Any "roof penetrating" modification is best installed at construction
time or during re-roofing. INEVITABLY, a retro-fit on an existing roof
will end up leaking, because nobody is willing to 'weave' shingles
anymore.

If the job were _really_ done right, it would be just as good on an
existing roof as on a new one. But that just never happens anymore in
today's G.A.S. construction trade.

Lloyd

My dad installed these. They leaked and ruined a bunch a sheet rock.
He was sorry he did it.

On another note, Tom would you send me your new email address
karltownsendATembarqmailDOTcom