Very rarely will a roof sewer vent need to be cleaned. On the occasion that you need to remove debris or a nest, the best way is to try a shop vac. Then you could try a hose to clear any other items...
on a main line they should be installed at a minimum of every 100'. Also any turning radius 135* or more. Every main line needs an end of a line clean out. Every fixture that is more than 5' from a...