I am plumbing storm water from my shed to a tank. Down pipe is 90mm . Just above the ground I want to fit a Test Opening in 100mm. To go from 90mm pipe to the 100 mm test opening T, should I use a 90/100adaptor, or a 90/100 collar?
Sorry, I forgot to add, this will all be inspected by the council plumbing inspector, so I want to get it right first go. I dont want to phone and ask him, as I did not enjoy my last discussion with him.