Roofing/venting 1921 open rafter tails hipped roof
Currently we have failing cedar shingles on a fir board base with gaps between the boards. The hipped roof has open rafter tails and no soffits. Ventilation is provided by two turbine exhaust vents and five flat pans (designed as exhaust vents) for "intake" vents. We do not have soffit vents or ridge vents, but we have been told by roofers that due to the 1921 construction they cannot put holes in the fascia boards nor install ridge vents because air will not reach to the attic. Most of the year we have adequate ventilation until the snow covers the pan vents; then we develop ice dams.
Question #1. Cedar shingles are no longer made from old growth wood thus seem to be too short lived to justify the high replacement costs. However, the breathability of cedar shingles over gapped fir boards maybe protecting our roof deck from moisture? Do you think this is true?
Question #2. Installers of asbestos shingles want to put a tight OSB layer over the existing fir boards for two reasons. One, so the old nails won't lift thru the shingles. Two, so the installer nail guns won't blow holes in the new shingles. Will there be a greater loss of intake air flow with a more tightly sealed roof using the current ventilation scheme?
Question #3. How do we install a new roof with adequate ventilation that will get us through the entire winter without ice damming?
You cannot get rid of ice damming. The reason ice damming occurs is because the snow is melting on the warm roof and as the water runs down to the bottom it refreezes there where it is colder. Sometimes the side facing the sun might minimize ice damming. Sometimes a house with a lot of heat loss keeps the attic warmer and that might minimize it but that costs a fortune for lost heat. Ice damming is the norm. Some people might feel compelled to do something about it if ice falls on them over their door as they walk out or odd things like that. Ice damming does not mean there is a problem. It's the opposite. Ice damming means the roof system is working properly.
If you can't get the attic or rafters ventilated don't worry about it. Not everyone has a condensation problem. Ventilation is a preventative thing but it doesn't necessarily mean that you need it.