I wonder if anyone can help with a small venting problem. A vent is needed at the top of the run and it is to go from the last w.c. inside the house, through the ceiling above, and out through the roof. It will be an obstruction in the room above and have to be boxed in. However, there is an unused chimney just behind the downstairs w.c. Is it permitted to vent through a chimney please? It won't be straight, but I don't see why a flexible pipe wouldn't work.