Grade and fall

During your site visit you will have determined the natural incline or slope of the ground on the site. You will also have confirmed the depth of the authority connection point. This information will assist you in calculating the required depth and slope of the drain trenches to ensure you achieve the minimum grade of the drain to maintain the self cleaning effect of the system.  

Open icon Grade

Grade or gradient is the term used to describe slope. In plumbing work the grade is the slope at which sanitary drainage pipes are to be laid in trenches.

Grade may be expressed as a percentage or ratio. For example, a grade of 2.00% is the same as a ratio of 1 in 50. This means over a distance of 50 m the trench or pipe will slope down, or fall, 1 m from the horizontal.

AS/NZS 3500.2 provides grade requirements as a percentage. Refer to the Pipe Grades Conversion Table, Appendix B in AS/NZS 3500.2 for the conversion of grade from percentage to ratio.

Graphic showing a grade of 1 in 50. Over the distance shown of 50 m, the pipe falls 1 metre from horizontal.
Open icon Drain grades

Table 3.2 in Section 3 of AS/NZS 3500.2:2003 provides advice on the minimum grades for vented and unvented drains. All main drains must be laid at a minimum grade of 1.65%.

A drain of DN 65 must be laid at a minimum grade of 2.50% or a ratio of 1 in 40.
A drain of DN 100 must be laid at a minimum grade of 1.65% or a ratio of 1 in 60.