How high above the roof should a soil vent extend?
The soil pipe, also known as the vent stack or stack pipe, provides air for the plumbing system. Depending on your region, the vent stack may protrude only a few inches above the surface of the roof. This may seem suitable, since the only purpose for the stack is to provide air and expel sewer gases.
Make sure the stack to protrude a minimum of 15 cm (6 inches) from the roof. Some builders recommend a minimum of 30 cm (12 inches). Plumbing books vary in their advice.
According to the Uniform Plumbing Code, the soil pipe, or "stack pipe," shall extend "vertically not less than six (6) inches... above the roof nor less than one (1) foot... from any vertical surface."
UK building regulations do not apply to soil pipes. You also do not require planning permission for a soil vent out of a roof. Listed buildings may have other restrictions.