royfellows said:YOU! I might have known
Brains said:Measuring sticks for cutting timber to length, better than a tape measure any day
A long hole is drilled in the roof and the small inner rod was fixed to the end by the wire and one of the clips top right. The outer tube is fixed near the roof by the other clip with the two overlapping, both showing green. If the roof starts to come away between the two fixings the inner rod will move to yellow then red, indicating failure. If the roof delaminates below the bottom fixing the outer tube will go yellow then red