and tools.

Clearing debris out of a gutter removes a blockage without changing the system. Changing the system is licensed trade work. The Queensland Building and Construction Commission (QBCC) lists metal fascias and gutters in the scope of its plumbing and drainage licence, and a separate metal fascias and gutters licence covers fabricating and installing rainwater goods. Call a licensed plumber or roof plumber for:

• Re-hanging or re-falling a gutter that sags or holds water.
• Adding downpipes, moving them, or fitting overflow measures.
• Replacing a rusted or split gutter, or upsizing to a bigger profile.
• Repairing a broken or collapsed underground stormwater pipe.

Ask for the tradesperson's QBCC licence number before the work starts.

The Queensland Government's prepare your home guidance says to act before storm, cyclone and monsoon season, and to clean gutters and downpipes so water can drain away as quickly as possible.

In Cairns the dry end of the year gives the window. Cairns Aero averages 34.1 mm of rain in September and 47.2 mm in October, against more than 400 mm a month from January to March. A clear then means the gutters start the wet empty, and any fault that is not a blockage shows up in the early storms rather than the worst ones.

Our pre-wet-season yard and gutter checklist puts the gutters alongside the other yard jobs worth finishing before the rain sets in.

If the overflow is a blockage, Trisim's gutter cleaning service clears the full gutter run and the downpipes rather than skimming the visible leaf, because a clear gutter feeding a blocked downpipe still overflows. Every job is quoted before it starts. Trisim carries public liability insurance, the team is police-checked, and the business works for property managers, agents and landlords across Cairns.

Trisim also mows lawns in beachside suburbs including Holloways Beach, Machans Beach and Clifton Beach, so ask about the gutters when you book the lawn. If the clear turns up a sagging run or a split joint, that is the point to bring in a licensed roof plumber.