By Sergio Olea, owner of Monkeys Roofing and qualified roof plumber (Certificate III in Roof Plumbing). Published 29 October 2026. Last checked 5 November 2026.
Twenty minutes of ordinary rain over Marrickville, nothing anyone would call a storm, and there is water sheeting across the back courtyard and a damp patch spreading up the laundry wall. The usual assumption is that it takes an extreme storm to do that. In most of the Inner West it does not, and the cause is far more often the roof drainage than the yard.
Why does water reach the house in rain that is not even heavy?
Because this part of Sydney floods in common storms, not only rare ones. Inner West Council’s Marrickville Valley Flood Study found 3,989 properties with more than 150 mm of water over them in a 1 per cent (100-year) storm, and 3,151 properties with that same depth in a storm as ordinary as a 2-year event.
The study area covers Petersham, Stanmore, Enmore, Newtown, St Peters, Tempe, Marrickville and Dulwich Hill. Read those two numbers next to each other: the gap between a once-in-a-century storm and a storm most houses see every couple of years is only about 800 properties. The ground is low and flat, the drainage empties slowly, and the system is already close to its limit in rain that barely makes the news. An undersized downpipe or a gutter packed with leaf litter does not wait for a disaster to show itself. If your house is in Marrickville, our page on roof repairs in Marrickville sets out which streets the council flood studies name and what that means for gutters and downpipes.
Where is the water actually coming from?
Overland flow across the ground, not a canal breaking its banks. In the Dobroyd and Hawthorne Canal catchments, 885 of 1,446 flood-affected lots in Dobroyd and 690 of 1,036 in Hawthorne are affected by overland flow only. How far you live from the canal tells you very little. Where your roof water is discharged tells you a great deal.
That catchment takes in Ashfield, Haberfield, Summer Hill and parts of Petersham, Lewisham, Dulwich Hill and Croydon. A roof is a large collecting surface with only a few controlled exits. When those exits work, the water leaves in a pipe. When a gutter is blocked or a downpipe discharges into a garden bed instead of a line to the kerb, the same water arrives at ground level, at the wall, as sheet flow. That is the mechanism the Council’s own study says dominates here.
Do the street trees make it worse?
They make blockages harder to notice. Inner West Council’s Ashfield street tree strategy records brush box as the dominant avenue tree in streets including Cecil, Oak, Federal, Ilford and Orpington, with paperbark melaleuca and crepe myrtle common elsewhere. Brush box is evergreen and sheds leaf and bark right through the year rather than in one autumn drop.
A plane tree gives you an obvious pile of broad leaves and an obvious cue to get up there. Brush box and melaleuca do the opposite: a slow, steady trickle of small leaf and shed bark that packs down into a dense mat in the base of the gutter. From the footpath the gutter looks clear. It is not clear, and the first time you learn that is when the water goes over the front edge instead of down the pipe.
Does adding a roof over the extension trigger council rules?
Yes, past a fairly small threshold. Marrickville DCP 2011, section 2.25, requires on-site detention for any development adding 40 square metres or more of roof or paved area. On lots over 200 square metres you can substitute a 5,000 litre retention tank; between 100 and 200 square metres it is 4,000 litres, and under 100 square metres it is 3,000 litres.
The retained water has to be plumbed to real uses: toilets, the laundry, outdoor taps and irrigation. A rear extension, a carport roof or a large pergola with a solid roof will often reach 40 square metres on its own, so the stormwater requirement arrives with the build rather than being an optional extra. Worth knowing before the plans are finalised, not after.
What is worth checking before the next wet stretch?
Three things, in order. Whether the gutter is actually clear at the base, not just at the lip. Whether every downpipe is connected to a stormwater line that reaches the kerb or the street system, rather than ending in a garden. And whether the roof water leaving your property is going where it was designed to go, given that overland flow is what does the damage around here.
Run a hose into the gutter after a clean and watch each downpipe discharge. It takes ten minutes and it catches the two failures that cause most internal water damage in these suburbs: a blocked outlet, and a downpipe that goes nowhere.
The short version
Inner West flooding is mostly overland flow in common storms, not canal flooding in rare ones. Evergreen street trees block gutters quietly, all year. Keep the gutters and downpipe outlets clear, make sure every downpipe reaches a stormwater line, and if you are adding 40 square metres or more of roof, budget for detention or a tank from the start.
Sources: Inner West Council, Marrickville Valley Flood Study (2015), Dobroyd Canal and Hawthorne Canal Floodplain Risk Management Study and Plan (2020), Marrickville DCP 2011, section 2.25 Stormwater Management, Inner West Council Street Tree Strategy, Ashfield.
We do gutter cleaning, gutter guard installation and roof repairs across the Inner West, with before and after photos on every job. Call 0452 223 327.
