Penetrating roof -- what sealant to use?

I have to make penetrations in my roof for an attic fan and a vent cap for a bathroom fan. The bathroom fan vent cap installation instructions recommend using roof tar to put under the flashing and to seal the roofing nails. The attic fan installation instructions recommend pure silicon sealant for the same thing.
Any recommendations as to which I should use?
Thanks!
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Bob in CT
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Bob in CT wrote:

I would recommend using what they recommend. I doubt if either is going to be a problem, but it is possible the materials used by each are different and the use of the other product may damage those materials.
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Joseph E. Meehan

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don't really need any sealer at all, just slide the flashing up under the shingles.
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It is asphalt shingles. I think I'll still put a little sealant up there, just in case!
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Bob in CT
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Paul James wrote:

I have seen it recommended by the manufacturer for use with asphalt shingles, not so much to seal the opening as to glue the roofing tab down.
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Joseph E. Meehan

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Are your flashings metal or plastic, if plastic most manufacturers don't recommend that you use any kind of tar based sealer. For metal if you have any nail heads showing it's a good idea to dab them with a little tar. I agree that if you break the seal on the shingles around your flashing a little dab of roof cement will stick them back down.
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