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Yes, the roof IS partially blocking your signals. With the TVFool report you posted (assumes 10 ft. mounting height), you should have really strong reception of all the channels you are interested in. Before you start swapping out antennas, you should mount at a location as free of local obstacles as possible. The few dB you might lose in cable loss will be more than balanced out by the up to 20 dB increase in received signal.
As far as yagi vs bow-tie, it depends! A lot of bow-tie designs use small elements that don't perform as well at the low end of UHF. The sweet spot seems to be 9" or so whiskers. Of course, a too-small yagi would suffer the same fate. Also, the VHF/UHF yagis are specifically designed to be sensitive to VHF. Most bow-tie arrays are not, with the one design I am familiar with being a do-it-yourself project.
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