SANITARY DRAINAGE
Gully trap installation rules in Victoria
A gully trap is the overflow relief point of the sanitary drain and a disconnector trap in one. Get the surround height and the grate level wrong and the drain will surcharge inside the house instead of out at the gully.
What a gully trap is for
A gully trap, often the overflow relief gully, does two jobs. It is a disconnector trap that takes waste from fixtures into the drain through a water seal that keeps drain air out, and it is the relief point for the whole drain. If the drain blocks or surcharges, the idea is that it spills outside at the gully grate before it ever backs up through the lowest fixture inside the building.
The two height principles
Two levels decide whether a gully works. The surround sits a set distance above the surrounding paving or finished ground, so surface water, grit and leaves do not wash into the drain. The grate then sits below the overflow level of the lowest fixture it serves, so the gully relieves first and the fixture stays dry. Get these two relationships right and the relief function works. AS/NZS 3500.2 sets the actual dimensions, so confirm the current clause rather than working from memory.
Set-out and clearance
The gully needs the trap and riser set out so the seal is maintained and the grate is accessible and clear for cleaning. Keep it away from where paving, garden beds or later landscaping could bury the surround or raise the surrounding level above it, because that quietly defeats the height relationship. The connection of fixture branches into the gully also has its own arrangement under the standard.
Common failures
- Surround flush with or below the paving, so the yard drains into the sanitary drain.
- Grate set above the lowest fixture's overflow, so the house floods before the gully relieves.
- Later paving or concrete raising the surrounding level after the fact.
- Surround cracked or grate missing, breaking the seal and relief function.
What to photograph
Take a side-on shot that shows the surround height against the surrounding paving or ground, a shot looking down on the grate, and a wider angle showing the gully relative to the nearest fixtures so the relief level reads. Elemetric keeps that with the job so the heights are provable later.
Common questions
What does a gully trap do?
A gully trap is a disconnector trap with an overflow relief function. It takes waste discharge from fixtures into the drain through a trap that holds a water seal, keeps drain air out of the property, and acts as the overflow relief point so the drain surcharges outside at the gully rather than back up through a fixture inside.
How high should the gully surround be?
Describe it as a principle rather than a fixed figure: the surround sits a set distance above the surrounding paving or ground so surface water and grit do not wash in, and the grate sits below the overflow level of the lowest fixture it serves so the gully relieves first. Check the current AS/NZS 3500.2 clause for the exact dimensions on your job.
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Download on the App Store →General information for licensed tradespeople, not legal or regulatory advice. The licensed plumber remains solely responsible for compliance. Refer to the current AS/NZS 3500 standards and the Building and Plumbing Commission (formerly the VBA) for authoritative requirements.