I understand that I might be able to make improvements to the our soil stack by bringing the top of it into the boxing-in in the bathroom and using an air admittance valve from Screwfix (95531).
The present stack is internal and exits the building through the roof above the bathroom. Where it passes through the roof tiles has been the source of a couple of serious leaks in the past that have damaged the bathroom ceiling so I would like to get rid of this weak point.
So, do these valves work? If they do, how low can I bring the top of the stack? Could it be, say, level with the top of the close-coupled cistern?
Removing the boxing-in in the upper part of the bathroom and reducing it to around 800mm in height would also have benefits as wall space is in short supply.