I have a uhf yagi uda antenna mounted on the roof I have all the element
(alu welding rods) clamped to a alu- boom except the active elements.
Does this effect the preformance of the antenna in that the reflector and
directors are all connected from an electric point of view?

Provided the lengths are corrected for the boom (connected or not connected)
there is no differance in the antenna. It might make some differance if the
connection is not 100% all the time as the elements could become loose and
not be connected . YOu can get all kinds of noise on signals if the
elements become loose. That is one plus for insulating the elements.
Unless welded to the boom many elements will loosen up some in time.