Stormwater Pit Cleaning: What to Clear Before Storm Season

Storm season across South East Queensland runs November through March, and the work that keeps water out of a house takes one afternoon in September. Start at the stormwater pit, because that’s where the roof ends up. Lift the grate, shovel the silt into a garbage bag, and look at the pit walls for cracks while the lid is off. Then work upward to the downpipe inlets and the gutters. The working threshold we use on stormwater pit cleaning across Brisbane, Logan and Ipswich: solids should never sit deeper than 75% of the pit. Past that, you’re into clogging territory, and the first heavy downpour will find out.

Most of the blocked stormwater pits we open have never been looked at. Not once.

Key takeaways

• Solids deeper than three quarters of the pit depth mean it needs clearing. On a 300mm-deep pit, that’s 225mm of build-up.
• The downpipe inlet blocks before the gutter does.
• Every downpipe should be running while it rains. One that isn’t is blocked.
• Water still sitting in a stormwater pit a week after rain points at the outlet.
• Charged systems have flush-out points for a reason. Almost nobody uses them.

Where does a stormwater system block first?

At the top of the downpipe. Debris washes off the roof, travels along the gutter to the stormwater gutter outlet, and packs into the throat where the opening narrows down to 90mm.

Which is why “my gutters are clean” and “my stormwater drains work” are two different statements. We’ve opened plenty of gutters that were spotless the length of the run, feeding a downpipe plugged solid with dried moss that came off the roof sheeting. Water backed up over the gutter line and went in through an upstairs window. Same house, different year, the opposite problem: the kerb-side stormwater gully was full of leaf litter, so water couldn’t leave the property at all and sheeted into the garage. Two chokepoints at opposite ends of the same system, and the same wet carpet either way.

Roof debris is more than leaves. Grit off old tiles, seed pods, moss, and after any roof work, offcuts and nails. One of the more predictable callouts we get is a downpipe that fails within a fortnight of a re-roof, because whatever the roofers left in the gutter went straight down the first time it rained.

Then there’s what grows into the line. A photo of a small tree sprouting out of a downpipe is funny right up until you’re the one paying for it. Underground, the same thing happens in a form nobody sees: fine roots find the joint of an old earthenware line, and inside a season you have a root mass sitting in reactive clay a foot down and several feet across. Pre-1980s suburbs get the worst of it. Figs, jacarandas and camphor laurels along the older streets of Brisbane’s inner north put out fine feeder roots that only need a hairline crack at a pipe junction.

How do I know if a stormwater pit needs clearing?

Lift the grate and measure the build-up against the depth of the pit. Silt sitting deeper than 75% of the pit is past due for a clean.

That number is the most useful thing on this page. A 300mm pit holding 225mm of sludge has almost no capacity left, and the outlet is likely already partly submerged in it. Below that line you have somewhere for a heavy flow to buffer. Above it, the pit stops behaving like a pit and starts behaving like a section of blocked pipe with a lid on top.

While the stormwater pit lid is off, look at the walls. Hairline cracking, a bulged corner, mortar washed out of a joint on a brick pit, roots feeding in through the outlet. Any of that is worth a photo before you put the grate back, because it changes the conversation from maintenance to a stormwater repair.

The clean itself is a shovel, a bucket, gloves and a garbage bag. Not a callout. Silt pits and gully pits across most Brisbane and Gold Coast blocks are shallow enough to reach from the surface, and the material coming out is wet leaf mulch and grit rather than anything hazardous. Hose the last of it through once the bulk is out and watch where the water goes. If it drains away as fast as you’re putting it in, the outlet is clear.

How do I find the stormwater pit in my backyard?

Look for a grate or a concrete lid sitting flush with the ground at the low point of the block, usually near a back fence, a paved edge, or the corner where two roof catchments meet.

Follow the downpipes if the grates aren’t obvious. Every downpipe on the house goes somewhere, and on most SEQ blocks built after the early eighties it’s a buried line running to a backyard stormwater pit and out to the kerb or an inter-allotment drain. On older Queenslanders, particularly the ones on stumps with limited under-house access, downpipes were often discharged to the surface or into stormwater soak pits that have long since silted up and been turfed over. A soft, spongy patch of lawn that stays wet three days after everyone else’s has dried out is worth probing.

Storm water drains in backyard settings get lost fast. Turf creeps over the grate, a garden bed gets built up around it, someone pavers over a pit and leaves a 40mm gap for access. If you find one buried, mark it.

Why is there standing water in my stormwater pit?

Some water in the base is normal on older pits, since many were built with a sump below the outlet to slow the flow and catch grit. Water still sitting there a week after rain is a different signal, and usually means the outlet is restricted.

There’s a genuine split on this among drainers, and it comes down to when the pit was built. The older approach treats a drainage sump as useful: heavy silt drops out in the bottom instead of travelling down the line, and you clean the sump rather than the pipe. The newer approach benches the base of the pit up to the invert level of the outflow pipe so nothing stands in it at all, on the argument that a permanent pool breeds mosquitoes and turns anaerobic. The pit still does its job on surcharge, which is what people forget: capacity in a storm comes from the volume above the outlet, not below it.

Either design will hold a little water. What matters is the level. If the water in the stormwater pit is sitting at or above the outlet invert on a dry week, that pipe isn’t clearing, and you have a partial blockage somewhere downstream of the pit rather than in it.

What can you only see while it’s raining?

Almost everything that matters. A blocked downpipe, a bad fall on a buried line, and a grading problem beside the house are all invisible on a dry day, and all of them announce themselves within two minutes of a decent shower.

Go out during rain and watch. Every downpipe should be discharging. One that’s dry, or that’s dribbling while the others run hard, is blocked at the top. That’s a sixty-second diagnostic requiring nothing but a raincoat.

Then look at the ground. No water should be pooling within a metre of the house, even mid-downpour. Water tracking across the roof in a direction that doesn’t match the fall suggests a flashing problem or a missing diverter. And check where each downpipe actually puts its water, because a clear gutter feeding a downpipe that discharges toward the low side of the house is soaking exactly the ground it was installed to protect.

You don’t need to do this every time it rains. Nobody in Brisbane in February has that kind of time. A few times a year and after anything the Bureau bothers to name is enough to catch the things that matter.

What about a charged stormwater system?

A charged system, sometimes called a wet system, runs the downpipe lines below the level of their outlet so the pipes sit permanently full of water. Pressure pushes the water back up to the discharge point, usually a rainwater tank. They need flushing, and they have flush-out points fitted for that purpose.

Nobody flushes them.

That’s not laziness so much as not knowing the flush points are there. Sludge accumulates in the bottom of a charged line the same way it accumulates in a silt pit, except you can’t see it and there’s no grate to lift. Organic matter sits in standing water for years. The result smells like sewage, and anyone who has taken a screw cap off a charged line under pressure has worn it. Take the cap off slowly, keep your face out of the arc, and don’t lose the gasket in the rush, because they’re small and they land in wet grass.

The fix worth making is mechanical. Replace the screw caps on the flush-out points with sliding gate valves, then flushing becomes a thirty-second job you’ll actually do twice a year. Open them fast when you do. A slow trickle carries nothing: you need enough flow to lift the sludge and push it out, so the valve wants to go from shut to fully open in one movement.

Should you wait until the leaves have finished dropping?

No. Clear the pits and gutters before the first storms, then again after the main leaf drop.

The objection is fair enough, since clearing a gutter in September only for a fig to empty itself into it in October feels like wasted work. But storm season doesn’t wait for the trees, and the November cells that come through the Gold Coast hinterland and the western suburbs don’t care that the leaves haven’t finished falling. Two passes, one before and one after, costs an extra afternoon and removes the argument entirely.

There’s a practical reason to book the first pass early too. Every gutter cleaner and drainage crew in South East Queensland is flat out from the first week of decent rain, and by then you’re joining a queue behind people who already have water inside. We take calls in October from homeowners who rang three other numbers first.

When does a blocked stormwater pit stop being a shovel job?

When the pit is clean and it still won’t drain. At that point the problem is in the line between the pit and the discharge point, and no amount of digging out the pit will change it.

The signs are consistent. A stormwater pit overflowing during moderate rain rather than only in a downpour. Water holding above the outlet invert on a dry week. Fine roots visible in the outlet when you clean it. Cracked or displaced pit walls. Or a pit that clogs again within weeks of being cleared, which usually means silt is washing back from a partially collapsed section downstream.

That’s the point for a camera. A pan-and-tilt CCTV inspection puts eyes on the actual condition of the line so the repair is priced against what’s there, and high-pressure water jetting at 4000 PSI clears a root mass that a snake will only bore a hole through. Where the line has cracked or a joint has failed under an established driveway or a mature yard, trenchless pipe relining rebuilds it from the inside without opening the surface. Excavation is still the right answer sometimes, particularly on a fully collapsed section, and we’ll say so when it is.

If a stormwater pit on your property is holding water, overflowing, or filling again within weeks of a clean, book a CCTV inspection before storm season rather than during it. Our crews cover Brisbane, Ipswich, Logan and the Gold Coast, and the camera tells you whether you’re looking at a clean, a patch, or a reline. Talk to a drainage specialist and we’ll get a time locked in while the calendar still has room in it.