Preventing ice dams on a properly designed roof is all about keeping heat off the underside of the deck. They are most often caused by leaks through the building envelope into the attic space.
I also do unvented roofs, attic's and crawlspaces. There is an enormous amount of bad information around.
Back in the 70's my parents built a ski house in Vermont -- simple Swiss looking building with long overhangs with a back slope roof on the garage/outbuilding. This was during the 70's energy crisis and everyone was trying new things -- most did not end well. Lots of plastic VB .. especially bad. We had a neighbor who was a german MIT educated engineer ... when my dad proudly showed him the design -- he said it would not work. he knew what he was talking about -- because he lived in the area where this design originated. He eliminated all the venting and we used what must have been an early foam board on deck -- all of these structures were cathedral with wood ceilings.
Venting can actually cause problems -- the original design was built by others in the area. On calm cold sunny days with dark exterior and large over hangs -- the sun will heat the siding and this heat can actually rise up and enter the vents under the eaves and into the attic .... causing the problem.
Read some of the building science stuff -- its crazy (double roofs). The USA is not the only place with snow and heated buildings.
I also do unvented roofs, attic's and crawlspaces. There is an enormous amount of bad information around.
Back in the 70's my parents built a ski house in Vermont -- simple Swiss looking building with long overhangs with a back slope roof on the garage/outbuilding. This was during the 70's energy crisis and everyone was trying new things -- most did not end well. Lots of plastic VB .. especially bad. We had a neighbor who was a german MIT educated engineer ... when my dad proudly showed him the design -- he said it would not work. he knew what he was talking about -- because he lived in the area where this design originated. He eliminated all the venting and we used what must have been an early foam board on deck -- all of these structures were cathedral with wood ceilings.
Venting can actually cause problems -- the original design was built by others in the area. On calm cold sunny days with dark exterior and large over hangs -- the sun will heat the siding and this heat can actually rise up and enter the vents under the eaves and into the attic .... causing the problem.
Read some of the building science stuff -- its crazy (double roofs). The USA is not the only place with snow and heated buildings.