And have a Plan B, in case the Lead-lok doesn't fit.
http://youtu.be/Ft0IV-Wvw4s
There should be an accessible stop-cock at the base of the riser. The stop-cock probably won't work and will start leaking after you move it, but the pipe there is quite likely the same size lead as on the roof, so you could get a micrometer or vernier calipers onto it there.
http://youtu.be/Ft0IV-Wvw4s
There should be an accessible stop-cock at the base of the riser. The stop-cock probably won't work and will start leaking after you move it, but the pipe there is quite likely the same size lead as on the roof, so you could get a micrometer or vernier calipers onto it there.