GENERAL PRINCIPLES APPLICABLE TO ANY DRAINAGE SCHEME
1. The diameter of the drains should be 4 in. for domestic buildings, and this size is usually proportionate to the volume of liquid expected through the pipes.
2. Where a number of houses are drained by a single pipe or in cases of large buildings, then a 6-in. pipe is generally required for part of the drainage system.
3. Self-cleansing velocities are required, and this means that the drains should be laid to gradients of 1 in 40 for 4-in. and 1 in 60 for 6-in. pipes, where practicable. It is harmful to lay pipes to an excessive gradient, and care should be exercised to obtain the right fall.
4. There should be means of inspection, usually obtained by the construction of inspection chambers suitable placed.