Step 1
Install the water tank in its permanent location. If your water tank is going underground, install it in its location but do not cover the top with soil.
Step 2
Measure the distance from the tank vent opening to the roof surface or any surface that will cover the water tank. If your water tank will not have any surface covering the roof, do not measure anything and use a pipe at least four inches long.
Step 3
Add four inches to the distance from the tank vent opening to the roof surface. This will give you the minimum height you can size the vent piping, as you must provide a minimum of four inches between the roof surface and the vent cap, which will go at the top of the vent piping. To be on the safe side, you can add two extra inches to your distance, as this will make sure the minimum distance is met.