The 2 degree drop does not start at the top of your drains. The drain first drop vertically at 0 degrees for a few inches through the foundation and the hydraulic pressure of the waste water falling will help to power the solid waste down the 2 degree grade. Water isn't compressible so a vertical drop builds up a lot of pressure when it collides with anything in the drain. This is why you can't have straight down waste water pipes in tall buildings. They limit their vertical height by redirecting it horizontally every few floors because the force of a straight down drop from 40 stories would absolutely burst the pipes and fitting at the lower floors of the building. Hydraulic pressure makes the system work well with only 2 degrees of grade for residential installations.
The 2 degree drop does not start at the top of your drains. The drain first drop vertically at 0 degrees for a few inches through the foundation and the hydraulic pressure of the waste water falling will help to power the solid waste down the 2 degree grade. Water isn't compressible so a vertical drop builds up a lot of pressure when it collides with anything in the drain. This is why you can't have straight down waste water pipes in tall buildings. They limit their vertical height by redirecting it horizontally every few floors because the force of a straight down drop from 40 stories would absolutely burst the pipes and fitting at the lower floors of the building. Hydraulic pressure makes the system work well with only 2 degrees of grade for residential installations.
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