SANITARY DRAINAGE

Minimum grades and falls for sanitary drainage

A sanitary drain has to fall at the right grade so the solids and the water move together and the line self-cleans. Too flat and it blocks, too steep and it can leave solids behind, so the grade is not a free choice.

Why grade matters

A sanitary drain works by carrying the solids along on a moving body of water so the line scours itself clean. That only happens inside a band of grades. Get it right and the drain stays clear for years. Get it wrong in either direction and you are creating a blockage that turns up later.

Too flat and too steep both fail

Too flat is the obvious one: the water sits and slows, the solids drop out, and the line silts up. Too steep is the one people forget. When a drain runs too hard the water can outrun the solids and leave them stranded, and over time that builds up the same way a flat line does. The standard sets a minimum grade so a line is not laid flat, and the principle is that the fall keeps liquid and solids travelling together.

The numbers and the principle

For a DN100 sanitary drain the well-established minimum grade is 1:60, one in fall for every sixty along. For other sizes the principle is that larger pipe can run a little flatter and still self-clean, while smaller branches need to be steeper to carry the same load. Rather than guess a percentage, take the required grade for the actual size and situation from the current AS/NZS 3500.2, and set it out to that.

What to photograph

Capture the trench or the set-out with a level or a grade tool on the pipe before backfill, so the fall is provable. Photograph the run end to end and any junctions where the grade changes. Once it is buried the grade is impossible to check without digging, so the record has to be made on the day. Elemetric keeps that with the job.

Common questions

What grade is a DN100 sanitary drain?

A minimum grade of 1:60 is the well-established figure for a DN100 sanitary drain. That is one unit of fall for every sixty units of length. Always confirm the current standard for the size and situation you are working on.

Can a drain be too steep?

Yes. If a drain is too steep the water can run away faster than the solids, leaving them behind and eventually building up. The grade has to keep the liquid and the solids moving together, which is why both a minimum and an upper limit matter.

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.