Im hoping someone on this forum is a retired roofer and can give some professional advice.
I have installed EPDM roof membrane on a roof before and know that there can not be gaps larger then 1/8" in the plywood. That being said, the roof I am planning on replacing is three layers of try ply covering the original metal roof which is over 100 years old. So naturally the wood planks underneath are the same age. The planks are not milled on the sides, so in some areas there are 1" gaps between parts of the planks. However, the planks are in very good condition.
My plan is to rip off the triply and metal roof and install 1 or 2" Polyiso Insulation boards. The boards would cover the gaps, add R value and make sure the EPDM doesn't come in contact with and tar products that may be on the wood.
I REALLY don't want to remove the planks and put down plywood. I have no experience with Polyiso but I am assuming that it will be stable enough to cover the gaps in the wood. I guess I'm really just looking for a confirmation.
1" or 2" iso will span 1" gaps. If the gaps are more then 2 5/8" then 1" iso is not recommended. Make sure to get the proper screws and plates to fasten the ISO and run a nailer around the outside edge.