This section is from the book "American Plumbing Practice", by The Engineering Record. Also available from Amazon: Plumbing: A working manual of American plumbing practice.
If the rain leader from the roof is carried inside of the house it should be of the same material as the soil pipe and should have an iron trap and brass cleanout on the house side close to the drain.
If outside leaders are used they should connect with an accessible iron trap inside of the house wall. See that leader lines are secure against settlement and leakage, as damp foundations and cellars frequently result from defective underground leaders.
Cleanoutsfor all large traps should be on the house side of the trap This gives added protection to the trap in case the covers are not made secure at any time.
All yard, area, and court drains may connect with the rain leader branch inside of the leader trap. When pipes pass through floors the opening should be sealed to prevent air and odors from the cellar and lower floors passing to the floors above.
Safe wastes, if provided, should discharge over a sink in the cellar or upon the cellar floor. In no case should they connect directly with the drains.