No, not if you have proper attic insulation and ventilation. Otherwise it could get up to 140 to 160 degrees. That is why attic insulation and ventilation are so important.
Improperly vented attics. Not enough insulation. Bathroom exhaust vents that are not vented through the roof. Also a furnace humidifier that is set at over 40% in winter. Insulation blocking the intake vents in the overhangs.
The warm air that gets by your attic insulation gets trapped if you don’t have enough insulation or improper ventilation. This warms up the bottom side of the roof decking, which in turn increases snow melt, which causes icicles and ice dams.
The roof board or plywood at the leading edge is cut back 2″. Then an aluminum vented drip edge is installed at the eave edge which will exhaust the heated attic air out either the roof vents or ridge vent. This will cool down the attic which can reach temperatures of 140 degrees which will then save you money on air conditing costs, eliminate moisture in the attic in the winter time, and reduce ice dams.
Yes, with adequate intake ventilation from each overhang. Unless it is a “hiproof” (4 sided), which does’nt normally have a very long peak, or if you have many gable dormers which have no overhangs, and would be where your intake soffit vents would go. It is important to have a “balanced” amount of intake ventilation from both sides.
If shingles are installed as per manufacturers recommended installation specification, with proper attic ventilation requirements, yes 2 layers are allowed and the warranty is intact.
Yes, with proper installation procedures strictly followed as per manufactures specifications, and with proper attic ventilation requirements being followed, a roof will last 25-50 years depending on the grade of shingle you select.
Most cities have adopted national building codes that only allow 2 roofs, due to weight bearing loads of the shingles themselves, and snow loads on the pre-engineered truss systems on 24″ centers.