This is my first post so please forgive me if I do not do it exactly right or if it is in the wrong location, but here is my question.
We live in Northern California and have an add on with a flat roof. A couple of years ago it started to leak, we tried and tried to fix it but it still leaked so we tore off the old roof leaving only the plywood base and replaced it with another flat roof. It still leaked so again we tore off the old roof leaving only the plywood base and this time replaced that with a shingle roof and it still leaked. So we once again tore off the old roof leaving only the plywood base and this time replaced it with a metal roof and it STILL leaks. AUGH.... What we have done so far is... black jacked old plywood seems, put felt on top of old plywood, put new plywood on top of felt offsetting the seams, black jacked all new seems, installed ALL new flashings trims etc. We have followed manufactures and suppliers directions to the "T". But it still leaks. THIS IS NOT A LITTLE LEAK IT IS LEAKING ACROSS THE WHOLE ROOM. We have now torn off the wet and soggy ceiling and can see it coming form the old seams. We believe it may be coming from where the addition meets the original roof and running down because it is locking metal roof sheets, how would the water get through that? We have done everything we can think of and it has cost us thousands of dollars. PLEASE any advise on what we're looking for and how to stop it.
I will download pictures as soon as I can... Thank you for any advice!