is there a maximum distance a class a pipe can be away from an exterior wall, assuming i can brace it properly? i know the min is 2". trying to go through the shed roof of an attached porch, straight up and outside of a soffit that extends 18" from the wall. the pipe would need to be about 2 feet out from the wall due to interior clearances. if i need to i could offset the black pipe inside to get the clearance to 2" from the soffit and attach the standard brace to it. does another foot or so matter since the pipe would already be 20" from the wall anyway? total run is 15' of class A. appreciate any insight