Gerry is reshingling an open beam low sloped roof that has always given him problems with ice dams. He wants to know if it is worth it to tear up the first four feet, install air sheets and replace the boards?
Actually it is a great idea to perfect the over-the-wall section of the roof space anytime you are reshingling because it is so easy to get at that area from the top while the shingles are gone. Air baffles are the minimum that should be installed but if you are actually going to open the roof deck and get total access to the area, I would recommend doing much better than just air baffles when you have had a history of ice dam problems.
In many older roof structures, the roof deck is placed very close to the top of the outer wall. This means that there is never really enough space for significant insulation and good air flow both. Shooting polyurethane foam just over the top of the wall can leave that air path and give an insulating value that is twice that of fiberglass, and at the same time it prevents all air flow over the wall header. This will have the added benefit of warming the inside wall at this point, reducing or eliminating any condensation and mould problems on the ceiling.
Improving an old roof design by this method is worth every penny it costs. If your roofer himself doesn't offer the service, coordinate the work between the roofer and the polyurethane installer.