We have a similar plastic board which Mas' describes, inner facing on a single leaf 3' high wall of a small double glazed porch on the NW face of house ... That wall used to be wet with condensation in low temperatures when kitchen door opened and warm moist air hit it -- Not with the plastic imitation planking tho', I guess tis the air gaps behind the plank effect .. I could not believe it, no trickles at the inner wall base either, it has been dry as bone for several years since fitting, not pretty like wood, but dead functional.
When my roofline was done I too thought about Mas's idea of a firm base to the roofline plastic edging, it would provide good support, the plastic being very flexible will bow etc between the rafters.
The pix below is similar but the angle is obviously suiting the underboard to the plastic and may not reflect 'real life'.
Venting is quite clever, there are several rows of plastic pins which locate and space the plastic off the top edge of the fascia board ( opposite side to the bird comb ), allegedly the pins are too closely packed to allow a queen wasp through (?) but provides full length ventilation.
A closer look at the 'venting' just visible at the top edge of the fascia board, (look past the 'end trim' which extends above the top edge of fascia in the pix) air flows up the front face of fascia behind guttering, under the plastic edging which is spaced off the board and through the 'pinned' gap, hopefully into roof space.
Anyway that's my 'tiny' lot Mick nothing more to offer mate .. good luck !
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