A house that was inspected had frost on the bottom 8 to 12 inches of the underside of the roof sheating. This is found on the north side of the house. the house is currently unoccupied , but the interior Temp. has been maintained at 60 Degrees.We have had below O Temps for the last 10 days.
the house was built 10 years ago. My Question iswhat could be the cause ?? is there a solution
Patrick;
The pictures you submitted seem to indicate the the proper vent is installed intermittently, and the possibility of the insulation not being properly installed at the wall/soffit intersection might exist.
The improper use and installation of the proper vents would create what you have witnessed.
Marcel 
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Any ice build-up in the attic is signs of warm air traveling upward because of its natural buoyancy. As it reaches the ceiling in the top floor, it seeks ways to rise even higher through cracks and gaps in the ceiling and walls. Some of those pathways are obvious; many others are not.
Those openings around and through recessed canister lights, whole-house fan installations, attic-access hatchways and pull-down stairs, and electrical boxes in the ceiling and uninsulated walls all provide conduits from the house into the attic.
Additionally, heat is conducted upward through the top-floor ceiling through inadequate attic floor insulation.
As others stated the problem is caused by the natural movement of moisture out of a home and compounded by improper ventilation.
Might I suggest you go to www.Airvent.com, look for the Principles of Attic Ventilation manual, download and study…
Now take what you know from the manual, do the math for the home and compare what it should have to what it does have…in my experience dealing with roofs and attics I think you will find the answer is obvious…the builder didn’t understand ventilation and therefore was not about to waste precious pennies on things that ‘don’t really matter’ like baffles in each rafter spacing, soffit with the correct NFV, and a ridge vent with an external baffle.
These problems too often go un-noticed such as in this case for 10 years…long past the homes warranty. Often when found the repairs made are only what is needed to nail on a new roof because the roofer is just that a roofer not a roofing and ventilation specialist who is brave enough to say your home was not constructed properly and that is why this happened. To do this breaks down the warm fuzzy feeling that the homeowner had about the builder and makes the bearer of the news a (insert your own word). The news and information given by the person who does know what the problem really is challenged by the homeowner, the homeowner then seeks out family members, clergy and people in the workplace for their opinion. Upon finding one person that put a roof on something or at least had a brother in law that put on a roof or two that assures them that it wasn’t done this way 30 years ago and that a shingle is still a shingle. The bearing of the truth is now chastised and run out town on a rail until repairs to the roof are made without corrections to the ventilation…Until one day the homeowner is in the attic looking for something and sees the problem again…and the SOB that told him the truth he didn’t want to hear or spend money to correct gets to say I told you so.
Sorry I rambled…just some flashbacks…just educate yourself about attic ventilation and be prepared to have your findings and recommendations challenged…by the un-informed.
missing vents at eave. each bay should have a baffle that allows venting from the soffit to the ridge or gable vent (hope there was a ridge or gable vent). the pic on the right shows only one baffle. not enough air flow. there are calcs avail for how much vent space per sq ft of attic space.
andy
Hey Pat, am I seeing things correctly on the first picture are the nail shafts rusty?
Just had the same situation in an occupied home. 2’x6’ area right above main 2nd floor bathroom. Soffit/attic vents only placed every 3rd or 4rth row. Was worse ice build up than your photos, so I suspected washroom exhaust duct may have been breached as well. Turned out to be the lack of vents.