Overflow Relief Gully: What It Is and Why It's Overflowing

An overflow relief gully is the small grated fitting in the ground outside most Sydney homes, usually near the bathroom or laundry. It's connected to your sewer, and its job is to let sewage spill out into the yard if the sewer blocks, rather than back up through your shower or floor waste.

If yours is overflowing, bubbling or smelling, it's almost always telling you the sewer downstream of it is blocked. Here's how it works, the rules around it, and what to do next.

What is an overflow relief gully?

Plumbers call it an ORG, and you'll also hear it called a gully trap or overflow gully. It's a short vertical riser with a water-sealed trap at the bottom, a loose grate on top, and a connection into the house sewer below.

Older homes often have a gully with basin and laundry waste pipes discharging over or into it. Newer homes usually have a plain riser with just the grate. Either way, the principle is the same.

The water sitting in the trap stops sewer gas coming up through the grate. The loose grate is the clever part: when the sewer backs up, the pressure lifts it and sewage escapes outside, at the lowest safe point, before it can reach anything inside the house.

What it protects

Without one, a blocked sewer fills until the water finds the lowest opening. In most houses that's the shower drain or a floor waste. That means sewage across the bathroom floor, into the grout, under the vanity and sometimes into the timber subfloor.

A mess on the lawn is unpleasant. A mess through the bathroom is a much bigger clean-up, and it's a health problem as well. The gully is there so you get the first one and not the second.

How high should an overflow relief gully be?

The plumbing code, AS/NZS 3500, sets the heights, and they matter because the whole system relies on gravity.

• The top of the grate must sit a set distance below the lowest fixture it protects, typically the shower or floor waste inside. That gap is what makes the gully overflow first. On most jobs that's at least 150 mm.
• The grate must also sit above the surrounding ground, so rainwater and garden run-off don't pour into the sewer. That's typically 75 mm above soil or lawn, and less where the area around it is paved.
• It has to stay accessible, with the grate loose enough to lift under pressure.

If your gully is too high relative to the inside floor, it may not overflow before the shower does. If it's too low or sitting in a hollow, it takes in stormwater every time it rains, which Sydney Water does not allow and which can overload the sewer.

Why is my overflow relief gully overflowing?

When the gully overflows, the problem is almost never the gully itself. It's doing its job. Water can't get past a point further down the line, so it's coming back up at the first escape route.

A few signs usually come with it:

• Toilets flushing slowly or rising before they drain
• Gurgling from the shower, basin or bath when something else is running
• Sewage smell in the yard, especially around the grate
• Water in the gully that doesn't drop between uses

Tree roots in old clay sewers

This is the big one in the Hills District. Much of Castle Hill, Baulkham Hills, Carlingford and Pennant Hills was built in the 1960s to 80s with earthenware sewer pipes. They were laid in short lengths with joints every metre or so, and those joints are where roots get in.

Established jacarandas, liquidambars, camphor laurels and gums send fine roots towards the moisture and nutrients inside the pipe. Once they're in, they grow into a mat that catches toilet paper and wipes until the line blocks. Wet winters make it worse. Our guide to tree roots in pipes goes into this in more detail.

Wipes, fat and foreign objects

"Flushable" wipes don't break down the way toilet paper does, and they're a common cause of a blocked sewer pipe even in newer PVC lines in Kellyville or The Ponds. Kitchen fat that sets further down the line does the same.

Collapsed or displaced pipe

Ground movement in clay soils, heavy vehicles over a shallow line, or old age can crack or shift a section of pipe. The line then holds water at the dip and collects everything that comes through.

A problem in the sewer main

Sometimes the blockage isn't on your property at all. If several neighbours have issues at the same time, or your gully is overflowing and nothing inside the house has been used, the main itself may be blocked.

Is a blocked sewer my responsibility or Sydney Water's?

This is the question we get asked most on these jobs, and the answer depends on where the blockage is.

In broad terms, you own and maintain the private sewer pipes on your property, running from the house down to where they connect to Sydney Water's sewer main. Sydney Water looks after the main itself. Some older properties have the main running through the backyard, which confuses things further.

Where the blockage is	Who is usually responsible
Inside the house (toilet, shower, basin waste)	Property owner
Private sewer line on your property	Property owner
At or in the Sydney Water sewer main	Sydney Water
Shared line serving more than one property	Can be shared, depends on the property

The practical answer: if only your house is affected, the blockage is most likely in your private line and you need a plumber. If your neighbours are affected too, or sewage is coming out of a Sydney Water maintenance hole, call Sydney Water's faults line first.

When we attend, we put a drain camera through the line. That shows exactly where the blockage is and whether it's on your side of the connection. If it's in the main, we'll tell you and you can hand it over to Sydney Water with that information.

Can I cover my gully grate?

No. This is one of the most common causes of the problem we find in the Hills and Hornsby Shire.

A new garden bed, a paved courtyard, a deck or a pile of mulch goes over the top of the gully, and nobody thinks about it until the sewer blocks. With the grate buried or sealed, the overflow has nowhere to go but inside. We've also seen grates glued down or screwed in place to stop smells, which defeats the entire purpose.

Keep the grate clear, visible and loose. If you're landscaping or renovating and the gully is in the way, it can be relocated or raised, but that's a job for a licensed plumber so the heights still work.

What you can safely do yourself

• Stop using water in the house. Every flush and shower adds to what's backing up.
• Check whether the grate is clear and can lift. Remove any mulch, soil or pavers sitting on it.
• Ask your neighbours whether they're having problems too.
• Keep children and pets away from any overflow, and wash your hands after being near it.
• If the gully smells but isn't overflowing, the trap may have dried out. Pouring a bucket of water in to refill it often fixes that.

What needs a licensed plumber

• Clearing a blocked sewer line, whether with a high-pressure jetter or an electric eel
• A camera inspection to find where and why it blocked
• Repairing or relining a cracked, root-damaged or collapsed pipe
• Raising, lowering or relocating the overflow relief gully
• Any change to how waste pipes connect into it

Sewer work in NSW has to be done by a licensed plumber, and the gully heights are part of what's checked on compliant work. Hiring a drain cleaning machine yourself can clear a blockage, but it can also push a root mass further down or damage an old clay pipe.

If your sewage smell is lingering without an overflow, our sewage smell help page walks through the other likely causes.

Getting it fixed

An overflowing gully rarely fixes itself. Root blockages come back, usually within months, because clearing the line trims the roots but doesn't seal the joint they came through. Finding out whether it's a one-off or an ongoing pipe problem is what saves you from calling us again next winter.

If your overflow relief gully is overflowing anywhere across the Hills District, Hornsby Shire or the Upper North Shore, we can usually get there the same day, find the blockage with a camera and clear it. There's no call-out fee, and you'll have a fixed price before we start. Have a look at our blocked drain and sewer clearing service for what's involved, or see how we price our work.