To the OP: Let the insurance adjuster dig his own hole. Get him to agree that the roof needs a solid, smooth surface to adhere. Get him to agree that decking less thickness than the original decking is sub standard. Then get him to agree to let you tear off the original roofing and examine the decking that remains. If it does not pas the requirements he has said is necessary, he will pay to re-deck. If you are right, then a new deck is put on. If he is right (which of course he won't be) the membrane goes over the old decking.
From a legal aspect, at least in the States, the insured needs to be made whole after a loss. That means putting whatever is insured in the same condition as before the loss. Is a roof not properly adhered to the decking and not smooth as before the loss putting the insured whole again?
