I have a soil stack into a clay elbow pipe. Water leaks from over the top of the clay pipe when waste goes down the soil stack, there is no seal. If I move the stack slightly so it completely covers the clay waste pipe, water builds up, if it is slightly ofset the waste goes down better. Where should the stack be positioned in relation to the waste pipe and Should there be a seal? if not how can i stop the waste coming out of the pipe, what seal or filler do i need? And how do I fit it in a very tight fitting, obviously not tight enough as it is leaking waste!!!!