Water proofing skylight

I'm building a hardwood sash to hold a double glazed unit that will be installed on top of a curb (pitch about 10 deg.) on a flat roof. The bottom rail of sash will be barefaced to allow the glass to run over and d/glazed unit will be probably be held in with beading. Any suggestions for waterproofing beading where glass meets sash. (I've come up with tape as used on glass roofs.)
thanks
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