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Carnation

Single sided 4 inch tape

3M 8067 all weather flashing tape 

This is for use for joining the various polyethylene waterproof membranes.  Get the 4" wide version because this can be used on top of 2" wide different tape (Venture tape) to form a double seal.

3M All Weather Flashing Tape #8067

It costs about $38 for a 75 foot roll.

http://solutions.3m.com/wps/portal/3M/en_US/3M-SpecialtyTapes/SpcltyTape/?PC_7_RJH9U5230GE3E02LECIE20KLR3_assetId=1180611197797

http://www.shop3m.com/3m-all-weather-flashing-tape-8067-tan-4-in-x-75-ft-slit-liner.html