When Sanitary phases are optimized using this method, a set of slope rules are applied as shown below:
| Pipe Diameter (mm) | Dwellings | EN 752 Units | Self Cleansing Slope (1:X) |
| < 100 | < 1 | < 25 | 40 |
| < 100 | >= 1 | >= 25 | 80 |
| > 100 and <= 150 | < 10 | < 25 | 60 |
| > 100 and <= 150 | >= 10 | >=25 | 150 |
| > 150 | > 0 | > 0 | 150 |
Gradients do not have to be steeper than those listed above in order to achieve minimum velocities by reference to EN 752, document H1 of The Building Regulations 2000 (Drainage and Waste Disposal) and Design and Construction Guide Approved Version 2.0 Sewerage Sector Guidance Appendix C (section B6, point 9). However, slopes may be increased to achieve consistent cover from upstream to downstream i.e. follow the surface gradient.
Specification for 100 mm pipes has been set by reference to EN 752 and document H1 of The Building Regulations 2000 (Drainage and Waste Disposal).
Specification for 150 mm pipes has been set by reference to Design and Construction Guide Approved Version 2.0 Sewerage Sector Guidance Appendix C (section B6, point 9).
Pipes of 150 mm or larger are not laid steeper than 1:60 when Optimize to Proportional Velocity at 1/3 Flow has been chosen. If for any reason this is not acceptable, then the slope may be edited. If pipes smaller than 150 mm diameter are present in the pipe file, then they will be chosen if they have sufficient capacity. However, the adopting authority may wish to use 150 mm pipes as a minimum and this may be achieved by omitting smaller diameters from the pipe size file. Also, 150 mm pipes may be permitted at flatter slopes than those selected for 100 mm pipes and excavation can be saved in some circumstances by using 150 mm pipes in lieu of smaller diameters.