Zero. The point of the shield is to provide an additional layer of protection to the decking in the event that water is able to back up on the roof. It should start at the edge of the roof. Having any wood showing completely defeats the purpose of installing it. Generally in our warmer climates usually just a couple runs is fine. In colder climates where snow build-up is common, they often do the whole roof.Can anyone tell me how much raw wood should be exposed between the ice and water shield underneath my shingles and the gutter?. The roofers tell me this is normal. I believe ice and water shield should cover all the exposed wood that might get splashed or have ice contact.