Mark Hindley wrote:

When fitting the insulation, I noticed that it was relatively easy to
dent the surface with a point load, even by just kneeling on them. As
there is work to be done next spring on a dormer in the adjoining
pitched roof, I was wondering if it was sensible to sheath the
insulation boards with a thin layer of ply on top, screwed through the
insulation to the deck, to provide them with a bit of protection?

snip

Would 4mm exterior ply be sufficient?

IIRC Kingspan sell a product comprising sheets of their insulation
bonded to sheets of 4mm ply, just for this purpose, so i would have
thought it should be fine

David