I had a new roof installed last year. I noticed that on one side of my house when it gets below freezing (I live in Philly) I get a significant amount of frost on the exposed side of the plywood that is under my roof. See attached picture. What's weird is that my house is symmetrical and I only see this frost on the front of my house. The plywood on the back of side of my house is completely frost free. I don't have a bathroom or anything else on the front side that would produce moisture. I was thinking of putting a dehumidifier in the crawl space under the front side but this will be a pain to empty all the time and I imagine that this could be a fire hazard. Any ideas or suguestions would be appreciated.

Yes you need more roof venting. The one side is probably related to how the sun hits the different parts of the roof..
Is this a pitched roof? And if it is do you have a ridge vent at the top of the peak or any soffits on the eves.
How is the insul in the ceiling below that frost area? If this is a slanted roof like the ones on the town houses you still need a roof vent in the roof.
If the insulation is not good below that area and it and no vapor barrier either than that is another thing to be checked.

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